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PLEASANT WAYS IN SCIENCE.

WORKS BY RICHARD A. PROCTOR.

LIGHT SCIENCE FOR LEISURE HOURS: Familiar Essays on Scientific Subjects. Crown 8vo, 3s. 6d.

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PLEASANT WAYS IN SCIENCE. Crown 8vo, 3s. 6d.

MYTHS AND MARVELS OF ASTRONOMY. Crown 8vo, 3s. 6d.

NATURE STUDIES. By Grant Allen, A. Wilson, T. Foster, E. Clodd, and R. A. Proctor. Crown 8vo, 3s. 6d.

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PLEASANT WAYS
IN SCIENCE

BY
RICHARD A. PROCTOR

AUTHOR OF
“ROUGH WAYS MADE SMOOTH,” “THE EXPANSE OF HEAVEN,” “OUR PLACE
AMONG INFINITIES,” “MYTHS AND MARVELS OF ASTRONOMY,”
ETC. ETC.

NEW IMPRESSION

LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY
1905

CONTENTS.

 

PAGE

Oxygen in the Sun 1 Sun-Spot, Storm, and Famine 28 New Ways of Measuring the Sun’s Distance 56 Drifting Light Waves 77 The New Star which faded into Star-Mist 106 Star-Grouping, Star-Drift, and Star-Mist 136 Mallet’s Theory of Volcanoes 151 Towards the North Pole 156 A Mighty Sea-Wave 178 Strange Sea Creatures 199 On some Marvels in Telegraphy 232 The Phonograph, or Voice-Recorder 274 The Gorilla and other Apes 296 The Use and Abuse of Food 330 Ozone 347 Dew 357 The Levelling Power of Rain 367 Ancient Babylonian Astrogony 388

PREFACE.

It is very necessary that all who desire to become really proficient in any department of science should follow the beaten track, toiling more or less painfully over the difficult parts of the high road which is their only trustworthy approach to the learning they desire to attain. But there are many who wish to learn about scientific discoveries without this special labour, for which some have, perhaps, little taste, while many have scant leisure. My purpose in the present work, as in my “Light Science for Leisure Hours,” the “Myths and Marvels of Astronomy,” the “Borderland of Science,” and “Science Byways,” has been to provide paths of easy access to the knowledge of some of the more interesting discoveries, researches, or inquiries of the science of the day. I wish it to be distinctly understood that my purpose is to interest rather than to instruct, in the strict sense of the word. But I may add that it seems to me even more necessary to be cautious, and accurate in such a work as the present than in advanced treatises. For in a scientific work the reasoning which accompanies the statements of fact affords the means of testing and sometimes of correcting such statements. In a work like the present, where explanation and description take the place of reasoning, there is no such check. For this reason I have been very careful in the accounts which I have given of the subjects here dealt with. I have been particularly careful not to present, as established truths, such views as are at present only matters of opinion.

The essays in the present volume are taken chiefly from the Contemporary Review, the Gentleman’s Magazine, the Cornhill Magazine, Belgravia, and Chambers’ Journal. The sixth, however, presents the substance (and official report) of a lecture which I delivered at the Royal Institution in May, 1870. It was then that I first publicly enunciated the views respecting the stellar universe which I afterwards more fully stated in my “Universe of Stars.” The same views have also been submitted to the Paris Academy of Science, as the results of his own investigations, by M. Flammarion, in words which read almost like translations of passages in the above-mentioned essay.

RICHARD A. PROCTOR.

PLEASANT WAYS IN SCIENCE.

OXYGEN IN THE SUN.

The most promising result of solar research since Kirchhoff in 1859 interpreted the dark lines of the sun’s spectrum has recently been announced from America. Interesting in itself, the discovery just made is doubly interesting in what it seems to promise in the future. Just as Kirchhoff’s great discovery, that a certain double dark line in the solar spectrum is due to the vapour of sodium in the sun’s atmosphere, was but the first of a long series of results which the spectroscopic analysis of the sun was to reveal, so the discovery just announced that a certain important gas—the oxygen present in our air and the chief chemical constituent of water—shows its presence in the sun by bright lines instead of dark, will in all probability turn out to be but the firstfruits of a new method of examining the solar spectrum. As its author, Dr. Henry Draper, of New York, remarks, further investigation in the direction he has pursued will lead to the discovery of other elements in the sun, but it was not “proper to conceal, for the sake of personal advantage, the principle on which such researches are to be conducted.” It may well happen, though I anticipate otherwise, that by thus at once describing his method of observation, Dr. Draper may enable others to add to the list of known solar elements some which yet remain to be detected; but if Dr. Draper should thus have added but one element to that list, he will ever be regarded as the physicist to whose acumen the method was due by which all were detected, and to whom, therefore, the chief credit of their discovery must certainly be attributed.

I propose briefly to consider the circumstances which preceded the great discovery which it is now my pleasing duty to describe, in order that the reader may the more readily follow the remarks by which I shall endeavour to indicate some of the results which seem to follow from the discovery, as well as the line along which, in my opinion, the new method may most hopefully be followed.

It is generally known that what is called the spectroscopic method of analyzing the sun’s substance had its origin in Kirchhoff’s interpretation of the dark lines in the solar spectrum. Until 1859 these dark lines had not been supposed to have any special significance, or rather it had not been supposed that their significance, whatever it might be, could be interpreted. A physicist of some eminence spoke of these phenomena in 1858 in a tone which ought by the way seldom to be adopted by the man of science. “The phenomena defy, as we have seen,” he said, “all attempts hitherto to reduce them within empirical laws, and no complete explanation or theory of them is possible. All that theory can be expected to do is this—it may explain how dark lines of any sort may arise within the spectrum.” Kirchhoff, in 1859, showed not only how dark lines of any sort may appear, but how and why they do appear, and precisely what they mean. He found that the dark lines of the solar spectrum are due to the vapours of various elements in the sun’s atmosphere, and that the nature of such elements may be determined from the observed position of the dark lines. Thus when iron is raised by the passage of the electric spark to so intense a degree of heat that it is vaporized, the light of the glowing vapour of iron is found to give a multitude of bright lines along the whole length of the spectrum—that is, some red, some orange, some yellow, and so on. In the solar spectrum corresponding dark lines are found along the whole length of the spectrum—that is, some in the red, some in the orange, yellow, etc., and precisely in those parts of these various spectral regions which the bright lines of glowing iron would occupy. Multitudes of other dark lines exist of course in the solar spectrum. But those corresponding to the bright lines of glowing iron are unquestionably there. They are by no means lost in the multitude, as might be expected; but, owing to the peculiarity of their arrangement, strength, etc., they are perfectly recognizable as the iron lines reversed, that is, dark instead of bright. Kirchhoff’s researches showed how this is to be interpreted. It means that the vapour of iron exists in the atmosphere of the sun, glowing necessarily with an intensely bright light; but, being cooler (however intensely hot) than the general mass of the sun within, the iron vapour absorbs more light than it emits, and the result is that the iron lines, instead of appearing bright, as they would if the iron vapour alone were shining, appear relatively dark on the bright rainbow-tinted background of the solar spectrum.

Thus was it shown that in the atmosphere of the sun there is the glowing vapour of the familiar metal, iron; and in like manner other metals, and one element (hydrogen) which is not ordinarily regarded as a metal, were shown to be present in the sun’s atmosphere. In saying that they are present in the sun’s atmosphere, I am, in point of fact, saying that they are present in the sun; for the solar atmosphere is, in fact, the outer part of the sun himself, since a very large part, if not by far the greater part, of the sun’s mass must be vaporous. But no other elements, except the metals iron, sodium, barium, calcium, magnesium, aluminium, manganese, chromium, cobalt, nickel, zinc, copper, and titanium, and the element hydrogen, were shown to be present in the sun, by this method of observing directly the solar dark lines. In passing, I may note that there are reasons for regarding hydrogen as a metallic element, strange though the idea may seem to those who regard hardness, brightness, malleability, ductility, plasticity, and the like, as the characteristic properties of metals, and necessarily fail to comprehend how a gas far rarer, under the same conditions, than the air we breathe, and which cannot possibly be malleable, ductile, or the like, can conceivably be regarded as a metal. But there is in reality no necessary connection between any one of the above properties and the metallic nature; many of the fifty-five metals are wanting in all of these properties; nor is there any reason why, as we have in mercury a metal which at ordinary temperatures is a liquid, so we might have in hydrogen a metal which, at all obtainable temperatures, and under all obtainable conditions of pressure, is gaseous. It was shown by the late Professor Graham (aided in his researches most effectively by Dr. Chandler Roberts) that hydrogen will enter into such combination with the metal palladium that it may be regarded as forming, for the time, with the palladium, an alloy; and as alloys can only be regarded as compounds of two or more metals, the inference is that hydrogen is in reality a metallic element.

Fourteen only of the elements known to us, or less than a quarter of the total number, were thus found to be present in the sun’s constitution; and of these all were metals, if we regard hydrogen as metallic. Neither gold nor silver shows any trace of its presence, nor can any sign be seen of platinum, lead, and mercury. But, most remarkable of all, and most perplexing, was the absence of all trace of oxygen and nitrogen, two gases which could not be supposed wanting in the substance of the great ruling centre of the planetary system. It might well be believed, indeed, that none of the five metals just named are absent from the sun, and indeed that every one of the forty metals not recognized by the spectroscopic method nevertheless exists in the sun. For according to the nebular hypothesis of the origin of our solar system, the sun might be expected to contain all the elements which exist in our earth. Some of these elements might indeed escape discovery, because existing only in small quantities; and others (as platinum, gold, and lead, for example), because but a small portion of their vaporous substance rose above the level of that glowing surface which is called the photosphere. But that oxygen, which constitutes so large a portion of the solid, liquid, and vaporous mass of our earth, should not exist in enormous quantities, and its presence be very readily discernable, seemed amazing indeed. Nitrogen, also, might well be expected to be recognizable in the sun. Carbon, again, is so important a constituent of the earth, that we should expect to discover clear traces of its existence in the sun. In less degree, similar considerations apply to sulphur, boron, silicon, and the other non-metallic elements.

It was not supposed, however, by any one at all competent to form an opinion on the subject, that oxygen, nitrogen, and carbon are absent from the sun. It was perceived that an element might exist in enormous quantities in the substance of the sun, and yet fail to give any evidence of its presence, or only give such evidence as might readily escape recognition. If we remember how the dark lines are really caused, we shall perceive that this is so. A glowing vapour in the atmosphere of the sun absorbs rays of the same colour as it emits. If then, it is cooler than the glowing mass of the sun which it enwraps, and if, notwithstanding the heat received from this mass, it remains cooler, then it suffers none of those rays to pass earthwards.1 It emits rays of the same kind (that is, of the same colour) itself, but, being cooler, the rays thus coming from it are feebler; or, to speak more correctly, the ethereal waves thus originated are feebler than those of the same order which would have travelled earthwards from the sun but for the interposed screen of vapour. Hence the corresponding parts of the solar spectrum are less brilliant, and contrasted with the rainbow-tinted streak of light, on which they lie as on a background, they appear dark.

In order, then, that any element may be detected by its dark lines, it is necessary that it should lie as a vaporous screen between the more intensely heated mass of the sun and the eye of the observer on earth. It must then form an enclosing envelope cooler than the sun within it. Or rather, some part of the vapour must be thus situated. For enormous masses of the vapour might be within the photospheric surface of the sun at a much higher temperature, which yet, being enclosed in the cooler vaporous shell of the same substance, would not be able to send its light rays earthwards. One may compare the state of things, so far as that particular element is concerned, to what is presented in the case of a metallic globe cooled on the outside but intensely hot within. The cool outside of such a globe is what determines the light and heat received from it, so long as the more heated mass within has not yet (by conduction) warmed the exterior shell. So in the case of a vapour permeating the entire mass, perhaps, of the sun, and at as high a temperature as the sun everywhere except on the outside: it is the temperature of the outermost part of such a vaporous mass which determines the intensity of the rays received from it—or in other words, determines whether the corresponding parts of the spectrum shall be darker or not than the rest of the spectrum. If the vapour does not rise above the photosphere of the sun in sufficient quantity to exercise a recognizable absorptive effect, its presence in the sun will not be indicated by any dark lines.

I dwell here on the question of quantity, which is sometimes overlooked in considering the spectroscopic evidence of the sun’s condition, but is in reality a very important factor in determining the nature of the evidence relating to each element in the solar mass. In some cases, the quantity of a material necessary to give unmistakable spectroscopic evidence is singularly small; insomuch that new elements, as thallium, cæsium, rubidium, and gallium, have been actually first recognized by their spectral lines when existing in such minute quantities in the substances examined as to give no other trace whatever of their existence. But it would be altogether a mistake to suppose that some element existing in exceedingly small quantities, or, more correctly, existing in the form of an exceedingly rare vapour in the sun’s atmosphere, would be detected by means of its dark lines, or by any other method depending on the study of the solar spectrum. When we place a small portion of some substance in the space between the carbon points of an electric lamp, and volatilize that substance in the voltaic arc, we obtain a spectrum including all the bright lines of the various elements contained in the substance; and if some element is contained in it in exceedingly small quantity, we may yet perceive its distinctive bright lines among the others (many of them far brighter) belonging to the elements present in greater quantities. But if we have (for example) a great mass of molten iron, the rainbow-tinted spectrum of whose light we examine from a great distance, and if a small quantity of sodium, or other substance which vaporizes at moderate temperatures, be cast into the molten iron so that the vapour of the added element presently rises above the glowing surface of the iron, no trace of the presence of this vapour would be shown in the spectrum observed from a distance. The part of the spectrum where the dark lines of sodium usually appear would, undoubtedly, be less brilliant than before, in the same sense that the sun may be said to be less brilliant when the air is in the least degree moist than when it is perfectly dry; but the loss of brilliancy is as utterly imperceptible in the one case as it is in the other. In like manner, a vapour might exist in the atmosphere of the sun (above the photosphere, that is), of whose presence not a trace would be afforded in the spectroscope, for the simple reason that the absorptive action of the vapour, though exerted to reduce the brightness of particular solar rays or tints, would not affect those rays sufficiently for the spectroscopist to recognize any diminution of their lustre.

There is another consideration, which, so far as I know, has not hitherto received much attention, but should certainly be taken into account in the attempt to interpret the real meaning of the solar spectrum. Some of the metals which are vaporized by the sun’s heat below the photosphere may become liquid or even solid at or near the level of the photosphere. Even though the heat at the level of the photosphere may be such that, under ordinary conditions of pressure and so forth, such metals would be vaporous, the enormous pressure which must exist not far below the level of the photosphere may make the heat necessary for complete vaporization far greater than the actual heat at that level. In that case the vapour will in part condense into liquid globules, or, if the heat is considerably less than is necessary to keep the substance in the form of vapour, then it may in part be solidified, the tiny globules of liquid metal becoming tiny crystals of solid metal. We see both conditions fulfilled within the limits of our own air in the case of the vapour of water. Low down the water is present in the air (ordinarily) in the form of pure vapour; at a higher level the vapour is condensed by cold into liquid drops forming visible clouds (cumulus clouds), and yet higher, where the cold is still greater, the minute water-drops turn into ice-crystals, forming those light fleecy clouds called cirrus clouds by the meteorologist. Now true clouds of either sort may exist in the solar atmosphere even above that photospheric level which forms the boundary of the sun we see. It may be said that the spectroscope, applied to examine matter outside the photosphere, has given evidence only of vaporous cloud masses. The ruddy prominences which tower tens of thousands of miles above the surface of the sun, and the sierra (or as it is sometimes unclassically called, the chromosphere) which covers usually the whole of the photosphere to a depth of about eight thousand miles, show only, under spectroscopic scrutiny, the bright lines indicating gaseity. But though this is perfectly true, it is also true that we have not here a particle of evidence to show that clouds of liquid particles, and of tiny crystals, may not float over the sun’s surface, or even that the ruddy clouds shown by the spectroscope to shine with light indicative of gaseity may not also contain liquid and crystalline particles. For in point of fact, the very principle on which our recognition of the bright lines depends involves the inference that matter whose light would not be resolved into bright lines would not be recognizable at all. The bright lines are seen, because by means of a spectroscope we can throw them far apart, without reducing their lustre, while the background of rainbow-tinted spectrum has its various portions similarly thrown further apart and correspondingly weakened. One may compare the process (the comparison, I believe, has not hitherto been employed) to the dilution of a dense liquid in which solid masses have been floating: the more we increase the quantity of the liquid in diluting it with water, the more transparent it becomes, but the solid masses in it are not changed, so that we only have to dilute the liquid sufficiently to see these masses. But if there were in the interstices of the solid masses particles of some substance which dissolved in the water, we should not recognize the presence of this substance by any increase in its visibility; for the very same process which thinned the liquid would thin this soluble substance in the same degree. In like manner, by dispersing and correspondingly weakening the sun’s light more and more, we can recognize the light of the gaseous matter in the prominences, for this is not weakened; but if the prominences also contain matter in the solid or liquid form (that is, drops or crystals), the spectroscopic method will not indicate the presence of such matter, for the spectrum of matter of this sort will be weakened by dispersion in precisely the same degree that the solar spectrum itself is weakened.

It is easy to see how the evidence of the presence of any element which behaved in this way would be weakened, if we consider what would happen in the case of our own earth, according as the air were simply moist but without clouds, or loaded with cumulus masses but without cirrus clouds, or loaded with cirrus clouds. For although there is not in the case of the earth a central glowing mass like the sun’s, on whose rainbow-tinted spectrum the dark lines caused by the absorptive action of our atmosphere could be seen by the inhabitant of some distant planet studying the earth from without, yet the sun’s light reflected from the surface of the earth plays in reality a similar part. It does not give a simple rainbow-tinted spectrum; for, being sunlight, it shows all the dark lines of the solar spectrum: but the addition of new dark lines to these, in consequence of the absorptive action of the earth’s atmosphere, could very readily be determined. In fact, we do thus recognize in the spectra of Mars, Venus, and other planets, the presence of aqueous vapour in their atmosphere, despite the fact that our own air, containing also aqueous vapour, naturally renders so much the more difficult the detection of that vapour in the atmosphere of remote planets necessarily seen through our own air. Now, a distant observer examining the light of our own earth on a day when, though the air was moist, there were no clouds, would have ample evidence of the presence of the vapour of water; for the light which he examined would have gone twice through our earth’s atmosphere, from its outermost thinnest parts to the densest layers close to the surface, then back again through the entire thickness of the air. But if the air were heavily laden with cumulus clouds (without any cirrus clouds at a higher layer), although we should know that there was abundant moisture in the air, and indeed much more moisture then there had been when there had been no clouds, our imagined observer would either perceive no traces at all of this moisture, or he would perceive traces so much fainter than when the air was clear that he would be apt to infer that the air was either quite dry, or at least very much drier than it had been in that case. For the light which he would receive from the earth would not in this case have passed through the entire depth of moisture-laden air twice, but twice only through that portion of the air which lay above the clouds, at whose surface the sun’s light would be reflected. The whole of the moisture-laden layer of the air would be snugly concealed under the cloud-layer, and would exercise no absorptive action whatever on the light which the remote observer would examine. If from the upper surface of the layer of cumulus clouds aqueous vapour rose still higher, and were converted in the cold upper regions of the atmosphere into clouds of ice-crystals, the distant observer would have still less chance of recognizing the presence of moisture in our atmosphere. For the layer of air between the cumulus clouds and the cirrus clouds would be unable to exert any absorptive action on the light which reached the observer. All such light would come to him after reflection from the layer of cirrus clouds. He would be apt to infer that there was no moisture at all in the air of our planet, at the very time when in fact there was so much moisture that not one layer only, but two layers of clouds enveloped the earth, the innermost layer consisting of particles of liquid water, the outermost of particles of frozen water. Using the words ice, water, and steam, to represent the solid, liquid, and vaporous states of water, we may fairly say that ice and water, by hiding steam, would persuade the remote observer that there was no water at all on the earth—at least if he trusted solely to the spectroscopic evidence then obtained.2

We might in like manner fail to obtain any spectroscopic evidence of the presence of particular elements in the sun, because they do not exist in sufficient quantity in the vaporous form in those outer layers which the spectroscope can alone deal with.

In passing, I must note a circumstance in which some of those who have dealt with this special part of the spectroscopic evidence have erred. It is true in one sense that some elements may be of such a nature that their vapours cannot rise so high in the solar atmosphere as those of other elements. But it must not be supposed that the denser vapours seek a lower level, the lighter vapours rising higher. According to the known laws of gaseous diffusion, a gas or vapour diffuses itself throughout a space occupied by another gas or several other gases, in the same way as though the space were not occupied at all. If we introduce into a vessel full of common air a quantity of carbonic acid gas (I follow the older and more familiar nomenclature), this gas, although of much higher specific gravity than either oxygen or nitrogen, does not take its place at the bottom of the vessel, but so diffuses itself that the air of the upper part of the vessel contains exactly the same quantity of carbonic acid gas as the air of the lower part. Similarly, if hydrogen is introduced, it does not seek the upper part of the vessel, but diffuses itself uniformly throughout the vessel. If we enclose the carbonic acid gas in a light silken covering, and the hydrogen in another (at the same pressure as the air in the vessel) one little balloon will sink and the other will rise; but this is simply because diffusion is prevented. It may be asked how this agrees with what I have said above, that some elements may not exist in sufficient quantity or in suitable condition above the sun’s photospheric level to give any spectroscope evidence of their nature. As to quantity, indeed, the answer is obvious: if there is only a small quantity of any given element in the entire mass of the sun, only a very small quantity can under any circumstances exist outside the photosphere. As regards condition, it must be remembered that the vessel of my illustrative case was supposed to contain air at a given temperature and pressure throughout. If the vessel was so large that in different parts of it the temperature and pressure were different, the diffusion would, indeed, still be perfect, because at all ordinary temperatures and pressures hydrogen and carbonic acid gas remain gaseous. But if the vapour introduced is of such a nature that at moderate temperatures and pressures it condenses, wholly or in part, or liquefies, the diffusion will not take place with the same uniformity. We need not go further for illustration than to the case of our own atmosphere as it actually exists. The vapour of water spreads uniformly through each layer of the atmosphere which is at such a temperature and pressure as to permit of such diffusion; but where the temperature is too low for complete diffusion (at the actual pressure) the aqueous vapour is condensed into visible cloud, diffusion being checked at this point as at an impassable boundary. In the case of the sun, as in the case of our own earth, it is not the density of an element when in a vaporous form which limits its diffusion, but the value of the temperature at which its vapour at given pressure condenses into liquid particles. It is in this way only that any separation can be effected between the various elements which exist in the sun’s substance. A separation of this sort is unquestionably competent to modify the spectroscopic evidence respecting different elements. But it would be a mistake to suppose that any such separation could occur as has been imagined by some—a separation causing in remote times the planets supposed to have been thrown off by the sun to be rarest on the outskirts of the solar system and densest close to the sun. The small densities of the outer family of planets, as compared with the densities of the so-called terrestrial planets, must certainly be otherwise explained.

But undoubtedly the chief circumstance likely to operate in veiling the existence of important constituents of the solar mass must be that which has so long prevented spectroscopists from detecting the presence of oxygen in the sun. An element may exist in such a condition, either over particular parts of the photosphere, or over the entire surface of the sun, that instead of causing dark lines in the solar spectrum it may produce bright lines. Such lines may be conspicuous, or they may be so little brighter than the background of the spectrum as to be scarcely perceptible or quite imperceptible.

In passing, I would notice that this interpretation of the want of all spectroscopic evidence of the presence of oxygen, carbon, and other elements in the sun, is not an ex post facto explanation. As will presently appear, it is now absolutely certain that oxygen, though really existing, and doubtless, in enormous quantities, in the sun, has been concealed from recognition in this way. But that this might be so was perceived long ago. I myself, in the first edition of my treatise on “The Sun,” pointed out, in 1870, with special reference to nitrogen and oxygen, that an element “may be in a condition enabling it to radiate as much light as it absorbs, or else very little more or very little less; so that it either obliterates all signs of its existence, or else gives lines so little brighter or darker than the surrounding parts of the spectrum that we can detect no trace of its existence.” I had still earlier given a similar explanation of the absence of all spectroscopic evidence of hydrogen in the case of the bright star Betelgeux.3

Let us more closely consider the significance of what we learn from the spectral evidence respecting the gas hydrogen. We know that when the total light of the sun is dealt with, the presence of hydrogen is constantly indicated by dark lines. In other words, regarding the sun as a whole, hydrogen constantly reduces the emission of rays of those special tints which correspond to the light of this element. When we examine the light of other suns than ours, we find that in many cases, probably in by far the greater number of cases, hydrogen acts a similar part. But not in every case. In the spectra of some stars, notably in those of Betelgeux and Alpha Herculis, the lines of hydrogen are not visible at all; while in yet others, as Gamma Cassiopeiæ, the middle star of the five which form the straggling W of this constellation, the lines of hydrogen show bright upon the relatively dark background of the spectrum. When we examine closely the sun himself, we find that although his light as a whole gives a spectrum in which the lines of hydrogen appear dark, the light of particular parts of his surface, if separately examined, occasionally shows the hydrogen lines bright as in the spectrum of Gamma Cassiopeiæ, while sometimes the light of particular parts gives, like the light of Betelgeux, no spectroscopic evidence whatever of the presence of hydrogen. Manifestly, if the whole surface of the sun were in the condition of the portions which give bright hydrogen lines, the spectrum of the sun would resemble that of Gamma Cassiopeiæ; while if the whole surface were in the condition of those parts which show no lines of hydrogen, the spectrum of the sun would resemble that of Betelgeux. Now if there were any reason for supposing that the parts of the sun which give no lines of hydrogen are those from which the hydrogen has been temporarily removed in some way, we might reasonably infer that in the stars whose spectra show no hydrogen lines there is no hydrogen. But the fact that the hydrogen lines are sometimes seen bright renders this supposition untenable. For we cannot suppose that the lines of hydrogen change from dark to bright or from bright to dark (both which changes certainly take place) without passing through a stage in which they are neither bright nor dark; in other words, we are compelled to assume that there is an intermediate condition in which the hydrogen lines, though really existent, are invisible because they are of precisely the same lustre as the adjacent parts of the spectrum. Hence the evanescence of the hydrogen lines affords no reason for supposing that hydrogen has become even reduced in quantity where the lines are not seen. And therefore it follows that the invisibility of the hydrogen lines in the spectrum of Betelgeux is no proof that hydrogen does not exist in that star in quantities resembling those in which it is present in the sun. And this, being demonstrated in the case of one gas, must be regarded as at least probable in the case of other gases. Wherefore the absence of the lines of oxygen from the spectrum of any star affords no sufficient reason for believing that oxygen is not present in that star, or that it may not be as plentifully present as hydrogen, or even far more plentifully present.

There are other considerations which have to be taken into account, as well in dealing with the difficulty arising from the absence of the lines of particular elements from the solar spectrum as in weighing the extremely important discovery which has just been effected by Dr. H. Draper.

I would specially call attention now to a point which I thus presented seven years ago:—“The great difficulty of interpreting the results of the spectroscopic analysis of the sun arises from the circumstance that we have no means of learning whence that part of the light comes which gives the continuous spectrum. When we recognize certain dark lines, we know certainly that the corresponding element exists in the gaseous form at a lower temperature than the substance which gives the continuous spectrum. But as regards that continuous spectrum itself we can form no such exact opinion.” It might, for instance, have its origin in glowing liquid or solid matter; but it might also be compounded of many spectra, each containing a large number of bands, the bands of one spectrum filling up the spaces which would be left dark between the bands of another spectrum, and so on until the entire range from the extreme visible red to the extreme visible violet were occupied by what appeared as a continuous rainbow-tinted streak. “We have, in fact, in the sun,” as I pointed out, “a vast agglomeration of elements, subject to two giant influences, producing in some sort opposing effects—viz., a temperature far surpassing any we can form any conception of, and a pressure (throughout nearly the whole of the sun’s globe) which is perhaps even more disproportionate to the phenomena of our experience. Each known element would be vaporized by the solar temperature at known pressures; each (there can be little question) would be solidified by the vast pressures, did these arise at known temperatures. Now whether, under these circumstances, the laws of gaseous diffusion prevail where the elements are gaseous in the solar globe; whether, where liquid matter exists it is in general bounded in a definite manner from the neighbouring gaseous matter; whether any elements at all are solid, and if so under what conditions their solidity is maintained and the limits of the solid matter defined—all these are questions which must be answered before we can form a satisfactory idea of the solar constitution; yet they are questions which we have at present no means of answering.” Again, we require to know whether any process resembling combustion can under any circumstances take place in the sun’s globe. If we could assume that some general resemblance exists between the processes at work upon the sun and those we are acquainted with, we might imagine that the various elements ordinarily exist in the sun’s globe in the gaseous form (chiefly) to certain levels, to others chiefly in the liquid form, and to yet others chiefly in the solid form. But even then that part of each element which is gaseous may exist in two forms, having widely different spectra (in reality in five, but I consider only the extreme forms). The light of one part is capable of giving characteristic spectra of lines or bands (which will be different according to pressure and may appear either dark or bright); that of the other is capable of giving a spectrum nearly or quite continuous.

It will be seen that Dr. H. Draper’s discovery supplies an answer to one of the questions, or rather to one of the sets of questions, thus indicated. I give his discovery as far as possible in his own words.

“Oxygen discloses itself,” he says, “by bright lines or bands in the solar spectrum, and does not give dark absorption-lines like the metals. We must therefore change our theory of the solar spectrum, and no longer regard it merely as a continuous spectrum with certain rays absorbed by a layer of ignited metallic vapours, but as having also bright lines and bands superposed on the background of continuous spectrum. Such a conception not only opens the way to the discovery of others of the non-metals, sulphur, phosphorus, selenium, chlorine, bromine, iodine, fluorine, carbon, etc., but also may account for some of the so-called dark lines, by regarding them as intervals between bright lines. It must be distinctly understood that in speaking of the solar spectrum here, I do not mean the spectrum of any limited area upon the disc or margin of the sun, but the spectrum of light from the whole disc.”

In support of the important statement here advanced, Dr. Draper submits a photograph of part of the solar spectrum with a comparison spectrum of air, and also with some of the lines of iron and aluminium. The photograph itself, a copy of which, kindly sent to me by Dr. Draper, lies before me as I write, fully bears out Dr. Draper’s statement. It is absolutely free from handwork or retouching, except that reference letters have been added in the negative. It shows the part of the solar spectrum between the well-known Fraunhofer lines G and H, of which G (an iron line) lies in the indigo, and H (a line of hydrogen) in the violet, so that the portion photographed belongs to that region of the spectrum whose chemical or actinic energy is strongest. Adjacent to this lies the photograph of the air lines, showing nine or ten well-defined oxygen lines or groups of lines, and two nitrogen bands. The exact agreement of the two spectra in position is indicated by the coincidence of bright lines of iron and aluminium included in the air spectrum with the dark lines of the same elements in the solar spectrum. “No close observation,” as Dr. Draper truly remarks, “is needed to demonstrate to even the most casual observer” (of this photograph) “that the oxygen lines are found in the sun as bright lines.” There is in particular one quadruple group of oxygen lines in the air spectrum, the coincidence of which with a group of bright lines in the solar spectrum is unmistakable.

“This oxygen group alone is almost sufficient,” says Dr. Draper, “to prove the presence of oxygen in the sun, for not only does each of the four components have a representative in the solar group, but the relative strength and the general aspect of the lines in each case is similar.4 I shall not attempt at this time,” he proceeds, “to give a complete list of the oxygen lines, ... and it will be noticed that some lines in the air spectrum which have bright anologues in the sun are not marked with the symbol of oxygen. This is because there has not yet been an opportunity to make the necessary detailed comparisons. In order to be certain that a line belongs to oxygen, I have compared, under various pressures, the spectra of air, oxygen, nitrogen, carbonic acid, carburetted hydrogen, hydrogen, and cyanogen.

“As to the spectrum of nitrogen and the existence of this element in the sun there is not yet certainty. Nevertheless, even by comparing the diffused nitrogen lines of this particular photograph, in which nitrogen has been sacrificed to get the best effect for oxygen, the character of the evidence appears. There is a triple band somewhat diffused in the photograph belonging to nitrogen, which has its appropriate representative in the solar spectrum, and another band of nitrogen is similarly represented.” Dr. Draper states that “in another photograph a heavy nitrogen line which in the present one lies opposite an insufficiently exposed part of the solar spectrum, corresponds to a comparison band in the sun.”

But one of the most remarkable points in Dr. Draper’s paper is what he tells us respecting the visibility of these lines in the spectrum itself. They fall, as I have mentioned, in a part of the spectrum where the actinic energy is great but the luminosity small; in fact, while this part of the spectrum is the very strongest for photography, it is close to the region of the visible spectrum,

“Where the last gleamings of refracted light Die in the fainting violet away.”

It is therefore to be expected that those, if any, of the bright lines of oxygen, will be least favourably shown for direct vision, and most favourably for what might almost be called photographic vision, where we see what photography records for us. Yet Dr. Draper states that these bright lines of oxygen can be readily seen. “The bright lines of oxygen in the spectrum of the solar disc have not been hitherto perceived, probably from the fact that in eye-observation bright lines on a less bright background do not make the impression on the mind that dark lines do. When attention is called to their presence they are readily enough seen, even without the aid of a reference spectrum. The photograph, however, brings them into greater prominence.” As the lines of oxygen are not confined to the indigo and violet, we may fairly hope that the bright lines in other parts of the spectrum of oxygen may be detected in the spectrum of the sun, now that spectroscopists know that bright lines and not dark lines are to be looked for.

Dr. Draper remarks that from purely theoretic considerations derived from terrestrial chemistry, and the nebular hypothesis, the presence of oxygen in the sun might have been strongly suspected; for this element is currently stated to form eight-ninths of the water of the globe, one-third of the crust of the earth, and one-fifth of the air, and should therefore probably be a large constituent of every member of the solar system. On the other hand, the discovery of oxygen, and probably other non-metals, in the sun gives increased strength to the nebular hypothesis, because to many persons the absence of this important group has presented a considerable difficulty. I have already remarked on the circumstance that we cannot, according to the known laws of gaseous diffusion, accept the reasoning of those who have endeavoured to explain the small density of the outer planets by the supposition that the lighter gases were left behind by the great contracting nebulous mass, out of which, on the nebular hypothesis, the solar system is supposed to have been formed. It is important to notice, now, that if on the one hand we find in the community of structure between the sun and our earth, as confirmed by the discovery of oxygen and nitrogen in the sun, evidence favouring the theory according to which all the members of that system were formed out of what was originally a single mass, we do not find evidence against the theory (as those who have advanced the explanation above referred to may be disposed to imagine) in the recognition in the sun’s mass of enormous quantities of one of these elements which, according to their view, ought to be found chiefly in the outer members of the solar system. If those who believe in the nebular hypothesis (generally, that is, for many of the details of the hypothesis as advanced by Laplace are entirely untenable in the present position of physical science) had accepted the attempted explanation of the supposed absence of the non-metallic elements in the sun, they would now find themselves in a somewhat awkward position. They would, in fact, be almost bound logically to reject the nebular hypothesis, seeing that one of the asserted results of the formation of our system, according to that hypothesis, would have been disproved. But so far as I know no supporter of the nebular hypothesis possessing sufficient knowledge of astronomical facts and physical laws to render his opinion of any weight, has ever given in his adhesion to the unsatisfactory explanation referred to.

The view which I have long entertained respecting the growth of the solar system—viz., that it had its origin, not in contraction only or chiefly, but in combined processes of contraction and accretion—seems to me to be very strongly confirmed by Dr. Draper’s discovery. This would not be the place for a full discussion of the reasons on which this opinion is based. But I may remark that I believe no one who applies the laws of physics, as at present known, to the theory of the simple contraction of a great nebulous mass formerly extending far beyond the orbit of Neptune, till, when planet after planet had been thrown off, the sun was left in his present form and condition in the centre, will fail to perceive enormous difficulties in the hypothesis, or to recognize in Dr. Draper’s discovery a difficulty added to those affecting the hypothesis so presented. Has it ever occurred, I often wonder, to those who glibly quote the nebular theory as originally propounded, to inquire how far some of the processes suggested by Laplace are in accordance with the now known laws of physics? To begin with, the original nebulous mass extending to a distance exceeding the earth’s distance from the sun more than thirty times (this being only the distance of Neptune), if we assign to it a degree of compression making its axial diameter half its equatorial diameter, would have had a volume exceeding the sun’s (roughly) about 120,000,000,000 times, and in this degree its mean density would have been less than the sun’s. This would correspond to a density equal (roughly) to about one-400,000th part of the density of hydrogen gas at atmospheric pressure. To suppose that a great mass of matter, having this exceedingly small mean density, and extending to a distance of three or four thousand millions of miles from its centre, could under any circumstances rotate as a whole, or behave in other respects after the fashion attributed to the gaseous embryon of the solar system in ordinary descriptions of the nebular hypothesis, is altogether inconsistent with correct ideas of physical and dynamical laws. It is absolutely a necessity of any nebular hypothesis of the solar system, that from the very beginning a central nucleus and subordinate nuclei should form in it, and grow according to the results of the motions (at first to all intents and purposes independent) of its various parts. Granting this state of things, we arrive, by considering the combined effects of accretion and contraction, at a process of development according fully as well as that ordinarily described with the general relations described by Laplace, and accounting also (in a general way) for certain peculiarities which are in no degree explained by the ordinary theory. Amongst these may specially be noted the arrangement and distribution of the masses within the solar system, and the fact that so far as spectroscopic evidence enables us to judge, a general similarity of structure exists throughout the whole of the system.

Inquiring as to the significance of his discovery, Dr. Draper remarks that it seems rather difficult “at first sight to believe that an ignited5 gas in the solar atmosphere should not be indicated by dark lines in the solar spectrum, and should appear not to act under the law, ‘a gas when ignited absorbs rays of the same refrangibility as those it emits.’ But, in fact, the substances hitherto investigated in the sun are really metallic vapours, hydrogen probably coming under that rule. The non-metals obviously may behave differently. It is easy to speculate on the causes of such behaviour; and it may be suggested that the reason of the non-appearance of a dark line may be that the intensity of the light from a great thickness of ignited oxygen overpowers the effect of the photosphere, just as, if a person were to look at a candle-flame through a yard thickness of sodium vapour, he would only see bright sodium lines, and no dark absorption.”

The reasoning here is not altogether satisfactory (or else is not quite correctly expressed). In the first place, the difficulty dealt with has no real existence. The law that a gas when glowing absorbs rays of the same refrangibility as it emits, does not imply that a gas between a source of light and the observer will show its presence by spectroscopic dark lines. A gas so placed does receive from the source of light rays corresponding to those which it emits itself, if it is cooler than the source of light; and it absorbs them, being in fact heated by means of them, though the gain of temperature may be dissipated as fast as received; but if the gas is hotter, it emits more of those rays than it absorbs, and will make its presence known by its bright lines. This is not a matter of speculation, but of experiment. On the other hand, the experiment suggested by Dr. Draper would not have the effect he supposes, if it were correctly made. Doubtless, if the light from a considerable area of dully glowing sodium vapour were received by the spectroscope at the same time as the light of a candle-flame seen through the sodium vapour, the light of the sodium vapour overcoming that of the candle-flame would indicate its presence by bright lines; but if light were received only from that portion of the sodium vapour which lay between the eye and the candle-flame, then I apprehend that the dark lines of sodium would not only be seen, but would be conspicuous by their darkness.

It is in no cavilling spirit that I indicate what appears to me erroneous in a portion of Dr. Draper’s reasoning on his great discovery. The entire significance of the discovery depends on the meaning attached to it, and therefore it is most desirable to ascertain what this meaning really is. There can be no doubt, I think, that we are to look for the true interpretation of the brightness of the oxygen lines in the higher temperature of the oxygen, not in the great depth of oxygen above the photospheric level. The oxygen which produces these bright lines need not necessarily be above the photosphere at all. (In fact, I may remark here that Dr. Draper, in a communication addressed to myself, mentions that he has found no traces at present of oxygen above the photosphere, though I had not this circumstance in my thoughts in reasoning down to the conclusion that the part of the oxygen effective in showing these bright lines lies probably below the visible photosphere.) Of course, if the photosphere were really composed of glowing solid and liquid matter, or of masses of gas so compressed and so intensely heated as to give a continuous spectrum, no gas existing below the photosphere could send its light through, nor could its presence, therefore, be indicated in any spectroscopic manner. But the investigations which have been made into the structure of the photosphere as revealed by the telescope, and in particular the observations made by Professor Langley, of the Alleghany Observatory, show that we have not in the photosphere a definite bounding envelope of the sun, but receive light from many different depths below that spherical surface, 425,000 miles from the sun’s centre, which we call the photospheric level. We receive more light from the centre of the solar disc, I feel satisfied, not solely because the absorptive layer through which we there see the sun is shallower, but partly, and perhaps chiefly, because we there receive light from some of the interior and more intensely heated parts of the sun.6 Should this prove to be the case, it may be found possible to do what heretofore astronomers have supposed to be impossible—to ascertain in some degree how far and in what way the constitution of the sun varies below the photosphere, which, so far as ordinary telescopic observation is concerned, seems to present a limit below which researches cannot be pursued.

I hope we shall soon obtain news from Dr. Huggins’s Observatory that the oxygen lines have been photographed, and possibly the bright lines of other elements recognized in the solar spectrum. Mr. Lockyer also, we may hope, will exercise that observing skill which enabled him early to recognize the presence of bright hydrogen lines in the spectrum of portions of the sun’s surface, to examine that spectrum for other bright lines.

I do not remember any time within the last twenty years when the prospects of fresh solar discoveries seemed more hopeful than they do at present. The interest which has of late years been drawn to the subject has had the effect of enlisting fresh recruits in the work of observation, and many of these may before long be heard of as among those who have employed Dr. Draper’s method successfully.

But I would specially call attention to the interest which attaches to Dr. Draper’s discovery and to the researches likely to follow from it, in connection with a branch of research which is becoming more and more closely connected year by year with solar investigations—I mean stellar spectroscopy. We have seen the stars divided into orders according to their constitution. We recognize evidence tending to show that these various orders depend in part upon age—not absolute but relative age. There are among the suns which people space some younger by far than our sun, others far older, and some in a late stage of stellar decrepitude. Whether as yet spectroscopists have perfectly succeeded in classifying these stellar orders in such sort that the connection between a star’s spectrum and the star’s age can be at once determined, may be doubtful. But certainly there are reasons for hoping that before long this will be done. Amongst the stars, and (strange to say) among celestial objects which are not stars, there are suns in every conceivable stage of development, from embryon masses not as yet justly to be regarded as suns, to masses which have ceased to fulfil the duties of suns. Among the more pressing duties of spectroscopic analysis at the present time is the proper classification of these various orders of stars. Whensoever that task shall have been accomplished, strong light, I venture to predict, will be thrown on our sun’s present condition, as well as on his past history, and on that future fate upon which depends the future of our earth.

SUN-SPOT, STORM, AND FAMINE.

During the last five or six years a section of the scientific world has been exercised with the question how far the condition of the sun’s surface with regard to spots affects our earth’s condition as to weather, and therefore as to those circumstances which are more or less dependent on weather. Unfortunately, the question thus raised has not presented itself alone, but in company with another not so strictly scientific, in fact, regarded by most men of science as closely related to personal considerations—the question, namely, whether certain indicated persons should or should not be commissioned to undertake the inquiry into the scientific problem. But the scientific question itself ought not to be less interesting to us because it has been associated, correctly or not, with the wants and wishes of those who advocate the endowment of science. I propose here to consider the subject in its scientific aspect only, and apart from any bias suggested by the appeals which have been addressed to the administrators of the public funds.

It is hardly necessary to point out, in the first place, that all the phenomena of weather are directly referable to the sun as their governing cause. His rays poured upon our air cause the more important atmospheric currents directly. Indirectly they cause modifications of these currents, because where they fall on water or on moist surfaces they raise aqueous vapour into the air, which, when it returns to the liquid form as cloud, gives up to the surrounding air the heat which had originally vaporized the water. In these ways, directly or indirectly, various degrees of pressure and temperature are brought about in the atmospheric envelope of the earth, and, speaking generally, all air currents, from the gentlest zephyr to the fiercest tornado, are the movements by which the equilibrium of the air is restored. Like other movements tending to restore equilibrium, the atmospheric motions are oscillatory. Precisely as when a spring has been bent one way, it flies not back only, but beyond the mean position, till it is almost equally bent the other way, so the current of air which rushes in towards a place of unduly diminished pressure does more than restore the mean pressure, so that presently a return current carries off the excess of air thus carried in. We may say, indeed, that the mean pressure at any place scarcely ever exists, and when it exists for a time the resulting calm is of short duration. Just as the usual condition of the sea surface is one of disturbance, greater or less, so the usual condition of the air at every spot on the earth’s surface is one of motion not of quiescence. Every movement of the air, thus almost constantly perturbed, is due directly or indirectly to the sun.

So also every drop of rain or snow, every particle of liquid or of frozen water in mist or in cloud, owes its birth to the sun. The questions addressed of old to Job, “Hath the rain a father? or who hath begotten the drops of dew? out of whose womb came the ice? and the hoary frost of heaven, who hath gendered it?” have been answered by modern science, and to every question the answer is, The Sun. He is parent of the snow and hail, as he is of the moist warm rains of summer, of the ice which crowns the everlasting hills, and of the mist which rises from the valleys beneath his morning rays.

Since, then, the snow that clothes the earth in winter as with a garment, and the clouds that in due season drop fatness on the earth, are alike gendered by the sun; since every movement in our air, from the health-bringing breeze to the most destructive hurricane, owns him as its parent; we must at the outset admit, that if there is any body external to the earth whose varying aspect or condition can inform us beforehand of changes which the weather is to undergo, the sun is that body. That for countless ages the moon should have been regarded as the great weather-breeder, shows only how prone men are to recognize in apparent changes the true cause of real changes, and how slight the evidence is on which they will base laws of association which have no real foundation in fact. Every one can see when the moon is full, or horned, or gibbous, or half-full; when her horns are directed upwards, or downwards, or sideways. And as the weather is always changing, even as the moon is always changing, it must needs happen that from time to time changes of weather so closely follow changes of the moon as to suggest that the two orders of change stand to each other in the relation of cause and effect. Thus rough rules (such as those which Aratus has handed down to us) came to be formed, and as (to use Bacon’s expression) men mark when such rules hit, and never mark when they miss, a system of weather lore gradually comes into being, which, while in one sense based on facts, has not in reality a particle of true evidence in its favour—every single fact noted for each relation having been contradicted by several unnoted facts opposed to the relation. There could be no more instructive illustration of men’s habits in such matters than the system of lunar weather wisdom in vogue to this day among seamen, though long since utterly disproved by science. But let it be remarked in passing, that in leaving the moon, which has no direct influence, and scarcely any indirect influence, on the weather, for the sun, which is all-powerful, we have not got rid of the mental habits which led men so far astray in former times. We shall have to be specially careful lest it lead us astray yet once more, perhaps all the more readily because of the confidence with which we feel that, at the outset anyway, we are on the right road.

I suppose there must have been a time when men were not altogether certain whether the varying apparent path of the sun, as he travels from east to west every day, has any special effect on the weather. It seems so natural to us to recognize in the sun’s greater mid-day elevation and longer continuance above the horizon in summer, the cause of the greater warmth which then commonly prevails, that we find it difficult to believe that men could ever have been in doubt on this subject. Yet it is probable that a long time passed after the position of the sun as ruler of the day had been noticed, before his power as ruler of the seasons was recognized. I cannot at this moment recall any passage in the Bible, for example, in which direct reference is made to the sun’s special influence in bringing about the seasons, or any passage in very ancient writings referring definitely to the fact that the weather changes with the changing position of the sun in the skies (as distinguished from the star-sphere), and with the changing length of the day. “While the earth remaineth,” we are told in Genesis, “seed-time and harvest, and cold and heat, and summer and winter, and day and night, shall not cease;” but there is no reference to the sun’s aspect as determining summer and winter. We find no mention of any of the celestial signs of the seasons anywhere in the Bible, I think, but such signs as are mentioned in the parable of the fig tree—“When his branch is yet tender, and putteth forth leaves, ye know that summer is nigh.” Whether this indicates or not that the terrestrial, rather than the celestial signs of the progress of the year were chiefly noted by men in those times, it is tolerably certain that in the beginning a long interval must have elapsed between the recognition of the seasons themselves, and the recognition of their origin in the changes of the sun’s apparent motions.

When this discovery was effected, men made the most important and, I think, the most satisfactory step towards the determination of cyclic associations between solar and terrestrial phenomena. It is for that reason that I refer specially to the point. In reality, it does not appertain to my subject, for seasons and sun-spots are not associated. But it admirably illustrates the value of cyclic relations. Men might have gone on for centuries, we may conceive, noting the recurrence of seed-time and harvest-time, summer and winter, recognizing the periodical returns of heat and cold, and (in some regions) of dry seasons and wet seasons, of calm and storm, and so forth, without perceiving that the sun runs through his changes of diurnal motion in the same cyclic period. We can imagine that some few who might notice the connection between the two orders of celestial phenomena would be anxious to spread their faith in the association among their less observant brethren. They might maintain that observatories for watching the motions of the sun would demonstrate either that their belief was just or that it was not so, would in fact dispose finally of the question. It is giving the most advantageous possible position to those who now advocate the erection of solar observatories for determining what connection, if any, may exist between sun-spots and terrestrial phenomena, thus to compare them to observers who had noted a relation which unquestionably exists. But it is worthy of notice that if those whom I have imagined thus urging the erection of an observatory for solving the question whether the sun rules the seasons, and to some degree regulates the recurrence of dry or rainy, and of calm or stormy weather, had promised results of material value from their observations, they would have promised more than they could possibly have performed. Even in this most favourable case, where the sun is, beyond all question, the efficient ruling body, where the nature of the cyclic change is most exactly determinable, and where even the way in which the sun acts can be exactly ascertained, no direct benefit accrues from the knowledge. The exact determination of the sun’s apparent motions has its value, and this value is great, but it is most certainly not derived from any power of predicting the recurrence of those phenomena which nevertheless depend directly on the sun’s action. The farmer who in any given year knows from the almanac the exact duration of daylight, and the exact mid-day elevation of the sun for every day in the year, is not one whit better able to protect his crops or his herds against storm or flood than the tiller of the soil or the tender of flocks a hundred thousand years or so ago, who knew only when seed-time and summer and harvest-time and winter were at hand or in progress.

The evidence thus afforded is by no means promising, then, so far as the prediction of special storms, or floods, or droughts is concerned. It would seem that if past experience can afford any evidence in such matters, men may expect to recognize cycles of weather change long before they recognize corresponding solar cycles (presuming always that such cycles exist), and that they may expect to find the recognition of such association utterly barren, so far as the possibility of predicting definite weather changes is concerned. It would seem that there is no likelihood of anything better than what Sir J. Herschel said might be hoped for hereafter. “A lucky hit may be made; nay, some rude approach to the perception of a ‘cycle of seasons’ may possibly be obtainable. But no person in his senses would alter his plans of conduct for six months in advance in the most trifling particular on the faith of any special prediction of a warm or a cold, a wet or a dry, a calm or a stormy, summer or winter”—far less of a great storm or flood announced for any special day.

But let us see what the cycle association between solar spots and terrestrial weather actually is, or rather of what nature it promises to be, for as yet the true nature of the association has not been made out.

It has been found that in a period of about eleven years the sun’s surface is affected by what may be described as a wave of sun-spots. There is a short time—a year or so—during which scarce any spots are seen; they become more and more numerous during the next four or five years, until they attain a maximum of frequency and size; after this they wane in number and dimensions, until at length, about eleven years from the time when he had before been freest from spots, he attains again a similar condition. After this the spots begin to return, gradually attain to a maximum, then gradually diminish, until after eleven more years have elapsed few or none are seen. It must not be supposed that the sun is always free from spots at the time of minimum spot frequency, or that he always shows many and large spots at the time of maximum spot frequency. Occasionally several very large spots, and sometimes singularly large spots, have been seen in the very heart of the minimum spot season, and again there have been occasions when scarcely any spots have been seen for several days in the very heart of the maximum spot season. But, taking the average of each year, the progression of the spots in number and frequency from minimum to maximum, and their decline from maximum to minimum, are quite unmistakeable.

Now there are some terrestrial phenomena which we might expect to respond in greater or less degree to the sun’s changes of condition with respect to spots. We cannot doubt that the emission both of light and of heat is affected by the presence of spots. It is not altogether clear in what way the emission is affected. We cannot at once assume that because the spots are dark the quantity of sunlight must be less when the spots are numerous; for it may well be that the rest of the sun’s surface may at such times be notably brighter than usual, and the total emission of light may be greater on the whole instead of less. Similarly of the emission of heat. It is certain that when there are many spots the surface of the sun is far less uniform in brightness than at other times. The increase of brightness all round the spots is obvious to the eye when the sun’s image, duly enlarged, is received upon a screen in a darkened room. Whether the total emission of light is increased or diminished has not yet been put to the test. Professor Langley, of the Alleghany Observatory, near Pittsburg, U.S., has carefully measured the diminution of the sun’s emission of light and heat on the assumption that the portion of the surface not marked by spots remains unchanged in lustre. But until the total emission of light and heat at the times of maximum and minimum has been measured, without any assumption of the kind, we cannot decide the question.

More satisfactory would seem to be the measurements which have been made by Professor Piazzi Smyth, at Edinburgh, and later by the Astronomer Royal at Greenwich, into the underground temperature of the earth. By examining the temperature deep down below the surface, all local and temporary causes of change are eliminated, and causes external to the earth can alone be regarded as effective in producing systematic changes. “The effect is very slight,” I wrote a few years ago, “indeed barely recognizable. I have before me as I write Professor Smyth’s sheet of the quarterly temperatures from 1837 to 1869 at depths of 3, 6, 12, and 24 French feet. Of course the most remarkable feature, even at the depth of 24 feet, is the alternate rise and fall with the seasons. But it is seen that, while the range of rise and fall remains very nearly constant, the crests and troughs of the waves lie at varying levels.” After describing in the essay above referred to, which appears in my “Science Byways,” the actual configuration of the curves of temperature both for seasons and for years, and the chart in which the sun-spot waves and the temperature waves are brought into comparison, I was obliged to admit that the alleged association between the sun-spot period and the changes of underground temperature did not seem to me very clearly made out. It appears, however, there is a slight increase of temperature at the time when the sun-spots are least numerous.

That the earth’s magnetism is affected by the sun’s condition with respect to spots, seems to have been more clearly made out, though it must be noted that the Astronomer Royal considers the Greenwich magnetic observations inconsistent with this theory. It seems to have been rendered at least extremely probable that the daily oscillation of the magnetic needle is greater when spots are numerous than when there are few spots or none. Magnetic storms are also more numerous at the time of maximum spot-frequency, and auroras are then more common. (The reader will not fall into the mistake of supposing that magnetic storms have the remotest resemblance to hurricanes, or rainstorms, or hailstorms, or even to thunderstorms, though the thunderstorm is an electrical phenomenon. What is meant by a magnetic storm is simply such a condition of the earth’s frame that the magnetic currents traversing it are unusually strong.)

Thus far, however, we have merely considered relations which we might fairly expect to find affected by the sun’s condition as to spots. A slight change in his total brightness and in the total amount of heat emitted by him may naturally be looked for under circumstances which visibly affect the emission of light, and presumably affect the emission of heat also, from portions of his surface. Nor can we wonder if terrestrial magnetism, which is directly dependent on the sun’s emission of heat, should be affected by the existence of spots upon his surface.

It is otherwise with the effects which have recently been associated with the sun’s condition. It may or may not prove actually to be the case that wind and rain vary in quantity as the sun-spots vary in number (at least when we take in both cases the average for a year, or for two or three years), but it cannot be said that any such relation was antecedently to be expected. When we consider what the sun actually does for our earth, it seems unlikely that special effects such as these should depend on relatively minute peculiarities of the sun’s surface. There is our earth, with her oceans and continents, turning around swiftly on her axis, and exposed to his rays as a whole. Or, inverting the way of viewing matters, there is the sun riding high in the heavens of any region of the earth, pouring down his rays upon that region. We can understand how in the one case that rotating orb of the earth may receive rather more or rather less heat from the sun when he is spotted than when he is not, or how in the other way of viewing matters, that orb of the sun may give to any region rather more or rather less heat according as his surface is more or less spotted. But that in special regions of that rotating earth storms should be more or less frequent or rainfall heavier or lighter, as the sun’s condition changes through the exceedingly small range of variation due to the formation of spots, seems antecedently altogether unlikely; and equally unlikely the idea that peculiarities affecting limited regions of the sun’s surface should affect appreciably the general condition of the earth. If a somewhat homely comparison may be permitted, we can well understand how a piece of meat roasting before a fire may receive a greater or less supply of heat on the whole as the fire undergoes slight local changes (very slight indeed they must be, that the illustration may be accurate); but it would be extremely surprising if, in consequence of such slight changes in the fire, the roasting of particular portions of the joint were markedly accelerated or delayed, or affected in any other special manner.

But of course all such considerations as to antecedent probabilities must give way before the actual evidence of observed facts. Utterly inconsistent with all that is yet known of the sun’s physical action, as it may seem, on à priori grounds, to suppose that spots, currents, or other local disturbances of the sun’s surface could produce any but general effects on the earth as a whole, yet if we shall find that particular effects are produced in special regions of the earth’s surface in cycles unmistakably synchronizing with the solar-spot-cycle, we must accept the fact, whether we can explain it or not. Only let it be remembered at the outset that the earth is a large place, and the variations of wind and calm, rain and drought, are many and various in different regions. Whatever place we select for examining the rainfall, for example, we are likely to find, in running over the records of the last thirty years or so, some seemingly oscillatory changes; in the records of the winds, again, we are likely to find other seemingly oscillatory changes; if none of these records provide anything which seems in any way to correspond with the solar spot-cycle, we may perchance find some such cycle in the relative rainfall of particular months, or in the varying wetness or dryness of particular winds, and so forth. Or, if we utterly fail to find any such relation in one place we may find it in another, or not improbably in half-a-dozen places among the hundreds which are available for the search. If we are content with imperfect correspondence between some meteorological process or another and the solar-spot cycle, we shall be exceedingly unfortunate indeed if we fail to find a score of illustrative instances. And if we only record these, without noticing any of the cases where we fail to find any association whatever—in other words, as Bacon puts it, if “we note when we hit and never note when we miss,” we shall be able to make what will seem a very strong case indeed. But this is not exactly the scientific method in such cases. By following such a course, indeed, we might prove almost anything. If we take, for instance, a pack of cards, and regard the cards in order as corresponding to the years 1825 to 1877, and note their colours as dealt once, we shall find it very difficult to show that there is any connection whatever between the colours of the cards corresponding to particular years and the number of spots on the sun’s face. But if we repeat the process a thousand times, we shall find certain instances among the number, in which red suits correspond to all the years when there are many spots on the sun, and black suits to all the years when there are few spots on the sun. If now we were to publish all such deals, without mentioning anything at all about the others which showed no such association, we should go far to convince a certain section of the public that the condition of the sun as to spots might hereafter be foretold by the cards; whence, if the public were already satisfied that the condition of the sun specially affects the weather of particular places, it would follow that the future weather of these places might also be foretold by the cards.

I mention this matter at the outset, because many who are anxious to find some such cycle of seasons as Sir John Herschel thought might be discovered, have somewhat overlooked the fact that we must not hunt down such a cycle per fas et nefas. “Surely in meteorology as in astronomy,” Mr. Lockyer writes, for instance, “the thing to hunt down is a cycle, and if that is not to be found in the temperate zone, then go to the frigid zones or the torrid zone to look for it; and if found, then above all things and in whatever manner, lay hold of, study, and read it, and see what it means.” There can be no doubt that this is the way to find a cycle, or at least to find what looks like a cycle, but the worth of a cycle found in this way will be very questionable.7

I would not have it understood, however, that I consider all the cycles now to be referred to as unreal, or even that the supposed connection between them and the solar cycle has no existence. I only note that there are thousands, if not tens of thousands, of relations among which cycles may be looked for, and that there are perhaps twenty or thirty cases in which some sort of cyclic association between certain meteorological relations and the period of the solar spots presents itself. According to the recognized laws of probability, some at least amongst these cases must be regarded as accidental. Some, however, may still remain which are not accidental.

Among the earliest published instances may be mentioned Mr. Baxendell’s recognition of the fact that during a certain series of years, about thirty, I think, the amount of rainfall at Oxford was greater under west and south-west winds than under south and south-east winds when sun-spots were most numerous, whereas the reverse held in years when there were no spots or few. Examining the meteorological records of St. Petersburg, he found that a contrary state of things prevailed there.

The Rev. Mr. Main, Director of the Radcliffe Observatory at Oxford, found that westerly winds were slightly more common (as compared with other winds) when sun-spots were numerous than when they were few.

Mr. Meldrum, of Mauritius, has made a series of statistical inquiries into the records of cyclones which have traversed the Indian Ocean between the equator and 34 degrees south latitude, in each year from 1856 to 1877, noting the total distances traversed by each, the sums of their radii and areas, their duration in days, the sums of their total areas, and their relative areas. His researches, be it marked in passing, are of extreme interest and value, whether the suggested connection between sun-spots and cyclones (in the region specified) be eventually found to be a real one or not. The following are his results, as described in Nature by a writer who manifestly favours very strongly the doctrine that an intimate association exists between solar maculation (or spottiness) and terrestrial meteorological phenomena:—

“The period embraces two complete, or all but complete, sun-spot periods, the former beginning with 1856 and ending in 1867, and the latter extending from 1867 to about the present time [1877]. The broad result is that the number of cyclones, the length and duration of their courses, and the extent of the earth’s surface covered by them all, reach the maximum in each sun-spot period during the years of maximum maculation, and fall to the minimum during the years of minimum maculation. The peculiar value of these results lies in the fact that the portion of the earth’s surface over which this investigation extends, is, from its geographical position and what may be termed its meteorological homogeneity, singularly well fitted to bring out prominently any connection that may exist between the condition of the sun’s surface and atmospheric phenomena.”

The writer proceeds to describe an instance in which Mr. Meldrum predicted future meteorological phenomena, though without specifying the exact extent to which Mr. Meldrum’s anticipations were fulfilled or the reverse. “A drought commenced in Mauritius early in November,” he says, “and Mr. Meldrum ventured (on December 21) to express publicly his opinion that probably the drought would not break up till towards the end of January, and that it might last till the middle of February, adding that up to these dates the rainfall of the island would probably not exceed 50 per cent. of the mean fall. This opinion was an inference grounded on past observations, which show that former droughts have lasted from about three to three and a half months, and that these droughts have occurred in the years of minimum sun-spots, or, at all events, in years when the spots were far below the average, such as 1842, 1843, 1855, 1856, 1864, 1866, and 1867, and that now we are near the minimum epoch of sun-spots. It was further stated that the probability of rains being brought earlier by a cyclone was but slight, seeing that the season for cyclones is not till February or March, and that no cyclone whatever visited Mauritius during 1853–56 and 1864–67, the years of minimum sun-spots. From the immense practical importance of this application of the connection between sun-spots and weather to the prediction of the character of the weather of the ensuing season, we shall look forward with the liveliest interest to a detailed statement of the weather which actually occurred in that part of the Indian Ocean from November to March last [1876].”

It was natural that the great Indian famine, occurring at a time when sun-spots were nearly at a minimum, should by some be directly associated with a deficiency of sun-spots. In this country, indeed, we have had little reason, during the last two or three years of few sun-spots, to consider that drought is one of the special consequences to be attributed to deficient solar maculation. But in India it may be different, or at least it may be different in Madras, for it has been satisfactorily proved that in some parts of India the rainfall increases in inverse, not in direct proportion, to the extent of solar maculation. Dr. Hunter has shown to the satisfaction of many that at Madras there is “a cycle of rainfall corresponding with the period of solar maculation.” But Mr. E. D. Archibald, who is also thoroughly satisfied that the sun-spots affect the weather, remarks that Dr. Hunter has been somewhat hasty in arguing that the same conditions apply throughout the whole of Southern India. “This hasty generalization from the results of one station situated in a vast continent, the rainfall of which varies completely, both in amount and the season in which it falls, according to locality, has been strongly contested by Mr. Blanford, the Government Meteorologist, who, in making a careful comparison of the rainfalls of seven stations, three of which (Madras, Bangalore, and Mysore) are in Southern India, the others being Bombay, Najpore, Jubbulpore, and Calcutta, finds that, with the exception of Najpore in Central India, which shows some slight approach to the same cyclical variation which is so distinctly marked in the Madras registers, the rest of the stations form complete exceptions to the rule adduced for Madras, in many of them the hypothetical order of relation being reversed. Mr. Blanford, however, shows that, underlying the above irregularities, a certain cyclical variation exists on the average at all the stations, the amount, nevertheless, being so insignificant (not more than 9 per cent. of the total falls) that it could not be considered of sufficient magnitude to become a direct factor in the production of famine. It thus appears that the cycle of rainfall which is considered to be the most important element in causing periodic famines has only been proved satisfactorily for the town of Madras. It may perhaps hold for the Carnatic and Northern Siccars, the country immediately surrounding Madras, though perhaps, owing to the want of rainfall registers in these districts, evidence with regard to this part is still wanting.” On this Mr. Archibald proceeds to remark that, though Dr. Hunter has been only partially successful, the value of his able pamphlet is not diminished in any way, “an indirect effect of which has been to stimulate meteorological inquiry and research in the same direction throughout India. The meteorology of this country (India), from its peculiar and tropical position, is in such complete unison with any changes that may arise from oscillations in the amount of solar radiation, and their effects upon the velocity and direction of the vapour-bearing winds, that a careful study of it cannot fail to discover meteorological periodicities in close connection with corresponding periods of solar disturbance.” So, indeed, it would seem.

The hope that famines may be abated, or, at least, some of their most grievous consequences forestalled by means of solar observatories, does not appear very clearly made out. Rather it would seem that the proper thing to do is to investigate the meteorological records of different Indian regions, and consider the resulting evidence of cyclic changes without any special reference to sun-spots; for if sun-spots may cause drought in one place, heavy rainfall in another, winds here and calms there, it seems conceivable that the effects of sun-spots may differ at different times, as they manifestly do in different places.

Let us turn, however, from famines to shipwrecks. Perhaps, if we admit that cyclones are more numerous, and blow more fiercely, and range more widely, even though it be over one large oceanic region only, in the sun-spot seasons than at other times, we may be assured, without further research, that shipwrecks will, on the whole, be more numerous near the time of sun-spot maxima than near the time of sun-spot minima.

The idea that this may be so was vaguely shadowed forth in a poem of many stanzas, called “The Meteorology of the Future: a Vision,” which appeared in Nature for July 5, 1877. I do not profess to understand precisely what the object of this poem may have been—I mean, whether it is intended to support or not the theory that sun-spots influence the weather. Several stanzas are very humorous, but the object of the humour is not manifest. The part referred to above is as follows:—Poor Jack lies at the bottom of the sea in 1881, and is asked in a spiritual way various questions as to the cause of his thus coming to grief. This he attributed to the rottenness of the ship in which he sailed, to the jobbery of the inspector, to the failure of the system of weather telegraphing, and so forth. But, says the questioner, there was one

“In fame to none will yield, He led the band who reaped renown On India’s famine field.

“Was he the man to see thee die? Thou wilt not tax him—come? The dead man groaned—‘I met my death Through a sun-spot maximum.’”

The first definite enunciation, however, of a relation between sun-spots and shipwrecks appeared in September, 1876. Mr. Henry Jeula, in the Times for September 19, stated that Dr. Hunter’s researches into the Madras rainfall had led him to throw together the scanty materials available relating to losses posted on Lloyd’s loss book, to ascertain if any coincidences existed between the varying number of such losses and Dr. Hunter’s results. “For,” he proceeds, “since the cycle of rainfall at Madras coincides, I am informed, with the periodicity of the cyclones in the adjoining Bay of Bengal” (a relation which is more than doubtful) “as worked out by the Government Astronomer at Mauritius” (whose researches, however, as we have seen, related to a region remote from the Bay of Bengal), “some coincidence between maritime casualties, rainfalls, and sun-spots appeared at least possible.” In passing, I may note that if any such relation were established, it would be only an extension of the significance of the cycle of cyclones, and could have no independent value. It would certainly follow, if the cycle of cyclones is made out, that shipwrecks being more numerous, merchants would suffer, and we should have the influence of the solar spots asserting itself in the Gazette. From the cyclic derangement of monetary and mercantile matters, again, other relations also cyclic in character would arise. But as all these may be inferred from the cycle of cyclones once this is established, we could scarcely find in their occurrence fresh evidence of the necessity of that much begged-for solar observatory. The last great monetary panic in this country, by the way, occurred in 1866, at a time of minimum solar maculation. Have we here a decisive proof that the sun rules the money market, the bank rate of discount rising to a maximum as the sun-spots sink to a minimum, and vice versâ? The idea is strengthened by the fact that the American panic in 1873 occurred when spots were very numerous, and its effects have steadily subsided as the spots have diminished in number; for this shows that the sun rules the money market in America on a principle diametrically opposed to that on which he (manifestly) rules the money market in England, precisely as the spots cause drought in Calcutta and plenteous rainfall at Madras, wet south-westers and dry south-westers at Oxford, and wet south-easters and dry south-easters at St. Petersburg. Surely it would be unreasonable to refuse to recognize the weight of evidence which thus tells on both sides at once.

To return, however, to the sun’s influence upon shipwrecks.

Mr. Jeula was “only able to obtain data for two complete cycles of eleven years, namely, from 1855 to 1876 inclusive, while the period investigated by Dr. Hunter extended from 1813 to 1876, and his observations related to Madras and its neighbourhood only, while the losses posted at Lloyd’s occurred to vessels of various countries, and happened in different parts of the world. It was necessary to bring these losses to some common basis of comparison, and the only available one was the number of ‘British registered vessels of the United Kingdom and Channel Islands’—manifestly an arbitrary one. I consequently cast out the percentage of losses posted each year upon the number of registered vessels for the same year, and also the percentage of losses posted in each of the eleven years of the two cycles upon the total posted in each complete cycle, thus obtaining two bases of comparison independent of each other.”

The results may be thus presented:—

Taking the four years of each cycle when sun-spots were least in number, Mr. Jeula found the mean percentage of losses in registered vessels of the United Kingdom and Channel Islands to be 11·13, and the mean percentage of losses in the total posted in the entire cycle of eleven years to be 8·64.

In the four years when sun-spots were intermediate in number, that is in two years following the minimum and in two years preceding the minimum, the respective percentages were 11·91 and 9·21.

Lastly, in the three years when sun-spots were most numerous, these percentages were, respectively, 12·49 and 9·53.

That the reader may more clearly understand what is meant here by percentages, I explain that while the numbers 11·13, 11·91 and 12·49 simply indicate the average number of wrecks (per hundred of all the ships registered) which occurred in the several years of the eleven-years cycle, the other numbers, 8·64, 19·21, and 9·53, indicate the average number of wrecks (per hundred of wrecks recorded) during eleven successive years, which occurred in the several years of the cycle. The latter numbers seem more directly to the purpose; and as the two sets agree pretty closely, we may limit our attention to them.

Now I would in the first place point out that it would have been well if the actual number or percentage had been indicated for each year of the cycle, instead of for periods of four years, four years, and three years. Two eleven-year cycles give in any case but meagre evidence, and it would have been well if the evidence had been given as fully as possible. If we had a hundred eleven-yearly cycles, and took the averages of wrecks for the four years of minimum solar maculation, the four intermediate years, and the three years of maximum maculation, we might rely with considerable confidence on the result, because accidental peculiarities one way or the other could be eliminated. But in two cycles only, such peculiarities may entirely mask any cyclic relation really existing, and appear to indicate a relation which has no real existence. If the percentages had been given for each year, the effect of such peculiarities would doubtless still remain, and the final result would not be more trustworthy than before; but we should have a chance of deciding whether such peculiarities really exist or not, and also of determining what their nature may be. As an instance in point, let me cite a case where, having only the results of a single cycle, we can so arrange them as to appear to indicate a cyclic association between sun-spots and rainfall, while, when we give them year by year, such an association is discredited, to say the least.

The total rainfall at Port Louis, between the years 1855 and 1868 inclusive, is as follows:—

In

Rainfall.

Condition of Sun.

1855

42·665

inches

Sun-spot minimum.

1856

46·230

„

1857

43·445

„

1858

35·506

„

1859

56·875

„

1860

45·166

„

Sun-spot maximum.

1861

68·733

„

1862

28·397

„

1863

33·420

„

1864

24·147

„

1865

44·730

„

1866

20·571

„

Sun-spot minimum.

1867

35·970

„

1868

64·180

„

I think no one, looking at these numbers as they stand, can recognize any evidence of a cyclic tendency. If we represent the rainfall by ordinates we get the accompanying figure, which shows the rainfall for eighteen years, and again I think it may be said that a very lively imagination is required to recognize anything resembling that wave-like undulation which the fundamental law of statistics requires where a cycle is to be made out from a single oscillation. Certainly the agreement between the broken curve of rainfall and the sun-spot curve indicated by the waved dotted line is not glaringly obvious. But when we strike an average for the rainfall, in the way adopted by Mr. Jeula for shipwrecks, how pleasantly is the theory of sun-spot influence illustrated by the Port Louis rainfall! Here is the result, as quoted by the high-priest of the new order of diviners, from the papers by Mr. Meldrum:—

Three minimum years—total rainfall

133·340

Three maximum years—total rainfall

170·774

Three minimum years—total rainfall

120·721

Nothing could be more satisfactory, but nothing, I venture to assert, more thoroughly inconsistent with the true method of statistical research.

May it not be that, underlying the broad results presented by Mr. Jeula, there are similar irregularities?

When we consider that the loss of ships depends, not only on a cause so irregularly variable (to all seeming) as wind-storms, but also on other matters liable to constant change, as the variations in the state of trade, the occurrence of wars and rumours of wars, special events, such as international exhibitions, and so forth, we perceive that an even wider range of survey is required to remove the effects of accidental peculiarities in their case, than in the case of rainfall, cyclones, or the like. I cannot but think, for instance, that the total number of ships lost in divers ways during the American war, and especially in its earlier years (corresponding with two of the three maximum years of sun-spots) may have been greater, not merely absolutely but relatively, than in other years. I think it conceivable, again, that during the depression following the great commercial panic of 1866 (occurring at a time of minimum solar maculation, as already noticed) the loss of ships may have been to some degree reduced, relatively as well as absolutely. We know that when trade is unusually active many ships have sailed, and perhaps may still be allowed to sail (despite Mr. Plimsoll’s endeavours), which should have been broken up; whereas in times of trade depression the ships actually afloat are likely to be, on the average, of a better class. So also, when, for some special reason, passenger traffic at sea is abnormally increased. I merely mention these as illustrative cases of causes not (probably) dependent on sun-spots, which may (not improbably) have affected the results examined by Mr. Jeula. I think it possible that those results, if presented for each year, would have indicated the operation of such causes, naturally masked when sets of four years, four years, and three years are taken instead of single years.

I imagine that considerations such as these will have to be taken into account and disposed of before it will be unhesitatingly admitted that sun-spots have any great effect in increasing the number of shipwrecks.

The advocates of the doctrine of sun-spot influence—or, perhaps it would be more correct to say, the advocates of the endowment of sun-spot research—think differently on these and other points. Each one of the somewhat doubtful relations discussed above is constantly referred to by them as a demonstrated fact, and a demonstrative proof of the theory they advocate. For instance, Mr. Lockyer, in referring to Meldrum’s statistical researches into the frequency of cyclones, does not hesitate to assert that according to these researches “the whole question of cyclones is merely a question of solar activity, and that if we wrote down in one column the number of cyclones in any given year, and in another column the number of sun-spots in any given year, there will be a strict relation between them—many sun-spots, many hurricanes; few sun-spots, few hurricanes.” ... And again, “Mr. Meldrum has since found” (not merely “has since found reason to believe,” but definitely, “has since found”) “that what is true of the storms which devastate the Indian Ocean is true of the storms which devastate the West Indies; and on referring to the storms of the Indian Ocean, Mr. Meldrum points out that at those years where we have been quietly mapping the sun-spot maxima, the harbours were filled with wrecks, and vessels coming in disabled from every part of the Indian Ocean.” Again, Mr. Balfour Stewart accepts Mr. Jeula’s statistics confidently as demonstrating that there are most shipwrecks during periods of maximum solar activity. Nor are the advocates of the new method of prediction at all doubtful as to the value of these relations in affording the basis of a system of prediction. They do not tell us precisely how we are to profit by the fact, if fact it is, that cyclones and shipwrecks mark the time of maximum solar maculation, and droughts and famine the time of minimum. “If we can manage to get at these things,” says Mr. Lockyer, “the power of prediction, that power which would be the most useful one in meteorology, if we could only get at it, would be within our grasp.” And Mr. Balfour Stewart, in a letter to the Times, says, “If we are on the track of a discovery which will in time enable us to foretell the cycle of droughts, public opinion should demand that the investigation be prosecuted with redoubled vigour and under better conditions. If forewarned be forearmed, then such research will ultimately conduce to the saving of life both at times of maximum and minimum sun-spot frequency.”

If these hopes are really justified by the facts of the case, it would be well that the matter should be as quickly as possible put to the test. No one would be so heartless, I think, as to reject, through an excess of scientific caution, a scheme which might issue in the saving of many lives from famine or from shipwreck. And on the other hand, no one, I think, would believe so ill of his fellow-men as to suppose for one moment that advantage could be taken of the sympathies which have been aroused by the Indian famine, or which may from time to time be excited by the record of great disasters by sea and land, to advocate bottomless schemes merely for purposes of personal advancement. We must now, perforce, believe that those who advocate the erection of new observatories and laboratories for studying the physics of the sun, have the most thorough faith in the scheme which they proffer to save our Indian population from famine and our seamen from shipwreck.

But they, on the other hand, should now also believe that those who have described the scheme as entirely hopeless, do really so regard it. If we exonerate them from the charge of responding to an appeal for food by offering spectroscopes, they in turn should exonerate us from the charge of denying spectroscopes to the starving millions of India though knowing well that the spectroscopic track leads straight to safety.

I must acknowledge I cannot for my own part see even that small modicum of hope in the course suggested which would suffice to justify its being followed. In my opinion, one ounce of rice would be worth more (simply because it would be worth something) than ten thousand tons of spectroscopes. For what, in the first place, has been shown as to the connection between meteorological phenomena and sun-spots? Supposing we grant, and it is granting a great deal, that all the cycles referred to have been made out. They one and all affect averages only. The most marked among them can so little be trusted in detail that while the maximum of sun-spots agrees in the main with an excess or defect of rain or wind, or of special rains with special winds, or the like, the actual year of maximum may present the exact reverse.

Of what use can it be to know, for instance, that the three years of least solar maculation will probably give a rainfall less than that for the preceding or following three years, if the middle year of the three, when the spots are most numerous of all, may haply show plenteous rainfall? Or it may be the first of the three, or the last, which is thus well supplied, while a defect in the other two, or in one of the others, brings the total triennial rainfall below the average. What provision could possibly be made under such circumstances to meet a contingency which may occur in any one of three years? or, at least, what provision could be made which would prove nearly so effective as an arrangement which could readily be made for keeping sufficient Government stores at suitable stations (that is, never allowing such stores to fall at the critical season in each year below a certain minimum), and sending early telegraphic information of unfavourable weather? Does any one suppose that the solar rice-grains are better worth watching for such a purpose than the terrestrial rice-grains, or that it is not well within the resources of modern science and modern means of communication and transport, to make sufficient preparation each year for a calamity always possible in India? And be it noticed that if, on the one hand, believers in solar safety from famine may urge that, in thus objecting to their scheme, I am opposing what might, in some year of great famine and small sun-spots, save the lives of a greater number than would be saved by any system of terrestrial watchfulness, I would point out, on the other, that the solar scheme, if it means anything at all, means special watchfulness at the minimum sun-spot season, and general confidence (so far as famine is concerned) at the season of maximum solar maculation; and that while as yet nothing has been really proved about the connection between sun-spots and famine, such confidence might prove to be a very dangerous mistake.

Supposing even it were not only proved that sun-spots exert such and such effects, but that this knowledge can avail to help us to measures of special precaution, how is the study of the sun going to advance our knowledge? In passing, let it be remarked that already an enormous number of workers are engaged in studying the sun in every part of the world. The sun is watched on every fine day, in every quarter of the earth, with the telescope, analyzed with the spectroscope, his prominences counted and measured, his surface photographed, and so forth. What more ought to be or could be done? But that is not the main point. If more could be done, what could be added to our knowledge which would avail in the way of prediction? “At present,” says Mr. Balfour Stewart, “the problem has not been pursued on a sufficiently large scale or in a sufficient number of places. If the attack is to be continued, the skirmishers should give way to heavy guns, and these should be brought to bear without delay now that the point of attack is known.” In other words, now that we know, according to the advocates of these views, that meteorological phenomena follow roughly the great solar-spot period, we should prosecute the attack in this direction, in order to find out—what? Minor periods, perhaps, with which meteorological phenomena may still more roughly synchronize. Other such periods are already known with which meteorological phenomena have never yet been associated. New details of the sun’s surface? No one has yet pretended that any of the details already known, except the spots, affect terrestrial weather, and the idea that peculiarities so minute as hitherto to have escaped detection can do so, is as absurd, on the face of it, as the supposition that minute details in the structure of a burning coal, such details as could only be detected by close scrutiny, can affect the general quality and effects of the heat transmitted by the coal, as part of a large fire, to the further side of a large room.

Lastly, I would urge this general argument against a theory which seems to me to have even less to recommend it to acceptance than the faith in astrology.8 If it requires, as we are so strongly assured, the most costly observations, the employment of the heaviest guns (and “great guns” are generally expensive), twenty or thirty years of time, and the closest scrutiny and research, to prove that sun-spots affect terrestrial relations in a definite manner, effects so extremely difficult to demonstrate cannot possibly be important enough to be worth predicting.

NEW WAYS OF MEASURING THE SUN’S DISTANCE.

It is strange that the problem of determining the sun’s distance, which for many ages was regarded as altogether insoluble, and which even during later years had seemed fairly solvable in but one or two ways, should be found, on closer investigation, to admit of many methods of solution. If astronomers should only be as fortunate hereafter in dealing with the problem of determining the distances of the stars, as they have been with the question of the sun’s distance, we may hope for knowledge respecting the structure of the universe such as even the Herschels despaired of our ever gaining. Yet this problem of determining star-distances does not seem more intractable, now, than the problem of measuring the sun’s distance appeared only two centuries ago. If we rightly view the many methods devised for dealing with the easier task, we must admit that the more difficult—which, by the way, is in reality infinitely the more interesting—cannot be regarded as so utterly hopeless as, with our present methods and appliances, it appears to be. True, we know only the distances of two or three stars, approximately, and have means of forming a vague opinion about the distances of only a dozen others, or thereabouts, while at distances now immeasurable lie six thousand stars visible to the eye, and twenty millions within range of the telescope. Yet, in Galileo’s time, men might have argued similarly against all hope of measuring the proportions of the solar system. “We know only,” they might have urged, “the distance of the moon, our immediate neighbour,—beyond her, at distances so great that hers, so far as we can judge, is by comparison almost as nothing, lie the Sun and Mercury, Venus and Mars; further away yet lie Jupiter and Saturn, and possibly other planets, not visible to the naked eye, but within range of that wonderful instrument, the telescope, which our Galileo and others are using so successfully. What hope can there be, when the exact measurement of the moon’s distance has so fully taxed our powers of celestial measurement, that we can ever obtain exact information respecting the distances of the sun and planets? By what method is a problem so stupendous to be attacked?” Yet, within a few years of that time, Kepler had formed already a rough estimate of the distance of the sun; in 1639, young Horrocks pointed to a method which has since been successfully applied. Before the end of the seventeenth century Cassini and Flamsteed had approached the solution of the problem more nearly, while Hailey had definitely formulated the method which bears his name. Long before the end of the eighteenth century it was certainly known that the sun’s distance lies between 85 millions of miles and 98 millions (Kepler, Cassini, and Flamsteed had been unable to indicate any superior limit). And lastly, in our own time, half a score of methods, each subdivisible into several forms, have been applied to the solution of this fundamental problem of observational astronomy.

I propose now to sketch some new and very promising methods, which have been applied already with a degree of success arguing well for the prospects of future applications of the methods under more favourable conditions.

In the first place, let us very briefly consider the methods which had been before employed, in order that the proper position of the new methods may be more clearly recognized.

The plan obviously suggested at the outset for the solution of the problem was simply to deal with it as a problem of surveying. It was in such a manner that the moon’s distance had been found, and the only difficulty in applying the method to the sun or to any planet consisted in the delicacy of the observations required. The earth being the only surveying-ground available to astronomers in dealing with this problem (in dealing with the problem of the stars’ distances they have a very much wider field of operations), it was necessary that a base-line should be measured on this globe of ours,—large enough compared with our small selves, but utterly insignificant compared with the dimensions of the solar system. The diameter of the earth being less than 8000 miles, the longest line which the observers could take for base scarcely exceeded 6000 miles; since observations of the same celestial object at opposite ends of a diameter necessarily imply that the object is in the horizon of both the observing stations (for precisely the same reason that two cords stretched from the ends of any diameter of a ball to a distant point touch the ball at those ends). But the sun’s distance being some 92 millions of miles, a base of 6000 miles amounts to less than the 15,000th part of the distance to be measured. Conceive a surveyor endeavouring to determine the distance of a steeple or rock 15,000 feet, or nearly three miles, from him, with a base-line one foot in length, and you can conceive the task of astronomers who should attempt to apply the direct surveying method to determine the sun’s distance,—at least, you have one of their difficulties strikingly illustrated, though a number of others remain which the illustration does not indicate. For, after all, a base one foot in length, though far too short, is a convenient one in many respects: the observer can pass from one end to the other without trouble—he looks at the distant object under almost exactly the same conditions from each end, and so forth. A base 6000 miles long for determining the sun’s distance is too short in precisely the same degree, but it is assuredly not so convenient a base for the observer. A giant 36,000 miles high would find it as convenient as a surveyor six feet high would find a one foot base-line; but astronomers, as a rule, are less than 36,000 miles in height. Accordingly the same observer cannot work at both ends of the base-line, and they have to send out expeditions to occupy each station. All the circumstances of temperature, atmosphere, personal observing qualities, etc., are unlike at the two ends of the base-line. The task of measuring the sun’s distance directly is, in fact, at present beyond the power of observational astronomy, wonderfully though its methods have developed in accuracy.

We all know how, by observations of Venus in transit, the difficulty has been so far reduced that trustworthy results have been obtained. Such observations belong to the surveying method, only Venus’s distance is made the object of measurement instead of the sun’s. The sun serves simply as a sort of dial-plate, Venus’s position while in transit across this celestial dial-plate being more easily measured than when she is at large upon the sky. The devices by which Halley and Delisle severally caused time to be the relation observed, instead of position, do not affect the general principle of the transit method. It remains dependent on the determination of position. Precisely as by the change of the position of the hands of a clock on the face we measure time, so by the transit method, as Halley and Delisle respectively suggested its use, we determine Venus’s position on the sun’s face, by observing the difference of the time she takes in crossing, or the difference of the time at which she begins to cross, or passes off, his face.

Besides the advantage of having a dial-face like the sun’s on which thus to determine positions, the transit method deals with Venus when at her nearest, or about 25 million miles from us, instead of the sun at his greater distance of from 90½ to 93½ millions of miles. Yet we do not get the entire advantage of this relative proximity of Venus. For the dial-face—the sun, that is—changes its position too—in less degree than Venus changes hers, but still so much as largely to reduce her seeming displacement. The sun being further away as 92 to 25, is less displaced as 25 to 92. Venus’s displacement is thus diminished by 25/92nds of its full amount, leaving only 67/92nds. Practically, then, the advantage of observing Venus, so far as distance is concerned, is the same as though, instead of being at a distance of only 25 million miles, her distance were greater as 92 to 67, giving as her effective distance when in transit some 34,300,000 miles.

All the methods of observing Venus in transit are affected in this respect. Astronomers were not content during the recent transit to use Halley’s and Delisle’s two time methods (which may be conveniently called the duration method and the epoch method), but endeavoured to determine the position of Venus on the sun’s face directly, both by observation and by photography. The heliometer was the instrument specially used for the former purpose; and as, in one of the new methods to be presently described, this is the most effective of all available instruments, a few words as to its construction will not be out of place.

The heliometer, then, is a telescope whose object-glass (that is, the large glass at the end towards the object observed) is divided into two halves along a diameter. When these two halves are exactly together—that is, in the position they had before the glass was divided—of course they show any object to which they may be directed precisely as they would have done before the glass was cut. But if, without separating the straight edges of the two semicircular glasses, one be made to slide along the other, the images formed by the two no longer coincide.9 Thus, if we are looking at the sun we see two overlapping discs, and by continuing to turn the screw or other mechanism which carries our half-circular glass past the other, the disc-images of the sun may be brought entirely clear of each other. Then we have two suns in the same field of view, seemingly in contact, or nearly so. Now, if we have some means of determining how far the movable half-glass has been carried past the other to bring the two discs into apparently exact contact, we have, in point of fact, a measure of the sun’s apparent diameter. We can improve this estimate by carrying back the movable glass till the images coincide again, then further back till they separate the other way and finally are brought into contact on that side. The entire range, from contact on one side to contact on the other side, gives twice the entire angular span of the sun’s diameter; and the half of this is more likely to be the true measure of the diameter, than the range from coincident images to contact either way, simply because instrumental errors are likely to be more evenly distributed over the double motion than over the movement on either side of the central position. The heliometer derived its name—which signifies sun-measurer—from this particular application of the instrument.

It is easily seen how the heliometer was made available in determining the position of Venus at any instant during transit. The observer could note what displacement of the two half-glasses was necessary to bring the black disc of Venus on one image of the sun to the edge of the other image, first touching on the inside and then on the outside. Then, reversing the motion, he could carry her disc to the opposite edge of the other image of the sun, first touching on the inside and then on the outside. Lord Lindsay’s private expedition—one of the most munificent and also one of the most laborious contributions to astronomy ever made—was the only English expedition which employed the heliometer, none of our public observatories possessing such an instrument, and official astronomers being unwilling to ask Government to provide instruments so costly. The Germans, however, and the Russians employed the heliometer very effectively.

Next in order of proximity, for the employment of the direct surveying method, is the planet Mars when he comes into opposition (or on the same line as the earth and sun) in the order

Sun____________________________Earth__________Mars,

at a favourable part of his considerably eccentric orbit. His distance then may be as small as 34½ millions of miles; and we have in his case to make no reduction for the displacement of the background on which his place is to be determined. That background is the star sphere, his place being measured from that of stars near which his apparent path on the heavens carries him; and the stars are so remote that the displacement due to a distance of six or seven thousand miles between two observers on the earth is to all intents and purposes nothing. The entire span of the earth’s orbit round the sun, though amounting to 184 millions of miles, is a mere point as seen from all save ten or twelve stars; how utterly evanescent, then, the span of the earth’s globe—less than the 23,000th part of her orbital range! Thus the entire displacement of Mars due to the distance separating the terrestrial observers comes into effect. So that, in comparing the observation of Mars in a favourable opposition with that of Venus in transit, we may fairly say that, so far as surveying considerations are concerned, the two planets are equally well suited for the astronomer’s purpose. Venus’s less distance of 25 millions of miles is effectively increased to 34⅓ millions by the displacement of the solar background on which we see her when in transit; while Mars’s distance of about 34½ millions of miles remains effectively the same when we measure his displacement from neighbouring fixed stars.

But in many respects Mars is superior to Venus for the purpose of determining the sun’s distance. Venus can only be observed at her nearest when in transit, and transit lasts but a few hours. Mars can be observed night after night for a fortnight or so, during which his distance still remains near enough to the least or opposition distance. Again, Venus being observed on the sun, all the disturbing influences due to the sun’s heat are at work in rendering the observation difficult. The air between us and the sun at such a time is disturbed by undulations due in no small degree to the sun’s action. It is true that we have not, in the case of Mars, any means of substituting time measures or time determinations for measures of position, as we have in Venus’s case, both with Halley’s and Delisle’s methods. But to say the truth, the advantage of substituting these time observations has not proved so great as was expected. Venus’s unfortunate deformity of figure when crossing the sun’s edge renders the determination of the exact moments of her entry on the sun’s face and of her departure from it by no means so trustworthy as astronomers could wish. On the whole, Mars would probably have the advantage even without that point in his favour which has now to be indicated.

Two methods of observing Mars for determining the sun’s distance are available, both of which, as they can be employed in applying one of the new methods, may conveniently be described at this point.

An observer far to the north of the earth’s equator sees Mars at midnight, when the planet is in opposition, displaced somewhat to the south of his true position—that is, of the position he would have as supposed to be seen from the centre of the earth. On the other hand, an observer far to the south of the equator sees Mars displaced somewhat to the north of his true position. The difference may be compared to different views of a distant steeple (projected, let us suppose, against a much more remote hill), from the uppermost and lowermost windows of a house corresponding to the northerly and southerly stations on the earth, and from a window on the middle story corresponding to a view of Mars from the earth’s centre. By ascertaining the displacement of the two views of Mars obtained from a station far to the north and another station far to the south, the astronomer can infer the distance of the planet, and thence the dimensions of the solar system. The displacement is determinable by noticing Mars’s position with respect to stars which chance to be close to him. For this purpose the heliometer is specially suitable, because, having first a view of Mars and some companion stars as they actually are placed, the observer can, by suitably displacing the movable half-glass, bring the star into apparent contact with the planet, first on one side of its disc, and then on the other side—the mean of the two resulting measures giving, of course, the distance between the star and the centre of the disc.

This method requires that there shall be two observers, one at a northern station, as Greenwich, or Paris, or Washington, the other at a southern station, as Cape Town, Cordoba, or Melbourne. The base-line is practically a north-and-south line; for though the two stations may not lie in the same, or nearly the same, longitude, the displacement determined is in reality that due to their difference of latitude only, a correction being made for their difference of longitude.

The other method depends, not on displacement of two observers north and south, or difference of latitude, but on displacement east and west. Moreover, it does not require that there shall be two observers at stations far apart, but uses the observations made at one and the same stations at different times. The earth, by turning on her axis, carries the observer from the west to the east of an imaginary line joining the earth’s centre and the centre of Mars. When on the west of that line, or in the early evening, he sees Mars displaced towards the east of the planet’s true position. After nine or ten hours the observer is carried as far to the east of that line, and sees Mars displaced towards the west of his true position. Of course Mars has moved in the interval. He is, in fact, in the midst of his retrograde career. But the astronomer knows perfectly well how to take that motion into account. Thus, by observing the two displacements, or the total displacement of Mars from east to west on account of the earth’s rotation, one and the same observer can, in the course of a single favourable night, determine the sun’s distance. And in passing, it may be remarked that this is the only general method of which so much can be said. By some of the others an astronomer can, indeed, estimate the sun’s distance without leaving his observatory—at least, theoretically he can do so. But many years of observation would be required before he would have materials for achieving this result. On the other hand, one good pair of observations of Mars, in the evening and in the morning, from a station near the equator, would give a very fair measure of the sun’s distance. The reason why the station should be near the equator will be manifest, if we consider that at the poles there would be no displacement due to rotation; at the equator the observer would be carried round a circle some twenty-five thousand miles in circumference; and the nearer his place to the equator the larger the circle in which he would be carried, and (cæteris paribus) the greater the evening and morning displacement of the planet.

Both these methods have been successfully applied to the problem of determining the sun’s distance, and both have recently been applied afresh under circumstances affording exceptionally good prospects of success, though as yet the results are not known.

It is, however, when we leave the direct surveying method to which both the observations of Venus in transit and Mars in opposition belong (in all their varieties), that the most remarkable, and, one may say, unexpected methods of determining the sun’s distance present themselves. Were not my subject a wide one, I would willingly descant at length on the marvellous ingenuity with which astronomers have availed themselves of every point of vantage whence they might measure the solar system. But, as matters actually stand, I must be content to sketch these other methods very roughly, only indicating their characteristic features.

One of them is in some sense related to the method by actual survey, only it takes advantage, not of the earth’s dimensions, but of the dimensions of her orbit round the common centre of gravity of herself and the moon. This orbit has a diameter of about six thousand miles; and as the earth travels round it, speeding swiftly onwards all the time in her path round the sun, the effect is the same as though the sun, in his apparent circuit round the earth, were constantly circling once in a lunar month around a small subordinate orbit of precisely the same size and shape as that small orbit in which the earth circuits round the moon’s centre of gravity. He appears then sometimes displaced about 3000 miles on one side, sometimes about 3000 miles on the other side of the place which he would have if our earth were not thus perturbed by the moon. But astronomers can note each day where he is, and thus learn by how much he seems displaced from his mean position. Knowing that his greatest displacement corresponds to so many miles exactly, and noting what it seems to be, they learn, in fact, how large a span of so many miles (about 3000) looks at the sun’s distance. Thus they learn the sun’s distance precisely as a rifleman learns the distance of a line of soldiers when he has ascertained their apparent size—for only at a certain distance can an object of known size have a certain apparent size.

The moon comes in, in another way, to determine the sun’s distance for us. We know how far away she is from the earth, and how much, therefore, she approaches the sun when new, and recedes from him when full. Calling this distance, roughly, a 390th part of the sun’s, her distance from him when new, her mean distance, and her distance from him when full, are as the numbers 389, 390, 391. Now, these numbers do not quite form a continued proportion, though they do so very nearly (for 389 is to 390 as 390 to 391-1/400). If they were in exact proportion, the sun’s disturbing influence on the moon when she is at her nearest would be exactly equal to his disturbing influence on the moon when at her furthest from him—or generally, the moon would be exactly as much disturbed (on the average) in that half of her path which lies nearer to the sun as in that half which lies further from him. As matters are, there is a slight difference. Astronomers can measure this difference; and measuring it, they can ascertain what the actual numbers are for which I have roughly given the numbers 389, 390, and 391; in other words, they can ascertain in what degree the sun’s distance exceeds the moon’s. This is equivalent to determining the sun’s distance, since the moon’s is already known.

Another way of measuring the sun’s distance has been “favoured” by Jupiter and his family of satellites. Few would have thought, when Römer first explained the delay which occurs in the eclipse of these moons while Jupiter is further from us than his mean distance, that that explanation would lead up to a determination of the sun’s distance. But so it happened. Römer showed that the delay is not in the recurrence of the eclipses, but in the arrival of the news of these events. From the observed time required by light to traverse the extra distance when Jupiter is nearly at his furthest from us, the time in which light crosses the distance separating us from the sun is deduced; whence, if that distance has been rightly determined, the velocity of light can be inferred. If this velocity is directly measured in any way, and found not to be what had been deduced from the adopted measure of the sun’s distance, the inference is that the sun’s distance has been incorrectly determined. Or, to put the matter in another way, we know exactly how many minutes and seconds light takes in travelling to us from the sun; if, therefore, we can find out how fast light travels we know how far away the sun is.

But who could hope to measure a velocity approaching 200,000 miles in a second? At a first view the task seems hopeless. Wheatstone, however, showed how it might be accomplished, measuring by his method the yet greater velocity of freely conducted electricity. Foucault and Fizeau measured the velocity of light; and recently Cornu has made more exact measurements. Knowing, then, how many miles light travels in a second, and in how many seconds it comes to us from the sun, we know the sun’s distance.

The first of the methods which I here describe as new methods must next be considered. It is one which Leverrier regards as the method of the future. In fact, so highly does he esteem it, that, on its account, he may almost be said to have refused to sanction in any way the French expeditions for observing the transit of Venus in 1874.

The members of the sun’s family perturb each other’s motions in a degree corresponding with their relative mass, compared with each other and with the sun. Now, it can be shown (the proof would be unsuitable to these pages,10 but I have given it in my treatise on “The Sun”) that no change in our estimate of the sun’s distance affects our estimate of his mean density as compared with the earth’s. His substance has a mean density equal to one-fourth of the earth’s, whether he be 90 millions or 95 millions of miles from us, or indeed whether he were ten millions or a million million miles from us (supposing for a moment our measures did not indicate his real distance more closely). We should still deduce from calculation the same unvarying estimate of his mean density. It follows that the nearer any estimate of his distance places him, and therefore the smaller it makes his estimated volume, the smaller also it makes his estimated mass, and in precisely the same degree. The same is true of the planets also. We determine Jupiter’s mass, for example (at least, this is the simplest way), by noting how he swerves his moons at their respective (estimated) distances. If we diminish our estimate of their distances, we diminish at the same time our estimate of Jupiter’s attractive power, and in such degree, it may be shown (see note), as precisely to correspond with our changed estimate of his size, leaving our estimate of his mean density unaltered. And the same is true for all methods of determining Jupiter’s mass. Suppose, then, that, adopting a certain estimate of the scale of the solar system, we find that the resulting estimate of the masses of the planets and of the sun, as compared with the earth’s mass, from their observed attractive influences on bodies circling around them or passing near them, accords with their estimated perturbing action as compared with the earth’s,—then we should infer that our estimate of the sun’s distance or of the scale of the solar system was correct. But suppose it appeared, on the contrary, that the earth took a larger or a smaller part in perturbing the planetary system than, according to our estimate of her relative mass, she should do,—then we should infer that the masses of the other members of the system had been overrated or underrated; or, in other words, that the scale of the solar system had been overrated or underrated respectively. Thus we should be able to introduce a correction into our estimate of the sun’s distance.

Such is the principle of the method by which Leverrier showed that in the astronomy of the future the scale of the solar system may be very exactly determined. Of course, the problem is a most delicate one. The earth plays, in truth, but a small part in perturbing the planetary system, and her influence can only be distinguished satisfactorily (at present, at any rate) in the case of the nearer members of the solar family. Yet the method is one which, unlike others, will have an accumulative accuracy, the discrepancies which are to test the result growing larger as time proceeds. The method has already been to some extent successful. It was, in fact, by observing that the motions of Mercury are not such as can be satisfactorily explained by the perturbations of the earth and Venus according to the estimate of relative masses deducible from the lately discarded value of the sun’s distance, that Leverrier first set astronomers on the track of the error affecting that value. He was certainly justified in entertaining a strong hope that hereafter this method will be exceedingly effective.

We come next to a method which promises to be more quickly if not more effectively available.

Venus and Mars approach the orbit of our earth more closely than any other planets, Venus being our nearest neighbour on the one side, and Mars on the other. Looking beyond Venus, we find only Mercury (and the mythical Vulcan), and Mercury can give no useful information respecting the sun’s distance. He could scarcely do so even if we could measure his position among the stars when he is at his nearest, as we can that of Mars; but as he can only then be fairly seen when he transits the sun’s face, and as the sun is nearly as much displaced as Mercury by change in the observer’s station, the difference between the two displacements is utterly insufficient for accurate measurement. But, when we look beyond the orbit of Mars, we find certain bodies which are well worth considering in connection with the problem of determining the sun’s distance. I refer to the asteroids, the ring of small planets travelling between the paths of Mars and Jupiter, but nearer (on the whole11) to the path of Mars than to that of Jupiter.

The asteroids present several important advantages over even Mars and Venus.

Of course, none of the asteroids approach so near to the earth as Mars at his nearest. His least distance from the sun being about 127 million miles, and the earth’s mean distance about 92 millions, with a range of about a million and a half on either side, owing to the eccentricity of her orbit, it follows that he may be as near as some 35 million miles (rather less in reality) from the earth when the sun, earth, and Mars are nearly in a straight line and in that order. The least distance of any asteroid from the sun amounts to about 167 million miles, so that their least distance from the earth cannot at any time be less than about 73,500,000 miles, even if the earth’s greatest distance from the sun corresponded with the least distance of one of these closely approaching asteroids. This, by the way, is not very far from being the case with the asteroid Ariadne, which comes within about 169 million miles of the sun at her nearest, her place of nearest approach being almost exactly in the same direction from the sun as the earth’s place of greatest recession, reached about the end of June. So that, whenever it so chances that Ariadne comes into opposition in June, or that the sun, earth, and Ariadne are thus placed—

Sun________Earth________Ariadne,

Ariadne will be but about 75,500,000 miles from the earth. Probably no asteroid will ever be discovered which approaches the earth much more nearly than this; and this approach, be it noticed, is not one which can occur in the case of Ariadne except at very long intervals.

But though we may consider 80 millions of miles as a fair average distance at which a few of the most closely approaching asteroids may be observed, and though this distance seems very great by comparison with Mars’s occasional opposition distance of 35 million miles, yet there are two points in which asteroids have the advantage over Mars. First, they are many, and several among them can be observed under favourable circumstances; and in the multitude of observations there is safety. In the second place, which is the great and characteristic good quality of this method of determining the sun’s distance, they do not present a disc, like the planet Mars, but a small star-like point. When we consider the qualities of the heliometric method of measuring the apparent distance between celestial objects, the advantage of points of light over discs will be obvious. If we are measuring the apparent distance between Mars and a star, we must, by shifting the movable object-glass, bring the star’s image into apparent contact with the disc-image of Mars, first on one side and then on the other, taking the mean for the distance between the centres. Whereas, when we determine the distance between a star and an asteroid, we have to bring two star-like points (one a star, the other the asteroid) into apparent coincidence. We can do this in two ways, making the result so much the more accurate. For consider what we have in the field of view when the two halves of the object-glass coincide. There is the asteroid and close by there is the star whose distance we seek to determine in order to ascertain the position of the asteroid on the celestial sphere. When the movable half is shifted, the two images of star and asteroid separate; and by an adjustment they can be made to separate along the line connecting them. Suppose, then, we first make the movable image of the asteroid travel away from the fixed image (meaning by movable and fixed images, respectively, those given by the movable and fixed halves of the object-glass), towards the fixed image of the star—the two points, like images, being brought into coincidence, we have the measure of the distance between star and asteroid. Now reverse the movement, carrying back the movable images of the asteroid and star till they coincide again with their fixed images. This movement gives us a second measure of the distance, which, however, may be regarded as only a reversed repetition of the preceding. But now, carrying on the reverse motion, the moving images of star and asteroid separate from their respective fixed images, the moving image of the star drawing near to the fixed image of the asteroid and eventually coinciding with it. Here we have a third measure of the distance, which is independent of the two former. Reversing the motion, and carrying the moving images to coincidence with the fixed images, we have a fourth measure, which is simply the third reversed. These four measures will give a far more satisfactory determination of the true apparent distance between the star and the asteroid than can, under any circumstances, be obtained in the case of Mars and a star. Of course, a much more exact determination is required to give satisfactory measures of the asteroid’s real distance from the earth in miles, for a much smaller error would vitiate the estimate of the asteroid’s distance than would vitiate to the same degree the estimate of Mars’s distance: for the apparent displacements of the asteroid as seen either from Northern and Southern stations, or from stations east and west of the meridian, are very much less than in the case of Mars, owing to his great proximity. But, on the whole, there are reasons for believing that the advantage derived from the nearness of Mars is almost entirely counterbalanced by the advantage derived from the neatness of the asteroid’s image. And the number of asteroids, with the consequent power of repeating such measurements many times for each occasion on which Mars has been thus observed, seem to make the asteroids—so long regarded as very unimportant members of the solar system—the bodies from which, after all, we shall gain our best estimate of the sun’s distance; that is, of the scale of the solar system.

* * * * *

Since the above pages were written, the results deduced from the observations made by the British expeditions for observing the transit of December 9, 1874, have been announced by the Astronomer Royal. It should be premised that they are not the results deducible from the entire series of British observations, for many of them can only be used effectively in combination with observations made by other nations. For instance, the British observations of the duration of the transit as seen from Southern stations are only useful when compared with observations of the duration of the transit as seen from Northern stations, and no British observations of this kind were taken at Northern stations, or could be taken at any of the British Northern stations except one, where chief reliance was placed on photographic methods. The only British results as yet “worked up” are those which are of themselves sufficient, theoretically, to indicate the sun’s distance, viz., those which indicated the epochs of the commencement of transit as seen from Northern and Southern stations, and those which indicated the epochs of the end of transit as seen from such stations. The Northern and Southern epochs of commencement compared together suffice of themselves to indicate the sun’s distance; so also do the epochs of the end of transit suffice of themselves for that purpose. Such observations belong to the Delislean method, which was the subject of so much controversy for two or three years before the transit took place. Originally it had been supposed that only observations by that method were available, and the British plans were formed upon that assumption. When it was shown that this assumption was altogether erroneous, there was scarcely time to modify the British plans so that of themselves they might provide for the other or Halleyan method. But the Southern stations which were suitable for that method were strengthened; and as other nations, especially America and Russia, occupied large numbers of Northern stations, the Halleyan method was, in point of fact, effectually provided for—a fortunate circumstance, as will presently be seen.

The British operations, then, thus far dealt with, were based on Delisle’s method; and as they were carried out with great zeal and completeness, we may consider that the result affords an excellent test of the qualities of this method, and may supply a satisfactory answer to the questions which were under discussion in 1872–74. Sir George Airy, indeed, considers that the zeal and completeness with which the British operations were carried out suffice to set the result obtained from them above all others. But this opinion is based rather on personal than on strictly scientific grounds. It appears to me that the questions to be primarily decided are whether the results are in satisfactory agreement (i) inter se and (ii) with the general tenor of former researches. In other words, while the Astronomer Royal considers that the method and the manner of its application must be considered so satisfactory that the results are to be accepted unquestionably, it appears to me that the results must be carefully questioned (as it were) to see whether the method, and the observations by it, are satisfactory. In the first place, the result obtained from Northern and Southern observations of the commencement ought to agree closely with the result obtained from Northern and Southern observations of the end of transit. Unfortunately, they differ rather widely. The sun’s distance by the former observations comes out about one million miles greater than the distance determined by the latter observations.

This should be decisive, one would suppose. But it is not all. The mean of the entire series of observations by Delisle’s method comes out nearly one million miles greater than the mean deduced by Professor Newcomb from many entire series of observations by six different methods, all of which may fairly be regarded as equal in value to Delisle’s, while three are regarded by most astronomers as unquestionably superior to it. Newcomb considers the probable limits of error in his evaluation from so many combined series of observations to be about 100,000 miles. Sir G. Airy will allow no wider limits of error for the result of the one series his observers have obtained than 200,000 miles. Thus the greatest value admitted by Newcomb falls short of the least value admitted by Sir G. Airy, by nearly 700,000 miles.

The obvious significance of this result should be, one would suppose, that Delisle’s method is not quite so effective as Sir G. Airy supposed; and the wide discordance between the several results, of which the result thus deduced is the mean, should prove this, one would imagine, beyond all possibility of question. The Astronomer Royal thinks differently, however. In his opinion, the wide difference between his result and the mean of all the most valued results by other astronomers, indicates the superiority of Delisle’s method, not its inadequacy to the purpose for which it has been employed.

Time will shortly decide which of these views is correct; but, for my own part, I do not hesitate to express my own conviction that the sun’s distance lies very near the limits indicated by Newcomb, and therefore is several hundred thousand miles less than the minimum distance allowed by the recently announced results.

DRIFTING LIGHT-WAVES.

The method of measuring the motion of very swiftly travelling bodies by noting changes in the light-waves which reach us from them—one of the most remarkable methods of observation ever yet devised by man—has recently been placed upon its trial, so to speak; with results exceedingly satisfactory to the students of science who had accepted the facts established by it. The method will not be unfamiliar to many of my readers. The principle involved was first noted by M. Doppler, but not in a form which promised any useful results. The method actually applied appears to have occurred simultaneously to several persons, as well theorists as observers. Thus Secchi claimed in March, 1868, to have applied it though unsuccessfully; Huggins in April, 1868, described his successful use of the method. I myself, wholly unaware that either of these observers was endeavouring to measure celestial motions by its means, described the method, in words which I shall presently quote, in the number of Fraser’s Magazine for January, 1868, two months before the earliest enunciation of its nature by the physicists just named.

It will be well briefly to describe the principle of this interesting method, before considering the attack to which it has been recently subjected, and its triumphant acquittal from defects charged against it. This brief description will not only be useful to those readers who chance not to be acquainted with the method, but may serve to remove objections which suggest themselves, I notice, to many who have had the principle of the method imperfectly explained to them.

Light travels from every self-luminous body in waves which sweep through the ether of space at the rate of 185,000 miles per second. The whole of that region of space over which astronomers have extended their survey, and doubtless a region many millions of millions of times more extended, may be compared to a wave-tossed sea, only that instead of a wave-tossed surface, there is wave-tossed space. At every point, through every point, along every line, athwart every line, myriads of light-waves are at all times rushing with the inconceivable velocity just mentioned.

It is from such waves that we have learned all we know about the universe outside our own earth. They bring to our shores news from other worlds, though the news is not always easy to decipher.

Now, seeing that we are thus immersed in an ocean, athwart which infinite series of waves are continually rushing, and moreover that we ourselves, and every one of the bodies whence the waves proceed either directly or after reflection, are travelling with enormous velocity through this ocean, the idea naturally presents itself that we may learn something about these motions (as well as about the bodies themselves whence they proceed), by studying the aspect of the waves which flow in upon us in all directions.

Suppose a strong swimmer who knew that, were he at rest, a certain series of waves would cross him at a particular rate—ten, for instance, in a minute—were to notice that when he was swimming directly facing them, eleven passed him in a minute: he would be able at once to compare his rate of swimming with the rate of the waves’ motion. He would know that while ten waves had passed him on account of the waves’ motion, he had by his own motion caused yet another wave to pass him, or in other words, had traversed the distance from one wave-crest to the next Thus he would know that his rate was one-tenth that of the waves. Similarly if, travelling the same way as the waves, he found that only nine passed him in a minute, instead of ten.

Again, it is not difficult to see that if an observer were at rest, and a body in the water, which by certain motions produced waves, were approaching or receding from the observer, the waves would come in faster in the former case, slower in the latter, than if the body were at rest. Suppose, for instance, that some machinery at the bows of a ship raised waves which, if the ship were at rest, would travel along at the rate of ten a minute past the observer’s station. Then clearly, if the ship approached him, each successive wave would have a shorter distance to travel, and so would reach him sooner than it otherwise would have done. Suppose, for instance, the ship travelled one-tenth as fast as the waves, and consider ten waves proceeding from her bows—the first would have to travel a certain distance before reaching the observer; the tenth, starting a minute later, instead of having to travel the same distance, would have to travel this distance diminished by the space over which the ship had passed in one minute (which the wave itself passes over in the tenth of a minute); instead, then, of reaching the observer one minute after the other, it would reach him nine-tenths of a minute after the first. Thus it would seem to him as though the waves were coming in faster than when the ship was at rest, in the proportion of ten to nine, though in reality they would be travelling at the same rate as before, only arriving in quicker succession, because of the continual shortening of the distance they had to travel, on account of the ship’s approach. If he knew precisely how fast they would arrive if the ship were at rest, and determined precisely how fast they did arrive, he would be able to determine at once the rate of the ship’s approach, at least the proportion between her rate and the rate of the waves’ motion. Similarly if, owing to the ship’s recession, the apparent rate of the waves’ motion were reduced, it is obvious that the actual change in the wave motion would not be a difference of rate; but, in the case of the approaching ship, the breadth from crest to crest would be reduced, while in the case of a receding ship the distance from crest to crest would be increased.

If the above explanation should still seem to require closer attention than the general reader may be disposed to give, the following, suggested by a friend of mine—a very skilful mathematician—will be found still simpler: Suppose a stream to flow quite uniformly, and that at one place on its banks an observer is stationed, while at another higher up a person throws corks into the water at regular intervals, say ten corks per minute; then these will float down and pass the other observer, wherever he may be, at the rate of ten per minute, if the cork-thrower is at rest. But if he saunters either up-stream or down-stream, the corks will no longer float past the other at the exact rate of ten per minute. If the thrower is sauntering down-stream, then, between throwing any cork and the next, he has walked a certain way down, and the tenth cork, instead of having to travel the same distance as the first before reaching the observer, has a shorter distance to travel, and so reaches that observer sooner. Or in fact, which some may find easier to see, this cork will be nearer to the first cork than it would have been if the thrower had remained still. The corks will lie at equal distances from each other, but these equal distances will be less than they would have been if the observer had been at rest. If, on the contrary, the cork-thrower saunters up-stream, the corks will be somewhat further apart than if he had remained at rest. And supposing the observer to know beforehand that the corks would be thrown in at the rate of ten a minute, he would know, if they passed him at a greater rate than ten a minute (or, in other words, at a less distance from each other than the stream traversed in the tenth of a minute), that the cork-thrower was travelling down-stream or approaching him; whereas, if fewer than ten a minute passed him, he would know that the cork-thrower was travelling away from him, or up-stream. But also, if the cork-thrower were at rest, and the observer moved up-stream—that is, towards him—the corks would pass him at a greater rate than ten a minute; whereas, if the observer were travelling down-stream, or from the thrower, they would pass him at a slower rate. If both were moving, it is easily seen that if their movement brought them nearer together, the number of corks passing the observer per minute would be increased, whereas if their movements set them further apart, the number passing him per minute would be diminished.

These illustrations, derived from the motions of water, suffice in reality for our purpose. The waves which are emitted by luminous bodies in space travel onwards like the water-waves or the corks of the preceding illustrations. If the body which emits them is rapidly approaching us, the waves are set closer together or narrowed; whereas, if the body is receding, they are thrown further apart or broadened. And if we can in any way recognize such narrowing or broadening of the light-waves, we know just as certainly that the source of light is approaching us or receding from us (as the case may be) as our observer in the second illustration would know from the distance between the corks whether his friend, the cork-thrower, was drawing near to him or travelling away from him.

But it may be convenient to give another illustration, drawn from waves, which, like those of light, are not themselves discernible by our senses—I refer to those aerial waves of compression and rarefaction which produce what we call sound. These waves are not only in this respect better suited than water-waves to illustrate our subject, but also because they travel in all directions through aerial space, not merely along a surface. The waves which produce a certain note, that is, which excite in our minds, through the auditory nerve, the impression corresponding to a certain tone, have a definite length. So long as the observer, and a source of sound vibrating in one particular period, remain both in the same place, the note is unchanged in tone, though it may grow louder or fainter according as the vibrations increase or diminish in amplitude. But if the source of sound is approaching the hearer, the waves are thrown closer together and the sound is rendered more acute (the longer waves giving the deeper sound); and, on the other hand, if the source of sound is receding from the hearer, the waves are thrown further apart and the sound is rendered graver. The rationale of these changes is precisely the same as that of the changes described in the preceding illustrations. It might, perhaps, appear that in so saying we were dismissing the illustration from sound, at least as an independent one, because we are explaining the illustration by preceding illustrations. But in reality, while there is absolutely nothing new to be said respecting the increase and diminution of distances (as between the waves and corks of the preceding illustration), the illustration from sound has the immense advantage of admitting readily of experimental tests. It is necessary only that the rate of approach or recession should bear an appreciable proportion to the rate at which sound travels. For waves are shortened or lengthened by approach or recession by an amount which bears to the entire length of the wave the same proportion which the rate of approach or recession bears to the rate of the wave’s advance. Now it is not very difficult to obtain rates of approach or recession fairly comparable with the velocity of sound—about 364 yards per second. An express train at full speed travels, let us say, about 1800 yards per minute, or 30 yards per second. Such a velocity would suffice to reduce all the sound-waves proceeding from a bell or whistle upon the engine, by about one-twelfth part, for an observer at rest on a station platform approached by the engine. On the contrary, after the engine had passed him, the sound-waves proceeding from the same bell or whistle would be lengthened by one-twelfth. The difference between the two tones would be almost exactly three semitones. If the hearer, instead of being on a platform, were in a train carried past the other at the same rate, the difference between the tone of the bell in approaching and its tone in receding would be about three tones. It would not be at all difficult so to arrange matters, that while two bells were sounding the same note—Mi, let us say—one bell on one engine the other on the other, a traveller by one should hear his own engine’s bell, the bell of the approaching engine, and the bell of the same engine receding, as the three notes—Do—Mi—Sol, whose wave-lengths are as the numbers 15, 12, and 10. We have here differences very easily to be recognized even by those who are not musicians. Every one who travels much by train must have noticed how the tone of a whistle changes as the engine sounding it travels past. The change is not quite sharp, but very rapid, because the other engine does not approach with a certain velocity up to a definite moment and then recede with the same velocity. It could only do this by rushing through the hearer, which would render the experiment theoretically more exact but practically unsatisfactory. As it rushes past instead of through him, there is a brief time during which the rate of approach is rapidly being reduced to nothing, followed by a similarly brief time during which the rate of recession gradually increases from nothing up to the actual rate of the engines’ velocities added together.12 The change of tone may be thus illustrated:—

A B representing the sound of the approaching whistle, B C representing the rapid degradation of sound as the engine rushes close past the hearer, and C D representing the sound of the receding whistle. When a bell is sounded on the engine, as in America, the effect is better recognized, as I had repeated occasion to notice during my travels in that country. Probably this is because the tone of a bell is in any case much more clearly recognized than the tone of a railway whistle. The change of tone as a clanging bell is carried swiftly past (by the combined motions of both trains) is not at all of such a nature as to require close attention for its detection.

However, the apparent variation of sound produced by rapid approach or recession has been tested by exact experiments. On a railway uniting Utrecht and Maarsen “were placed,” the late Professor Nichol wrote, “at intervals of something upwards of a thousand yards, three groups of musicians, who remained motionless during the requisite period. Another musician on the railway sounded at intervals one uniform note; and its effects on the ears of the stationary musicians have been fully published. From these, certainly—from the recorded changes between grave and the more acute, and vice versâ,—confirming, even numerically, what the relative velocities might have enabled one to predict, it appears justifiable to conclude that the general theory is correct; and that the note of any sound may be greatly modified, if not wholly changed, by the velocity of the individual hearing it,” or, he should have added, by the velocity of the source of sound: perhaps more correct than either, is the statement that the note may be altered by the approach or recession of the source of sound, whether that be caused by the motion of the sounding body, or of the hearer himself, or of both.

It is difficult, indeed, to understand how doubt can exist in the mind of any one competent to form an opinion on the matter, though, as we shall presently see, some students of science and one or two mathematicians have raised doubts as to the validity of the reasoning by which it is shown that a change should occur. That the reasoning is sound cannot, in reality, be questioned, and after careful examination of the arguments urged against it by one or two mathematicians, I can form no other opinion than that these arguments amount really but to an expression of inability to understand the matter. This may seem astonishing, but is explained when we remember that some mathematicians, by devoting their attention too particularly to special departments, lose, to a surprising degree, the power of dealing with subjects (even mathematical ones) outside their department. Apart from the soundness of the reasoning, the facts are unmistakably in accordance with the conclusion to which the reasoning points. Yet some few still entertain doubts, a circumstance which may prove a source of consolation to any who find themselves unable to follow the reasoning on which the effects of approach and recession on wave-lengths depend. Let such remember, however, that experiment in the case of the aerial waves producing sound, accords perfectly with theory, and that the waves which produce light are perfectly analogous (so far as this particular point is concerned) with the waves producing sound.

Ordinary white light, and many kinds of coloured light, may be compared with noise—that is, with a multitude of intermixed sounds. But light of one pure colour may be compared to sound of one determinate note. As the aerial waves producing the effect of one definite tone are all of one length, so the ethereal waves producing light of one definite colour are all of one length. Therefore if we approach or recede from a source of light emitting such waves, effects will result corresponding with what has been described above for the case of water-waves and sound-waves. If we approach the source of light, or if it approaches us, the waves will be shortened; if we recede from it, or if it recedes from us, the waves will be lengthened. But the colour of light depends on its wave-length, precisely as the tone of sound depends on its wave-length. The waves producing red light are longer than those producing orange light, these are longer than the waves producing yellow light; and so the wave-lengths shorten down from yellow to green, thence to blue, to indigo, and finally to violet. Thus if a body shining in reality with a pure green colour, approached the observer with a velocity comparable with that of light, it would seem blue, indigo, or violet, according to the rate of approach; whereas if it rapidly receded, it would seem yellow, orange, or red, according to the rate of recession.

Unfortunately in one sense, though very fortunately in many much more important respects, the rates of motion among the celestial bodies are not comparable with the velocity of light, but are always so much less as to be almost rest by comparison. The velocity of light is about 187,000 miles per second, or, according to the measures of the solar system at present in vogue (which will shortly have to give place to somewhat larger measures, the result of observations made upon the recent transit of Venus), about 185,000 miles per second. The swiftest celestial motion of which we have ever had direct evidence was that of the comet of the year 1843, which, at the time of its nearest approach to the sun, was travelling at the rate of about 350 miles per second. This, compared with the velocity of light, is as the motion of a person taking six steps a minute, each less than half a yard long, to the rush of the swiftest express train. No body within our solar system can travel faster than this, the motion of a body falling upon the sun from an infinite distance being only about 370 miles per second when it reaches his surface. And though swifter motions probably exist among the bodies travelling around more massive suns than ours, yet of such motions we can never become cognizant. All the motions taking place among the stars themselves would appear to be very much less in amount. The most swiftly moving sun seems to travel but at the rate of about 50 or 60 miles per second.

Now let us consider how far a motion of 100 miles per second might be expected to modify the colour of pure green light—selecting green as the middle colour of the spectrum. The waves producing green light are of such a length, that 47,000 of them scarcely equal in length a single inch. Draw on paper an inch and divide it carefully into ten equal parts, or take such parts from a well-divided rule; divide one of these tenths into ten equal parts, as nearly as the eye will permit you to judge; then one of these parts, or about half the thickness of an average pin, would contain 475 of the waves of pure green light. The same length would equal the length of 440 waves of pure yellow light, and of 511 waves of pure blue light. (The green, yellow, and blue, here spoken of, are understood to be of the precise colour of the middle of the green, yellow, and blue parts of the spectrum.) Thus the green waves must be increased in the proportion of 475 to 440 to give yellow light, or reduced in the proportion of 511 to 475 to give blue light. For the first purpose, the velocity of recession must bear to the velocity of light the proportion which 30 bears to 475, or must be equal to rather more than one-sixteenth part of the velocity of light—say 11,600 miles per second. For the second purpose, the velocity of approach must bear to the velocity of light the proportion which 36 bears to 475, or must be nearly equal to one-thirteenth part of the velocity of light—say 14,300 miles per second. But the motions of the stars and other celestial bodies, and also the motions of matter in the sun, and so forth, are very much less than these. Except in the case of one or two comets (and always dismissing from consideration the amazing apparent velocities with which comets’ tails seem to be formed), we may take 100 miles per second as the extreme limit of velocity with which we have to deal, in considering the application of our theory to the motions of recession and approach of celestial bodies. Thus in the case of recession the greatest possible change of colour in pure green light would be equivalent to the difference between the medium green of the spectrum, and the colour 1-116th part of the way from medium green to medium yellow; and in the case of approach, the change would correspond to the difference between the medium green and the colour 1-143rd part of the way from medium green to medium blue. Let any one look at a spectrum of fair length, or even at a correctly tinted painting of the solar spectrum, and note how utterly unrecognizable to ordinary vision is the difference of tint for even the twentieth part of the distance between medium green and medium yellow on one side or medium blue on the other, and he will recognize how utterly hopeless it would be to attempt to appreciate the change of colour due to the approach or recession of a luminous body shining with pure green light and moving at the tremendous rate of 100 miles per second. It would be hopeless, even though we had the medium green colour and the changed colour, either towards yellow or towards blue, placed side by side for comparison—how much more when the changed colour would have to be compared with the observer’s recollection of the medium colour, as seen on some other occasion!

But this is the least important of the difficulties affecting the application of this method by noting change of colour, as Doppler originally proposed. Another difficulty, which seems somehow to have wholly escaped Doppler’s attention, renders the colour test altogether unavailable. We do not get pure light from any of the celestial bodies except certain gaseous clouds or nebulæ. From every sun we get, as from our own sun, all the colours of the rainbow. There may be an excess of some colours and a deficiency of others in any star, so as to give the star a tint, or even a very decided colour. But even a blood-red star, or a deep-blue or violet star, does not shine with pure light, for the spectroscope shows that the star has other colours than those producing the prevailing tint, and it is only the great excess of red rays (all kinds of red, too) or of blue rays (of all kinds), and so on, which makes the star appear red, or blue, and so on, to the eye. By far the greater number of stars or suns show all the colours of the rainbow nearly equally distributed, as in the case of our own sun. Now imagine for a moment a white sun, which had been at rest, to begin suddenly to approach us so rapidly (travelling more than 10,000 miles per second) that the red rays became orange, the orange became yellow, the yellow green, the green blue, the blue indigo, the indigo violet, while the violet waves became too short to affect the sense of sight. Then, if that were all, that sun, being deprived of the red part of its light, would shine with a slightly bluish tinge, owing to the relative superabundance of rays from the violet end of the spectrum. We should be able to recognize such a change, yet not nearly so distinctly as if that sun had been shining with a pure green light, and suddenly beginning to approach us at the enormous rate just mentioned, changed in colour to full blue. Though, if that sun were all the time approaching us at the enormous rate imagined, we should be quite unable to tell whether its slightly bluish tinge were due to such motion of approach or to some inherent blueness in the light emitted by the star. Similarly, if a white sun suddenly began to recede so rapidly that its violet rays were turned to indigo, the indigo to blue, and so on, the orange rays turning to red, and the red rays disappearing altogether, then, if that were all, its light would become slightly reddish, owing to the relative superabundance of light from the red end of the spectrum; and we might distinguish the change, yet not so readily as if a sun shining with pure green light began to recede at the same enormous rate, and so shone with pure yellow light. Though, if that sun were all the time receding at that enormous rate, we should be quite unable to tell whether its slightly reddish hue were due to such motion of recession or to some inherent redness in its own lustre. But in neither case would that be all. In the former, the red rays would indeed become orange; but the rays beyond the red, which produce no effect upon vision, would be converted into red rays, and fill up the part of the spectrum deserted by the rays originally red. In the latter, the violet rays would indeed become indigo; but the rays beyond the violet, ordinarily producing no visible effect, would be converted into violet rays, and fill up the part of the spectrum deserted by the rays originally violet. Thus, despite the enormous velocity of approach in one case and of recession in the other, there would be no change whatever in the colour of the sun in either case. All the colours of the rainbow would still be present in the sun’s light, and it would therefore still be a white sun.

Doppler’s method would thus fail utterly, even though the stars were travelling hither and thither with motions a hundred times greater than the greatest known stellar motions.

This objection to Doppler’s theory, as originally proposed, was considered by me in an article on “Coloured Suns” in Fraser’s Magazine for January, 1868. His theory, indeed, was originally promulgated not as affording a means of measuring stellar motions, but as a way of accounting for the colours of double stars. It was thus presented by Professor Nichol, in a chapter of his “Architecture of the Heavens,” on this special subject:—“The rapid motion of light reaches indeed one of those numbers which reason owns, while imagination ceases to comprehend them; but it is also true that the swiftness with which certain individuals of the double stars sweep past their perihelias, or rather their periasters, is amazing; and in this matter of colours, it must be recollected that the question solely regards the difference between the velocities of the waves constituent of colours, at those different stellar positions. Still it is a bold—even a magnificent idea; and if it can be reconciled with the permanent colours of the multitude of stars surrounding us—stars which too are moving in great orbits with immense velocities—it may be hailed almost as a positive discovery. It must obtain confirmation, or otherwise, so soon as we can compare with certainty the observed colorific changes of separate systems with the known fluctuations of their orbital motions.”

That was written a quarter of a century ago, when spectroscopic analysis, as we now know it, had no existence. Accordingly, while the fatal objection to Doppler’s original theory is overlooked on the one hand, the means of applying the principle underlying the theory, in a much more exact manner than Doppler could have hoped for, is overlooked on the other. Both points are noted in the article above referred to, in the same paragraph. “We may dismiss,” I there stated, “the theory started some years ago by the French astronomer, M. Doppler.” But, I presently added, “It is quite clear that the effects of a motion rapid enough to produce such a change” (i.e. a change of tint in a pure colour) “would shift the position of the whole spectrum—and this change would be readily detected by a reference to the spectral lines.” This is true, even to the word “readily.” Velocities which would produce an appreciable change of tint would produce “readily” detectible changes in the position of the spectral lines; the velocities actually existing among the star-motions would produce changes in the position of these lines detectible only with extreme difficulty, or perhaps in the majority of instances not detectible at all.

It has been in this way that the spectroscopic method has actually been applied.

It is easy to perceive the essential difference between this way of applying the method and that depending on the attempted recognition of changes of colour. A dark line in the spectrum marks in reality the place of a missing tint. The tints next to it on either side are present, but the tint between them is wanting. They are changed in colour—very slightly, in fact quite inappreciably—by motions of recession or approach, or, in other words, they are shifted in position along the spectrum, towards the red end for recession, towards the violet end for approach; and of course the dark space between is shifted along with them. One may say that the missing tint is changed. For in reality that is precisely what would happen. If the light of a star at rest gave every tint of the spectrum, for instance, except mid-green alone, and that star approached or receded so swiftly that its motion would change pure green light to pure yellow in one case, or pure blue in the other, then the effect on the spectrum of such a star would be to throw the dark line from the middle of the green part of the spectrum to the middle of the yellow part in one case, or to the middle of the blue part in the other. The dark line would be quite notably shifted in either case. With the actual stellar motions, though all the lines are more or less shifted, the displacement is always exceedingly minute, and it becomes a task of extreme difficulty to recognize, and still more to measure, such displacement.

When I first indicated publicly (January, 1868) the way in which Doppler’s principle could alone be applied, two physicists, Huggins in England and Secchi in Italy, were actually endeavouring, with the excellent spectroscopes in their possession, to apply this method. In March, 1868, Secchi gave up the effort as useless, publicly announcing the plan on which he had proceeded and his failure to obtain any results except negative ones. A month later Huggins also publicly announced the plan on which he had been working, but was also able to state that in one case, that of the bright star Sirius, he had succeeded in measuring a motion in the line of sight, having discovered that Sirius was receding from the earth at the rate of 41·4 miles per second. I say was receding, because a part of the recession at the time of observation was due to the earth’s orbital motion around the sun. I had, at his request, supplied Huggins with the formula for calculating the correction due to this cause, and, applying it, he found that Sirius is receding from the sun at the rate of about 29½ miles per second, or some 930 millions of miles per annum.

I am not here specially concerned to consider the actual results of the application of this method since the time of Huggins’s first success; but the next chapter of the history of the method is one so interesting to myself personally that I feel tempted briefly to refer to details. So soon as I had heard of Huggins’s success with Sirius, and that an instrument was being prepared for him wherewith he might hope to extend the method to other stars, I ventured to make a prediction as to the result which he would obtain whensoever he should apply it to five stars of the seven forming the so-called Plough. I had found reason to feel assured that these five form a system drifting all together amid stellar space. Satisfied for my own part as to the validity of the evidence, I submitted it to Sir J. Herschel, who was struck by its force. The apparent drift of those stars was, of course, a thwart drift; but if they really were drifting in space, then their motions in the line of sight must of necessity be alike. My prediction, then, was that whensoever Huggins applied to those stars the new method he would find them either all receding at the same rate, or all approaching at the same rate, or else that all alike failed to give any evidence at all either of recession or approach. I had indicated the five in the first edition of my “Other Worlds”—to wit, the stars of the Plough, omitting the nearest “pointer” to the pole and the star marking the third horse (or the tip of the Great Bear’s tail). So soon as Huggins’s new telescope and its spectroscopic adjuncts were in working order, he re-examined Sirius, determined the motions of other stars; and at last on one suitable evening he tested the stars of the Plough. He began with the nearest pointer, and found that star swiftly approaching the earth. He turned to the other pointer, and found it rapidly receding from the earth. Being under the impression that my five included both pointers, he concluded that my prediction had utterly failed, and so went on with his observations, altogether unprejudiced in its favour, to say the least. The next star of the seven he found to be receding at the same rate as the second pointer; the next at the same rate, the next, and the next receding still at the same rate, and lastly the seventh receding at a different rate. Here, then, were five stars all receding at a common rate, and of the other two one receding at a different rate, the other swiftly approaching. Turning next to the work containing my prediction, Huggins found that the five stars thus receding at a common rate were the five whose community of motion I had indicated two years before. Thus the first prediction ever made respecting the motions of the so-called fixed stars was not wanting in success. I would venture to add that the theory of star-drift, on the strength of which the prediction was made, was in effect demonstrated by the result.

The next application of the new method was one of singular interest. I believe it was Mr. Lockyer who first thought of applying the method to measure the rate of solar hurricanes as well as the velocities of the uprush and downrush of vaporous matter in the atmosphere of the sun. Another spectroscopic method had enabled astronomers to watch the rush of glowing matter from the edge of the sun, by observing the coloured flames and their motions; but by the new method it was possible to determine whether the flames at the edge were swept by solar cyclones carrying them from or towards the eye of the terrestrial observer, and also to determine whether glowing vapours over the middle of the visible disc were subject to motion of uprush, which of course would carry them towards the eye, or of downrush, which would carry them from the eye. The result of observations directed to this end was to show that at least during the time when the sun is most spotted, solar hurricanes of tremendous violence take place, while the uprushing and downrushing motions of solar matter sometimes attain a velocity of more than 100 miles per second.

It was this success on the part of an English spectroscopist which caused that attack on the new method against which it has but recently been successfully defended, at least in the eyes of those who are satisfied only by experimental tests of the validity of a process. The Padre Secchi had failed, as we have seen, to recognize motions of recession and approach among the stars by the new method. But he had taken solar observation by spectroscopic methods under his special charge, and therefore when the new results reached his ears he felt bound to confirm or invalidate them. He believed that the apparent displacement of dark lines in the solar spectrum might be due to the heat of the sun causing changes in the delicate adjustments of the instrument—a cause of error against which precautions are certainly very necessary. He satisfied himself that when sufficient precautions are taken no displacements take place such as Lockyer, Young, and others claimed to have seen. But he submitted the matter to a further test. As the sun is spinning swiftly on his axis, his mighty equator, more than two and a half millions of miles in girth, circling once round in about twenty-four days, it is clear that on one side the sun’s surface is swiftly moving towards, and on the other side as swiftly moving from, the observer. By some amazing miscalculation, Secchi made the rate of this motion 20 miles per second, so that the sum of the two motions in opposite directions would equal 40 miles per second. He considered that he ought to be able by the new method, if the new method is trustworthy at all, to recognize this marked difference between the state of the sun’s eastern and western edges; he found on trial that he could not do so; and accordingly he expressed his opinion that the new method is not trustworthy, and that the arguments urged in its favour are invalid.

The weak point in his reasoning resided in the circumstance that the solar equator is only moving at the rate of about 1¼ miles per second, so that instead of a difference of 40 miles per second between the two edges, which should be appreciable, the actual difference (that is, the sum of the two equal motions in opposite directions) amounts only to 2½ miles per second, which certainly Secchi could not hope to recognize with the spectroscopic power at his disposal. Nevertheless, when the error in his reasoning was pointed out, though he admitted that error, he maintained the justice of his conclusion; just as Cassini, having mistakenly reasoned that the degrees of latitude should diminish towards the pole instead of increasing, and having next mistakenly found, as he supposed, that they do diminish, acknowledged the error of his reasoning, but insisted on the validity of his observations,—maintaining thenceforth, as all the world knows, that the earth is extended instead of flattened at the poles.

Huggins tried to recognize by the new method the effects of the sun’s rotation, using a much more powerful spectroscope than Secchi’s. The history of the particular spectroscope he employed is in one respect specially interesting to myself, as the extension of spectroscopic power was of my own devising before I had ever used or even seen a powerful spectroscope. The reader is aware that spectroscopes derive their light-sifting power from the prisms forming them. The number of prisms was gradually increased, from Newton’s single prism to Fraunhofer’s pair, and to Kirchhoff’s battery of four, till six were used, which bent the light round as far as it would go. Then the idea occurred of carrying the light to a higher level (by reflections) and sending it back through the same battery of prisms, doubling the dispersion. Such a battery, if of six prisms, would spread the spectral colours twice as widely apart as six used in the ordinary way, and would thus have a dispersive power of twelve prisms. It occurred to me that after taking the rays through six prisms, arranged in a curve like the letter C, an intermediate four-cornered prism of a particular shape (which I determined) might be made to send the rays into another battery of six prisms, the entire set forming a double curve like the letter S, the rays being then carried to a higher level and back through the double battery. In this way a dispersive power of nineteen prisms could be secured. My friend, Mr. Browning, the eminent optician, made a double battery of this kind,13 which was purchased by Mr. W. Spottiswoode, and by him lent to Mr. Huggins for the express purpose of dealing with the task Secchi had set spectroscopists. It did not, however, afford the required evidence. Huggins considered the displacement of dark lines due to the sun’s rotation to be recognizable, but so barely that he could not speak confidently on the point.

There for a while the matter rested. Vögel made observations confirming Huggins’s results relative to stellar motions; but Vögel’s instrumental means were not sufficiently powerful to render his results of much weight.

But recently two well-directed attacks have been made upon this problem, one in England, the other in America, and in both cases with success. Rather, perhaps, seeing that the method had been attacked and was supposed to require defence, we may say that two well-directed assaults have been made upon the attacking party, which has been completely routed.

Arrangements were made not very long ago, by which the astronomical work of Greenwich Observatory, for a long time directed almost exclusively to time observations, should include the study of the sun, stars, planets, and so forth. Amongst other work which was considered suited to the National Observatory was the application of spectroscopic analysis to determine motions of recession and approach among the celestial bodies. Some of these observations, by the way, were made, we are told, “to test the truth of Doppler’s principle,” though it seems difficult to suppose for an instant that mathematicians so skilful as the chief of the Observatory and some of his assistants could entertain any doubt on that point. Probably it was intended by the words just quoted to imply simply that some of the observations were made for the purpose of illustrating the principle of the method. We are not to suppose that on a point so simple the Greenwich observers have been in any sort of doubt.

At first their results were not very satisfactory. The difficulties which had for a long time foiled Huggins, and which Secchi was never able to master, rendered the first Greenwich measures of stellar motions in the line of sight wildly inconsistent, not only with Huggins’s results, but with each other.

Secchi was not slow to note this. He renewed his objections to the new method of observation, pointing and illustrating them by referring to the discrepancies among the Greenwich results. But recently a fresh series of results has been published, showing that the observers at Greenwich have succeeded in mastering some at least among the difficulties which they had before experienced. The measurements of star-motions showed now a satisfactory agreement with Huggins’s results, and their range of divergence among themselves was greatly reduced. The chief interest of the new results, however, lay in the observations made upon bodies known to be in motion in the line of sight at rates already measured. These observations, though not wanted as tests of the accuracy of the principle, were very necessary as tests of the qualities of the instruments used in applying it. It is here and thus that Secchi’s objections alone required to be met, and here and thus they have been thoroughly disposed of. Let us consider what means exist within the solar system for thus testing the new method.

The earth travels along in her orbit at the rate of about 18⅓ miles in every second of time. Not to enter into niceties which could only properly be dealt with mathematically, it may be said that with this full velocity she is at times approaching the remoter planets of the system, and at times receding from them; so that here at once is a range of difference amounting to about 37 miles per second, and fairly within the power of the new method of observation. For it matters nothing, so far as the new method is concerned, whether the earth is approaching another orb by her motion, or that orb approaching by its own motion. Again, the plant Venus travels at the rate of about 21½ miles per second, but as the earth travels only 3 miles a second less swiftly, and the same way round, only a small portion of Venus’s motion ever appears as a motion of approach towards or recession from the earth. Still, Venus is sometimes approaching and sometimes receding from the earth, at a rate of more than 8 miles per second. Her light is much brighter than that of Jupiter or Saturn, and accordingly this smaller rate of motion would be probably more easily recognized than the greater rate at which the giant planets are sometimes approaching and at other times receding from the earth. At least, the Greenwich observers seem to have confined their attention to Venus, so far as motions of planets in the line of sight are concerned. The moon, as a body which keeps always at nearly the same distance from us, would of course be the last in the world to be selected to give positive evidence in favour of the new method; but she serves to afford a useful test of the qualities of the instruments employed. If when these were applied to her they gave evidence of motions of recession or approach at the rate of several miles per second, when we know as a matter of fact that the moon’s distance never14 varies by more than 30,000 miles during the lunar month, her rate of approach or recession thus averaging about one-fiftieth part of a mile per second, discredit would be thrown on the new method—not, indeed, as regards its principle, which no competent reasoner can for a moment question, but as regards the possibility of practically applying it with our present instrumental means.

Observations have been made at Greenwich, both on Venus and on the moon, by the new method, with results entirely satisfactory. The method shows that Venus is receding when she is known to be receding, and that she is approaching when she is known to be approaching. Again, the method shows no signs of approach or recession in the moon’s case. It is thus in satisfactory agreement with the known facts. Of course these results are open to the objection that the observers have known beforehand what to expect, and that expectation often deceives the mind, especially in cases where the thing to be observed is not at all easy to recognize. It will presently be seen that the new method has been more satisfactorily tested, in this respect, in other ways. It may be partly due to the effect of expectation that in the case of Venus the motions of approach and recession, tested by the new method, have always been somewhat too great. A part of the excess may be due to the use of the measure of the sun’s distance, and therefore the measures of the dimensions of the solar system, in vogue before the recent transit. These measures fall short to some degree of those which result from the observations made in December, 1874, on Venus in transit, the sun’s distance being estimated at about 91,400,000 miles instead of 92,000,000 miles, which would seem to be nearer the real distance. Of course all the motions within the solar system would be correspondingly under-estimated. On the other hand, the new method would give all velocities with absolute correctness if instrumental difficulties could be overcome. The difference between the real velocities of Venus approaching and receding, and those calculated according to the present inexact estimate of the sun’s distance, is however much less than the observed discrepancy, doubtless due to the difficulties involved in the application of this most difficult method. I note the point, chiefly for the sake of mentioning the circumstance that theoretically the method affords a new means of measuring the dimensions of the solar system. Whensoever the practical application of the method has been so far improved that the rate of approach or recession of Venus, or Mercury, or Jupiter, or Saturn (any one of these planets), can be determined on any occasion, with great nicety, we can at once infer the sun’s distance with corresponding exactness. Considering that the method has only been invented ten years (setting aside Doppler’s first vague ideas respecting it), and that spectroscopic analysis as a method of exact observation is as yet little more than a quarter of a century old, we may fairly hope that in the years to come the new method, already successfully applied to measure motions of recession and approach at the rate of 20 or 30 miles per second, will be employed successfully in measuring much smaller velocities. Then will it give us a new method of measuring the great base-line of astronomical surveying—the distance of our world from the centre of the solar system.

That this will one day happen is rendered highly probable, in my opinion, by the successes next to be related.

Besides the motions of the planets around the sun, there are their motions of rotation, and the rotation of the sun himself upon his axis. Some among these turning motions are sufficiently rapid to be dealt with by the new method. The most rapid rotational motion with which we are acquainted from actual observation is that of the planet Jupiter. The circuit of his equator amounts to about 267,000 miles, and he turns once on his axis in a few minutes less than ten hours, so that his equatorial surface travels at the rate of about 26,700 miles an hour, or nearly 7½ miles per second. Thus between the advancing and retreating sides of the equator there is a difference of motion in the line of sight amounting to nearly 15 miles. But this is not all. Jupiter shines by reflecting sunlight. Now it is easily seen that where his turning equator meets the waves of light from the sun, these are shortened, in the same sense that waves are shortened for a swimmer travelling to meet them, while these waves, already shortened in this way, are further shortened when starting from the same advancing surface of Jupiter, on their journey to us after reflection. In this way the shortening of the waves is doubled, at least when the earth is so placed that Jupiter lies in the same direction from us as from the sun, the very time, in fact, when Jupiter is most favourably placed for ordinary observation, or is at his highest due south, when the sun is at his lowest below the northern horizon—that is, at midnight. The lengthening of the waves is similarly doubled at this most favourable time for observation; and the actual difference between the motion of the two sides of Jupiter’s equator being nearly 15 miles per second, the effect on the light-waves is equivalent to that due to a difference of nearly 30 miles per second. Thus the new method may fairly be expected to indicate Jupiter’s motion of rotation. The Greenwich observers have succeeded in applying it, though Jupiter has not been favourably situated for observation. Only on one occasion, says Sir G. Airy, was the spectrum of Jupiter “seen fairly well,” and on that occasion “measures were obtained which gave a result in remarkable agreement with the calculated value.” It may well be hoped that when in the course of a few years Jupiter returns to that part of his course where he rises high above the horizon, shining more brightly and through a less perturbed air, the new method will be still more successfully applied. We may even hope to see it extended to Saturn, not merely to confirm the measures already made of Saturn’s rotation, but to resolve the doubts which exist as to the rotation of Saturn’s ring-system.

Lastly, there remains the rotation of the sun, a movement much more difficult to detect by the new method, because the actual rate of motion even at the sun’s equator amounts only to about 1 mile per second.

In dealing with this very difficult task, the hardest which spectroscopists have yet attempted, the Greenwich observers have achieved an undoubted success; but unfortunately for them, though fortunately for science, another observatory, far smaller and of much less celebrity, has at the critical moment achieved success still more complete.

The astronomers at our National Observatory have been able to recognize by the new method the turning motion of the sun upon his axis. And here we have not, as in the case of Venus, to record merely that the observers have seen what they expected to see because of the known motion of the sun. “Particular care was taken,” says Airy, “to avoid any bias from previous knowledge of the direction in which a displacement” (of the spectral lines) “was to be expected,” the side of the sun under observation not being known by the observer until after the observation was completed.

But Professor Young, at Dartmouth College, Hanover, N.H., has done much more than merely obtain evidence by the new method that the sun is rotating as we already knew. He has succeeded so perfectly in mastering the instrumental and observational difficulties, as absolutely to be able to rely on his measurement (as distinguished from the mere recognition) of the sun’s motion of rotation. The manner in which he has extended the powers of ordinary spectroscopic analysis, cannot very readily be described in these pages, simply because the principles on which the extension depends require for their complete description a reference to mathematical considerations of some complexity. Let it be simply noted that what is called the diffraction spectrum, obtained by using a finely lined plate, results from the dispersive action of such a plate, or grating as it is technically called, and this dispersive power can be readily combined with that of a spectroscope of the ordinary kind. Now Dr. Rutherfurd, of New York, has succeeded in ruling so many thousand lines on glass within the breadth of a single inch as to produce a grating of high dispersive power. Availing himself of this beautiful extension of spectroscopic powers, Professor Young has succeeded in recognizing effects of much smaller motions of recession and approach than had before been observable by the new method. He has thus been able to measure the rotation-rate of the sun’s equatorial regions. His result exceeds considerably that inferred from the telescopic observation of the solar spots. For whereas from the motion of the spots a rotation-rate of about 1¼ mile per second has been calculated for the sun’s equator, Professor Young obtains from his spectroscopic observations a rate of rather more than 1⅖ mile, or about 300 yards per second more than the telescopic rate.

If Young had been measuring the motion of the same matter which is observed with the telescope, there could of course be no doubt that the telescope was right and the spectroscope wrong. We might add a few yards per second for the probably greater distance of the sun resulting from recent transit observations. For of course with an increase in our estimate of the sun’s distance there comes an increase in our estimate of the sun’s dimensions, and of the velocity of the rotational motion of his surface. But only about 12 yards per second could be allowed on this account; the rest would have to be regarded as an error due to the difficulties involved in the spectroscopic method. In reality, however, the telescopist and the spectroscopist observe different things in determining by their respective methods the sun’s motion of rotation. The former observes the motion of the spots belonging to the sun’s visible surface; the latter observes the motion of the glowing vapours outside that surface, for it is from these vapours, not from the surface of the sun, that the dark lines of the spectrum proceed. Now so confident is Professor Young of the accuracy of his spectroscopic observations, that he is prepared to regard the seeming difference of velocity between the atmosphere and surface of the sun as real. He believes that “the solar atmosphere really sweeps forward over the underlying surface, in the same way that the equatorial regions outstrip the other parts of the sun’s surface.” This inference, important and interesting in itself, is far more important in what it involves. For if we can accept it, it follows that the spectroscopic method of measuring the velocity of motions in the line of sight is competent, under favourable conditions, to obtain results accurate within a few hundred yards per second, or 10 or 12 miles per minute. If this shall really prove to be true for the method now, less than ten years after it was first successfully applied, what may we not hope from the method in future years? Spectroscopic analysis itself is in its infancy, and this method is but a recent application of spectroscopy. A century or so hence astronomers will smile (though not disdainfully) at these feeble efforts, much as we smile now in contemplating the puny telescopes with which Galileo and his contemporaries studied the star-depths. And we may well believe that largely as the knowledge gained by telescopists in our own time surpasses that which Galileo obtained, so will spectroscopists a few generations hence have gained a far wider and deeper insight into the constitution and movements of the stellar universe than the spectroscopists of our own day dare even hope to attain.

I venture confidently to predict that, in that day, astronomers will recognize in the universe of stars a variety of structure, a complexity of arrangement, an abundance of every form of cosmical vitality, such as I have been led by other considerations to suggest, not the mere cloven lamina of uniformly scattered stars more or less resembling our sun, and all in nearly the same stage of cosmical development, which the books of astronomy not many years since agreed in describing. The history of astronomical progress does not render it probable that the reasoning already advanced, though in reality demonstrative, will convince the generality of science students until direct and easily understood observations have shown the real nature of the constitution of that part of the universe over which astronomical survey extends. But the evidence already obtained, though its thorough analysis may be “caviare to the general,” suffices to show the real nature of the relations which one day will come within the direct scope of astronomical observation.

THE NEW STAR WHICH FADED INTO STAR-MIST.

The appearance of a new star in the constellation of the Swan in the autumn of 1876 promises to throw even more light than was expected on some of the most interesting problems with which modern astronomy has to deal. It was justly regarded as a circumstance of extreme interest that so soon after the outburst of the star which formed a new gem in the Northern Crown in May, 1866, another should have shone forth under seemingly similar conditions. And when, as time went on, it appeared that in several respects the new star in the Swan differed from the new star in the Crown, astronomers found fresh interest in studying, as closely as possible, the changes presented by the former as it gradually faded from view. But they were not prepared to expect what has actually taken place, or to recognize so great a difference of character between these two new stars, that whereas one seemed throughout its visibility to ordinary eyesight, and even until the present time, to be justly called a star, the other should so change as to render it extremely doubtful whether at any time it deserved to be regarded as a star or sun.

Few astronomical phenomena, even of those observed during this century (so fruitful in great astronomical discoveries), seem better worthy of thorough investigation and study than those presented by the two stars which appeared in the Crown and in the Swan, in 1866 and 1876 respectively. A new era seems indeed to be beginning for those departments of astronomy which deal with stars and star-cloudlets on the one hand, and with the evolution of solar systems and stellar systems on the other.

Let us briefly consider the history of the star of 1866 in the first place, and then turn our thoughts to the more surprising and probably more instructive history of the star which shone out in November, 1876.

In the first place, however, I would desire to make a few remarks on the objections which have been expressed by an observer to whom astronomy is indebted for very useful work, against the endeavour to interpret the facts ascertained respecting these so-called new stars. M. Cornu, who made some among the earliest spectroscopic observations of the star in Cygnus, after describing his results, proceeded as follows:—“Grand and seductive though the task may be of endeavouring to draw from observed facts inductions respecting the physical state of this new star, respecting its temperature, and the chemical reactions of which it may be the scene, I shall abstain from all commentary and all hypothesis on this subject. I think that we do not yet possess the data necessary for arriving at useful conclusions, or at least at conclusions capable of being tested: however attractive hypotheses may be, we must not forget that they are outside the bounds of science, and that, far from serving science, they seriously endanger its progress.” This, as I ventured to point out at the time, is utterly inconsistent with all experience. M. Cornu’s objection to theorizing when he did not see his way to theorizing justly, is sound enough; but his general objection to theorizing is, with all deference be it said, sheerly absurd. It will be noticed that I say theorizing, not hypothesis-framing; for though he speaks of hypotheses, he in reality is describing theories. The word hypothesis is too frequently used in this incorrect sense—perhaps so frequently that we may almost prefer sanctioning the use to substituting the correct word. But the fact really is, that many, even among scientific writers, when they hear the word hypothesis, think immediately of Newton’s famous “hypotheses non fingo,” a dictum relating to real hypotheses, not to theories. It would, in fact, be absurd to suppose that Newton, who had advanced, advocated, and eventually established, the noblest scientific theory the world has known, would ever have expressed an objection to theorizing, as he is commonly understood to have done by those who interpret his “hypotheses non fingo” in the sense which finds favour with M. Cornu. But apart from this, Newton definitely indicates what he means by hypotheses. “I frame no hypotheses,” he says, “for whatever is not deduced from phenomena is to be called an hypothesis.” M. Cornu, it will be seen, rejects the idea of deducing from phenomena what he calls an hypothesis, but what would not be an hypothesis according to Newton’s definition: “Malgré tout ce qu’il y aurait de séduisant et de grandiose à tirer de ce fait des inductions, etc., je m’abstiendrai de tout commentaire et de toute hypothèse à ce sujet.” It is not thus that observed scientific facts are to be made fruitful, nor thus that the points to which closer attention must be given are to be ascertained.

Since the preceding paragraph was written, my attention has been attracted to the words of another observer more experienced than M. Cornu, who has not only expressed the same opinion which I entertain respecting M. Cornu’s ill-advised remark, but has illustrated in a very practical way, and in this very case, how science gains from commentary and theory upon observed facts. Herr Vögel considers “that the fear that an hypothesis” (he, also, means a theory here) “might do harm to science is only justifiable in very rare cases: in most cases it will further science. In the first place, it draws the attention of the observer to things which but for the hypothesis might have been neglected. Of course if the observer is so strongly influenced that in favour of an hypothesis he sees things which do not exist—and this may happen sometimes—science may for a while be arrested in its progress, but in that case the observer is far more to blame than the author of the hypothesis. On the other hand, it is very possible that an observer may, involuntarily, arrest the progress of science, even without originating an hypothesis, by pronouncing and publishing sentences which have a tendency to diminish the general interest in a question, and which do not place its high significance in the proper light.” (This is very neatly put.) He is “almost inclined to think that such an effect might follow from the reading of M. Cornu’s remark, and that nowhere better than in the present case, where in short periods colossal changes showed themselves occurring upon a heavenly body, might the necessary data be obtained for drawing useful conclusions, and tests be applied to those hypotheses which have been ventured with regard to the condition of heavenly bodies.” It was, as we shall presently see, in thus collecting data and applying tests, that Vögel practically illustrated the justice of his views.

The star which shone out in the Northern Crown in May, 1866, would seem to have grown to its full brightness very quickly. It is not necessary that I should here consider the history of the star’s discovery; but I think all who have examined that history agree in considering that whereas on the evening of May 12, 1866, a new star was shining in the Northern Crown with second-magnitude brightness, none had been visible in the same spot with brightness above that of a fifth-magnitude star twenty-four hours earlier. On ascertaining, however, the place of the new star, astronomers found that there had been recorded in Argelander’s charts and catalogue a star of between the ninth and tenth magnitude in this spot. The star declined very rapidly in brightness. On May 13th it appeared of the third magnitude; on May 16th it had sunk to the fourth magnitude; on the 17th to the fifth; on the 19th to the seventh; and by the end of the month it shone only as a telescopic star of the ninth magnitude. It is now certainly not above the tenth magnitude.

Examined with the spectroscope, this star was found to be in an abnormal condition. It gave the rainbow-tinted streak crossed by dark lines, which is usually given by stars (with minor variations, which enable astronomers to classify the stars into several distinct orders). But superposed upon this spectrum, or perhaps we should rather say shining through this spectrum, were seen four brilliant lines, two of which certainly belonged to glowing hydrogen. These lines were so bright as to show that the greater part of the light of the star at the time came from the glowing gas or gases giving these lines. It appeared, however, that the rainbow-tinted spectrum on which these lines were seen was considerably brighter than it would otherwise have been, in consequence of the accession of heat indicated by and probably derived from the glowing hydrogen.

Unfortunately, we have not accordant accounts of the changes which the spectrum of this star underwent as the star faded out of view. Wolf and Rayet, of the Paris Observatory, assert that when there remained scarcely any trace of the continuous spectrum, the four bright lines were still quite brilliant. But Huggins affirms that this was not the case in his observations; he was “able to see the continuous spectrum when the bright lines could be scarcely distinguished.” As the bright lines certainly faded out of view eventually, we may reasonably assume that the French observers were prevented by the brightness of the lines from recognizing the continuous spectrum at that particular stage of the diminution of the star’s light when the continuous spectrum had faded considerably but the hydrogen lines little. Later, the continuous spectrum ceased to diminish in brightness, while the hydrogen lines rapidly faded. Thereafter the continuous spectrum could be discerned, and with greater and greater distinctness as the hydrogen lines faded out.

Now, in considering the meaning of the observed changes in the so-called “new star,” we have two general theories to consider.

One of these theories is that to which Dr. Huggins would seem to have inclined, though he did not definitely adopt it—the theory, namely, that in consequence of some internal convulsion enormous quantities of hydrogen and other gases were evolved, which in combining with some other elements ignited on the surface of the star, and thus enveloped the whole body suddenly in a sheet of flame.

“The ignited hydrogen gas in burning produced the light corresponding to the two bright bands in the red and green; the remaining bright lines were not, however, coincident with those of oxygen, as might have been expected. According to this theory, the burning hydrogen must have greatly increased the heat of the solid matter of the photosphere and brought it into a state of more intense incandescence and luminosity, which may explain how the formerly faint star could so suddenly assume such remarkable brilliance; the liberated hydrogen became exhausted, the flame gradually abated, and with the consequent cooling the photosphere became less vivid, and the star returned to its original condition.”

According to the other theory, advanced by Meyer and Klein, the blazing forth of this new star may have been occasioned by the violent precipitation of some great mass, perhaps a planet, upon a fixed star, “by which the momentum of the falling mass would be changed into molecular motion,” and result in the emission of light and heat.

“It might even be supposed that the new star, through its rapid motion, may have come in contact with one of the nebulæ which traverse in great numbers the realms of space in every direction, and which from their gaseous condition must possess a high temperature; such a collision would necessarily set the star in a blaze, and occasion the most vehement ignition of its hydrogen.”

If we regard these two theories in their more general aspect, considering one as the theory that the origin of disturbance was within the star, and the other as the theory that the origin of disturbance was outside the star, they seem to include all possible interpretations of the observed phenomena. But, as actually advanced, neither seems satisfactory. The sudden pouring forth of hydrogen from the interior, in quantities sufficient to explain the outburst, seems altogether improbable. On the other hand, as I have pointed out elsewhere, there are reasons for rejecting the theory that the cause of the heat which suddenly affected this star was either the downfall of a planet on the star or the collision of the star with a star-cloudlet or nebula, traversing space in one direction, while the star rushed onwards in another.

A planet could not very well come into final conflict with its sun at one fell swoop. It would gradually draw nearer and nearer, not by the narrowing of its path, but by the change of the path’s shape. The path would, in fact, become more and more eccentric; until at length, at its point of nearest approach, the planet would graze its primary, exciting an intense heat where it struck, but escaping actual destruction that time. The planet would make another circuit, and again graze the sun, at or near the same part of the planet’s path. For several circuits this would continue, the grazes not becoming more and more effective each time, but rather less. The interval between them, however, would grow continually less and less; at last the time would come when the planet’s path would be reduced to the circular form, its globe touching the sun’s all the way round, and then the planet would very quickly be reduced to vapour and partly burned up, its substance being absorbed by its sun. But all successive grazes would be indicated to us by accessions of lustre, the period between each seeming outburst being only a few months at first, and gradually becoming less and less (during a long course of years, perhaps even of centuries) until the planet was finally destroyed. Nothing of this sort has happened in the case of any so-called new star. As for the rush of a star through a nebulous mass, that is a theory which would scarcely be entertained by any one acquainted with the enormous distances separating the gaseous star-clouds properly called nebulæ. There may be small clouds of the same sort scattered much more freely through space; but we have not a particle of evidence that this is actually the case. All we certainly know about star-cloudlets suggests that the distances separating them from each other are comparable with those which separate star from star, in which case the idea of a star coming into collision with a star-cloudlet, and still more the idea of this occurring several times in a century, is wild in the extreme.

But while thus advancing objections, which seem to me irrefragable, against the theory that either a planet or a nebula (still less another small star) had come into collision with the orb in Corona which shone out so splendidly for a while, I advanced another view which seemed to me then and seems now to correspond well with phenomena, and to render the theory of action from without on the whole preferable to the theory of outburst from within. I suggested that, far more probably, an enormous flight of large meteoric masses travelling around the star had come into partial collision with it in the same way that the flight of November meteors comes into collision with our earth thrice in each century, and that other meteoric flights may occasionally come into collision with our sun, producing the disturbances which occasion the sun-spots. As I pointed out, in conceiving this we are imagining nothing new. A meteoric flight capable of producing the suggested effects would differ only in kind from meteoric flights which are known to circle around our own sun. The meteors which produce the November displays of falling stars follow in the track of a comet barely visible to the naked eye.

“May we not reasonably assume that those glorious comets which have not only been visible but conspicuous, shining even in the day-time, and brandishing around tails, which like that of the ‘wonder in heaven, the great dragon,’ seemed to ‘draw the third part of the stars of heaven,’ are followed by much denser flights of much more massive meteors? Some of these giant comets have paths which carry them very close to our sun. Newton’s comet, with its tail a hundred millions of miles in length, all but grazed the sun’s globe. The comet of 1843, whose tail, says Sir John Herschel, ‘stretched half-way across the sky,’ must actually have grazed the sun, though but lightly, for its nucleus was within 80,000 miles of his surface, and its head was more than 160,000 miles in diameter. And these are only two among the few comets whose paths are known. At any time we might be visited by a comet mightier than either, travelling in an orbit intersecting the sun’s surface, followed by flights of meteoric masses enormous in size and many in number, which, falling on the sun’s globe with enormous velocity corresponding to their vast orbital range and their near approach to the sun—a velocity of some 360 miles per second—would, beyond all doubt, excite his whole frame, and especially his surface regions, to a degree of heat far exceeding what he now emits.”

This theory corresponds far better also with observed facts than the theory of Meyer and Klein, in other respects than simply in antecedent probability. It can easily be shown that if a planet fell upon a sun in such sort as to become part of his mass, or if a nebula in a state of intense heat excited the whole frame of a star to a similar degree of heat, the effects would be of longer duration than the observed accession of heat and light in the case of all the so-called “new stars.” It has been calculated by Mr. Croll (the well-known mathematician to whom we owe the most complete investigations yet made into the effect of the varying eccentricity of the earth’s orbit on the climate of the earth) that if two suns, each equal in mass to one-half of our sun, came into collision with a velocity of 476 miles per second, light and heat would be produced which would cover the present rate of the sun’s radiation for fifty million years. Now although it certainly does not follow from this that such a collision would result in the steady emission of so much light and heat as our sun gives out, for a period of fifty million years, but is, on the contrary, certain that there would be a far greater emission at first and a far smaller emission afterwards, yet it manifestly must be admitted that such a collision could not possibly produce so short-lived an effect as we see in the case of every one of the so-called new stars. The diminution in the emission of light and heat from the maximum to one-half the maximum would not occupy fifty millions of years, or perhaps even five million or five hundred thousand years; but it would certainly require thousands of years; whereas we have seen that the new stars in the Crown and in the Swan have lost not one-half but ninety-nine hundredths of their maximum lustre in a few months.

This has been urged as an objection even to the term star as applied to these suddenly appearing orbs. But the objection is not valid; because there is no reason whatever for supposing that even our own sun might not be excited by the downfall of meteoric or cometic matter upon it to a sudden and short-lasting intensity of splendour and of heat. Mr. Lockyer remarks that, if any star, properly so called, were to become a “a world on fire,” or “burst into flames,” or, in less poetical language, were to be driven either into a condition of incandescence absolutely, or to have its incandescence increased, there can be little doubt that thousands or millions of years would be necessary for the reduction of its light to its original intensity. This must, however, have been written in forgetfulness of some facts which have been ascertained respecting our sun, and which indicate pretty clearly that the sun’s surface might be roused to a temporary intensity of splendour and heat without any corresponding increase in the internal heat, or in the activity of the causes, whatever they may be, to which the sun’s steady emissions of light and heat are due.

For instance, most of my readers are doubtless familiar with the account (an oft-told tale, at any rate) of the sudden increase in the splendour of a small portion of the sun’s surface on September 1, 1859, observed by two astronomers independently. The appearances described corresponded exactly with what we should expect if two large meteoric masses travelling side by side had rushed, with a velocity originally amounting to two or three hundred miles per second, through the portions of the solar atmosphere lying just above, at, and just below the visible photosphere. The actual rate of motion was measured at 120 miles per second as the minimum, but may, if the direction of motion was considerably inclined to the line of sight, have amounted to more than 200 miles per second. The effect was such, that the parts of the sun thus suddenly excited to an increased emission of light and heat appeared like bright stars upon the background of the glowing photosphere itself. One of the observers, Carrington, supposed for a moment that the dark glass screen used to protect the eye had broken. The increase of splendour was exceedingly limited in area, and lasted only for a few minutes—fortunately for the inhabitants of earth. As it was, the whole frame of the earth sympathized with the sun. Vivid auroras were seen, not only in both hemispheres, but in latitudes where auroras are seldom seen. They were accompanied by unusually great electro-magnetic disturbances.

“In many places,” says Sir J. Herschel, “the telegraph wires struck work. At Washington and Philadelphia, the electric signalmen received severe electric shocks. At a station in Norway, the telegraphic apparatus was set fire to, and at Boston, in North America, a flame of fire followed the pen of Bain’s electric telegraph, which writes down the message upon chemically prepared paper.”

We see, then, that most certainly the sun can be locally excited to increased emission of light and heat, which nevertheless may last but for a very short time; and we have good reason for believing that the actual cause of the sudden change in his condition was the downfall of meteoric matter upon a portion of his surface. We may well believe that, whatever the cause may have been, it was one which might in the case of other suns, or even in our sun’s own case, affect a much larger portion of the photosphere. If this happened there would be just such an accession of splendour as we recognize in the case of the new stars. And as the small local accession of brilliancy lasted only a few minutes, we can well believe that an increase of surface brilliancy affecting a much larger portion of the photosphere, or even the entire photosphere, might last but for a few days or weeks.

All that can be said in the way of negative evidence, so far as our own sun is concerned, is that we have no reason for believing that our sun has, at any time within many thousands of years, been excited to emit even for a few hours a much greater amount of light and heat than usual; so that it has afforded no direct evidence in favour of the belief that other suns may be roused to many times their normal splendour, and yet very quickly resume that usual lustre. But we know that our sun, whether because of his situation in space, or of his position in time (that is, the stage of solar development to which he has at present attained), belongs to the class of stars which shine with steady lustre. He does not vary like Betelgeux, for example, which is not only a sun like him as to general character, but notably a larger and more massive orb. Still less is he like Mira, the Wonderful Star; or like that more wonderful variable star, Eta Argûs, which at one time shines with a lustre nearly equalling that of the bright Sirius, and anon fades away almost into utter invisibility. He is a variable sun, for we cannot suppose that the waxing and waning of the sun-spot period leaves his lustre, as a whole, altogether unaffected. But his variation is so slight that, with all ordinary methods of photometric measurement by observers stationed on worlds which circle around other suns, it must be absolutely undiscernible. We do not, however, reject Betelgeux, or Mira, or even Eta Argûs, from among stars because they vary in lustre. We recognize the fact that, as in glory, so in condition and in changes of condition, one star differeth from another.

Doubtless there are excellent reasons for rejecting the theory that a massive body like a planet, or a nebulous mass like those which are found among the star-depths (the least of which would exceed many times in volume a sphere filling the entire space of the orbit of Neptune), fell on some remote sun in the Northern Crown. But there are no sufficient reasons for rejecting or even doubting the theory that a comet, bearing in its train a flight of many millions of meteoric masses, falling directly upon such a sun, might cause it to shine with many times its ordinary lustre, but only for a short time, a few months or weeks, or a few days, or even hours. In the article entitled “Suns in Flames,” in my “Myths and Marvels of Astronomy,” before the startling evidence recently obtained from the star in Cygnus had been thought of, I thus indicated the probable effects of such an event:—“When the earth has passed through the richer portions (not the actual nuclei be it remembered) of meteor systems, the meteors visible from even a single station have been counted by tens of thousands, and it has been computed that millions must have fallen upon the whole earth. These were meteors following in the trains of very small comets. If a very large comet followed by no denser a flight of meteors, but each meteoric mass much larger, fell directly upon the sun, it would not be the outskirts but the nucleus of the meteoric train which would impinge upon him. They would number thousands of millions. The velocity of downfall of each mass would be more than 360 miles per second. And they would continue to pour in upon him for several days in succession, millions falling every hour. It seems not improbable that under this tremendous and long-continued meteoric hail, his whole surface would be caused to glow as intensely as that small part whose brilliancy was so surprising in the observation made by Carrington and Hodgson. In that case our sun, seen from some remote star whence ordinarily he is invisible, would shine out as a new sun for a few days, while all things living, on our earth and whatever other members of the solar system are the abodes of life, would inevitably be destroyed.”

There are, indeed, reasons for believing, not only, as I have already indicated, that the outburst in the sun was caused by the downfall of meteoric masses, but that those masses were following in the train of a known comet, precisely as the November meteors follow in the train of Tempel’s comet (II., 1866). For we know that November meteoric displays have been witnessed for five or six years after the passage of Tempel’s comet, in its thirty-three year orbit, while the August meteoric displays have been witnessed fully one hundred and twenty years after the passage of their comet (II., 1862).15 Now only sixteen years before the solar outburst witnessed by Carrington and Hodgson, a magnificent comet had passed even closer to the sun than either Tempel’s comet or the second comet of 1862 approached the earth’s orbit. That was the famous comet of the year 1843. Many of us remember that wonderful object. I was but a child myself when it appeared, but I can well remember its amazing tail, which in March, 1843, stretched half-way across the sky.

“Of all the comets on record,” says Sir J. Herschel, “that approached nearest the sun; indeed, it was at first supposed that it had actually grazed the sun’s surface, but it proved to have just missed by an interval of not more than 80,000 miles—about a third of the distance of the moon from the earth, which (in such a matter) is a very close shave indeed to get clear off.”

We can well believe that the two meteors which produced the remarkable outburst of 1859 may have been stragglers from the main body following after that glorious comet. I do not insist upon the connection. In fact, I rather incline to the belief that the disturbance in 1859, occurring as it did about the time of maximum sun-spot frequency, was caused by meteors following in the train of some as yet undiscovered comet, circuiting the sun in about eleven years, the spots themselves being, I believe, due in the main to meteoric downfalls. There is greater reason for believing that the great sun-spot which appeared in June, 1843, was caused by the comet which three months before had grazed the sun’s surface. As Professor Kirkwood, of Bloomington, Indiana, justly remarks, had this comet approached a little nearer, the resistance of the solar atmosphere would probably have brought the comet’s entire mass to the solar surface. Even at its actual distance, it must have produced considerable atmospheric disturbance. But the recent discovery that a number of comets are associated with meteoric matter travelling in nearly the same orbits, suggests the inquiry whether an enormous meteorite following in the comet’s train, and having a somewhat less perihelion distance, may not have been precipitated upon the sun, thus producing the great disturbance observed so shortly after the comet’s perihelion passage.

Let us consider now the evidence obtained from the star in Cygnus, noting especially in what points it resembles, and in what points it differs from, the evidence afforded by the star in the Crown.

The new star was first seen by Professor Schmidt at a quarter to six on the evening of November 24. It was then shining as a star of the third magnitude, in the constellation of the Swan, not very far from the famous but faint star 61 Cygni—which first of all the stars in the northern heavens had its distance determined by astronomers. The three previous nights had unfortunately been dark; but Schmidt is certain that on November 20 the star was not visible. At midnight, November 24, its light was very yellow, and it was somewhat brighter than the well-known star Eta Pegasi, which marks the forearm of the Flying Horse. Schmidt sent news of the discovery to Leverrier, at Paris; but neither he nor Leverrier telegraphed the news, as they should have done, to Greenwich, Berlin, or the United States. Many precious opportunities for observing the spectrum of the new-comer at the time of its greatest brilliancy were thus lost.

The observers at Paris did their best to observe the spectrum of the star and the all-important changes in the spectrum. But they had unfavourable weather. It was not till December 2 that the star was observed at Paris, by which time the colour, which had been very yellow on November 24, had become “greenish, almost blue.” The star had also then sunk from the third to far below the fourth magnitude. It is seldom that science has to regret a more important loss of opportunity than this. What we want specially to know is the nature of the spectrum given by this star when its light was yellow; and this we can now never know. Nor are the outbursts of new stars so common that we may quickly expect another similar opportunity, even if any number of other new stars should present the same series of phenomena as the star in Cygnus.

On December 2, the spectrum, as observed by M. Cornu, consisted almost entirely of bright lines. On December 5, he determined the position of these lines, though clouds still greatly interfered with his labours. He found three bright lines of hydrogen, the strong double sodium line in the orange-yellow, the triple magnesium line in the yellow-green, and two other lines—one of which seemed to agree exactly in position with a bright line belonging to the solar corona. All these lines were shining upon the rainbow-tinted background of the spectrum, which was relatively faint. He drew the conclusion that in chemical constitution the atmosphere of the new star was constituted exactly like the solar sierra.

Herr Vögel’s observations commenced on December 5, and were continued at intervals until March 10, when the star had sunk to below the eighth magnitude.

Vögel’s earlier observations agreed well with Cornu’s. He remarks, however, that Cornu’s opinion as to the exact resemblance of the chemical constitution of the star’s atmosphere with that of the sierra is not just, for both Cornu and himself noticed one line which did not correspond with any line belonging to the solar sierra; and this line eventually became the brightest line of the whole spectrum. Comparing his own observations with those of Cornu, Vögel points out that they agree perfectly with regard to the presence of the three hydrogen lines, and that of the brightest line of the air spectrum (belonging to nitrogen),—which is the principal line of the spectrum of nebulæ. This is the line which has no analogue in the spectrum of the sierra.

We have also observations by F. Secchi, at Rome, Mr. Copeland, at Dunecht, and Mr. Backhouse, of Sunderland, all agreeing in the main with the observations made by Vögel and Cornu. In particular, Mr. Backhouse observed, as Vögel had done, that whereas in December the greenish-blue line of hydrogen, F, was brighter than the nitrogen line (also in the green-blue, but nearer the red end than F), on January 6 the nitrogen line was the brightest of all the lines in the spectrum of the new star.

Vögel, commenting on the results of his observations up to March 10, makes the following interesting remarks (I quote, with slight verbal alterations, from a paraphrase in a weekly scientific journal):—“A stellar spectrum with bright lines is always a highly interesting phenomenon for any one acquainted with stellar spectrum analysis, and well worthy of deep consideration. Although in the chromosphere (sierra) of our sun, near the limb, we see numerous bright lines, yet only dark lines appear in the spectrum whenever we produce a small star-like image of the sun, and examine it through the spectroscope. It is generally believed that the bright lines in some few star-spectra result from gases which break forth from the interior of the luminous body, the temperature of which is higher than that of the surface of the body—that is, the phenomenon is the same sometimes observed in the spectra of solar spots, where incandescent hydrogen rushing out of the hot interior becomes visible above the cooler spots through the hydrogen lines turning bright. But this is not the only possible explanation. We may also suppose that the atmosphere of a star, consisting of incandescent gases, as is the case with our own sun, is on the whole cooler than the nucleus, but with regard to the latter is extremely large. I cannot well imagine how the phenomenon can last for any long period of time if the former hypothesis be correct. The gas breaking forth from the hot interior of the body will impart a portion of its heat to the surface of the body, and thus raise the temperature of the latter; consequently, the difference of temperature between the incandescent gas and the surface of the body will soon be insufficient to produce bright lines; and these will disappear from the spectrum. This view applies perfectly to stars which suddenly appear and soon disappear again, or at least increase considerably in intensity—that is, it applies perfectly to so-called new stars in the spectra of which bright lines are apparent, if the hypothesis presently to be mentioned is admitted for their explanation. For a more stable state of things the second hypothesis seems to be far better adapted. Stars like Beta Lyræ, Gamma Cassiopeiæ, and others, which show the hydrogen lines and the sierra D line bright on a continuous spectrum, with only slight changes of intensity, possess, according to this theory, atmospheres very large relatively to their own volume—the atmospheres consisting of hydrogen and that unknown element which produces the D line.16 With regard to the new star, Zöllner, long before the progress lately made in stellar physics by means of spectrum analysis, deduced from Tycho’s observations of the star called after him, that on the surface of a star, through the constant emission of heat, the products of cooling, which in the case of our sun we call sun-spots, accumulate: so that finally the whole surface of the body is covered with a colder stratum, which gives much less light or none at all. Through a sudden and violent tearing up of this stratum, the interior incandescent materials which it encloses must naturally break forth, and must in consequence, according to the extent of their eruption, cause larger or smaller patches of the dark envelope of the body to become luminous again. To a distant observer such an eruption from the hot and still incandescent interior of a heavenly body must appear as the sudden flashing-up of a new star. That this evolution of light may under certain conditions be an extremely powerful one, could be explained by the circumstance that all the chemical compounds which, under the influence of a lower temperature, had already formed upon the surface, are again decomposed through the sudden eruption of these hot materials; and that this decomposition, as in the case of terrestrial substances, takes place under evolution of light and heat. Thus the bright flashing-up is not only ascribed to the parts of the surface which through the eruption of the incandescent matter have again become luminous, but also to a simultaneous process of combustion, which is initiated through the colder compounds coming into contact with the incandescent matter.”

Vögel considers that Zöllner’s hypothesis has been confirmed in its essential points by the application of spectrum analysis to the stars. We can recognize from the spectrum different stages in the process of cooling, and in some of the fainter stars we perceive indeed that chemical compounds have already formed, and still exist. As to new stars, again, says Vögel, Zöllner’s theory seems in nowise contradicted “by the spectral observations made on the two new stars of 1866 and 1876. The bright continuous spectrum, and the bright lines only slightly exceeding it at first” (a description, however, applying correctly only to the star of 1876), “could not be well explained if we only suppose a violent eruption from the interior, which again rendered the surface wholly or partially luminous; but are easily explained if we suppose that the quantity of light is considerably augmented through a simultaneous process of combustion. If this process is of short duration, then the continuous spectrum, as was the case with the new star of 1876, will very quickly decrease in intensity down to a certain limit, while the bright lines in the spectrum, which result from the incandescent gases that have emanated in enormous quantities from the interior, will continue for some time.”

It thus appears that Herr Vögel regarded the observations which had been made on this remarkable star up to March 10 as indicating that first there had been an outburst of glowing gaseous matter from the interior, producing the part of the light which gave the bright lines indicative of gaseity, and that then there had followed, as a consequence, the combustion of a portion of the solid and relatively cool crust, causing the continuous part of the spectrum. We may compare what had taken place, on this hypothesis, with the outburst of intensely hot gases from the interior of a volcanic crater, and the incandescence of the lips of the crater in consequence of the intense heat of the out-rushing gases. Any one viewing such a crater from a distance, with a spectroscope, would see the bright lines belonging to the out-rushing gases superposed upon the continuous spectrum due to the crater’s burning lips. Vögel further supposes that the burning parts of the star soon cooled, the majority of the remaining light (or at any rate the part of the remaining light spectroscopically most effective) being that which came from the glowing gases which had emanated in vast quantities from the star’s interior.

“The observations of the spectrum show, beyond doubt,” he says, “that the decrease in the light of the star corresponds with the cooling of its surface. The violet and blue parts decreased more rapidly in intensity than the other parts; and the absorption-bands which crossed the spectrum have gradually become darker and darker.”

The reasoning, however, if not altogether unsatisfactory, is by no means so conclusive as Herr Vögel appears to think. It is not clear how the incandescent portion of the surface could possibly cool in any great degree while enormous quantities of gas more intensely heated (by the hypothesis) remained around the star. The more rapid decrease in the violet and blue parts of the spectrum than in the red and orange is explicable as an effect of absorption, at least as readily as by the hypothesis that burning solid or liquid matter had cooled. Vögel himself could only regard the other bands which crossed the spectrum as absorption-bands. And the absorption of light from the continuous spectrum in these parts (that is, not where the bright lines belonging to the gaseous matter lay) could not possibly result from absorption produced by those gases. If other gases were in question, gases which, by cooling with the cooling surface, had become capable of thus absorbing light from special parts of the spectrum, how is it that before, when these gases were presumably intensely heated, they did not indicate their presence by bright bands? Bright bands, indeed, were seen, which eventually faded out of view, but these bright bands did not occupy the position where, later on, absorption-bands appeared.

The natural explanation of what had thus far been observed is different from that advanced by Vögel, though we must not assume that because it is the natural, it is necessarily the true explanation. It is this—that the source of that part of the star’s light which gave the bright-line spectrum, or the spectrum indicative of gaseity, belongs to the normal condition of the star, and not to gases poured forth, in consequence of some abnormal state of things, from the sun’s interior. We should infer naturally, though again I say not therefore correctly, that if a star spectroscope had been directed upon the place occupied by the new star before it began to shine with unusual splendour, the bright-line spectrum would have been observed. Some exceptional cause would then seem to have aroused the entire surface of the star to shine with a more intense brightness, the matter thus (presumably) more intensely heated being such as would give out the combined continuous and bright-line spectrum, including the bright lines which, instead of fading out, shone with at least relatively superior brightness as the star faded from view. The theory that, on the contrary, the matter giving these more persistent lines was that whose emission caused the star’s increase of lustre, seems at least not proven, and I would go so far as to say that it accords ill with the evidence.

The question, be it noted, is simply whether we should regard the kind of light which lasts longest in this star as it fades out of view as more probably belonging to the star’s abnormal brightness or to its normal luminosity. It seems to me there can be little doubt that the persistence of this part of the star’s light points to the latter rather than to the former view.

Let it also be noticed that the changes which had been observed thus far were altogether unlike those which had been observed in the case of the star in the Northern Crown, and therefore cannot justly be regarded as pointing to the same explanation. As the star in the Crown faded from view, the bright lines indicative of glowing hydrogen died out, and only the ordinary stellar spectrum remained. In the case of the star in the Swan, the part of the spectrum corresponding to stellar light faded gradually from view, and bright lines only were left, at least as conspicuous parts of the star’s spectrum. So that whereas one orb seemed to have faded into a faint star, the other seemed fading out into a nebula—not merely passing into such a condition as to shine with light indicative of gaseity, but actually so changing as to shine with light of the very tints (or, more strictly, of the very wave-lengths) observed in all the gaseous nebulæ.

The strange eventful history of the new star in Cygnus did not end here, however. We may even say, indeed, that it has not ended yet. But another chapter can already be written.

Vögel ceased from observing the star in March, precisely when observation seemed to promise the most interesting results. At most other observatories, also, no observations were made for about half a year. At the Dunecht Observatory17 pressure of work relating to Mars interfered with the prosecution of those observations which had been commenced early in the year. But on September 3, Lord Lindsay’s 15-inch reflector was directed upon the star. A star was still shining where the new star’s yellow lustre had been displayed in November, 1876; but now the star shone with a faint blue colour. Under spectroscopic examination, however, the light from this seeming blue star was found not to be starlight, properly speaking, at all. It formed no rainbow-tinted spectrum, but gave light of only a single colour. The single line now seen was that which at the time of Vögel’s latest observation had become the strongest of the bright lines of the originally complex spectrum of the so-called new star. It is the brightest of the lines given by the gaseous nebulæ. In fact, if nothing had been known about this body before the spectroscopic observation of September 3 was made, the inference from the spectrum given by the blue star would undoubtedly have been that the object is in reality a small nebula of the planetary sort, very similar to that one close by the pole of the ecliptic, which gave Huggins the first evidence of the gaseity of nebulæ, but very much smaller. I would specially direct the reader’s attention, in fact, to Huggins’s account of his observation of that planetary nebula in the Dragon. “On August 19, 1864,” he says, “I directed the telescope armed with the spectrum apparatus to this nebula. At first I suspected some derangement of the instrument had taken place, for no spectrum was seen, but only” a single line of light. “I then found that the light of this nebula, unlike any other extra-terrestrial light which had yet been subjected by me to prismatic analysis, was not composed of light of different refrangibilities, and therefore could not form a spectrum. A great part of the light from this nebula is monochromatic, and after passing through the prisms remains concentrated in a bright line.” A more careful examination showed that not far from the bright line was a much fainter line; and beyond this, again, a third exceedingly faint line was seen. The brightest of the three lines was a line of nitrogen corresponding in position with the brightest of the lines in the spectrum of our own air. The faintest corresponded in position with a line of hydrogen. The other has not yet been associated with a known line of any element. Besides the faint lines, Dr. Huggins perceived an exceedingly faint continuous spectrum on both sides of the group of bright lines; he suspected, however, that this faint spectrum was not continuous, but crossed by dark spaces. Later observations on other nebulæ induced him to regard this faint continuous spectrum as due to the solid or liquid matter of the nucleus, and as quite distinct from the bright lines into which nearly the whole of the light from the nebula is concentrated. The fainter parts of the spectrum of the gaseous nebulæ, in fact, correspond to those parts of the spectrum of the “new star” in Cygnus which last remained visible, before the light assumed its present monochromatic colour.

Now let us consider the significance of the evidence afforded by this discovery—not perhaps hoping at once to perceive the full meaning of the discovery, but endeavouring to advance as far as we safely can in the direction in which it seems to point.

We have, then, these broad facts: where no star had been known, an object has for a while shone with stellar lustre, in this sense, that its light gave a rainbow-tinted spectrum not unlike that which is given by a certain order of stars; this object has gradually parted with its new lustre, and in so doing the character of its spectrum has slowly altered, the continuous portion becoming fainter, and the chief lustre of the bright-line portion shifting from the hydrogen lines to a line which, there is every reason to believe, is absolutely identical with the nebula nitrogen line: and lastly, the object has ceased to give any perceptible light, other than that belonging to this nitrogen line.

Now it cannot, I think, be doubted that, accompanying the loss of lustre in this orb, there has been a corresponding loss of heat. The theory that all the solid and liquid materials of the orb have been vaporized by intense heat, and that this vaporization has caused the loss of the star’s light (as a lime-light might die out with the consumption of the lime, though the flame remained as hot as ever), is opposed by many considerations. It seems sufficient to mention this, that if a mass of solid matter, like a dead sun or planet, were exposed to an intense heat, first raising it to incandescence, and eventually altogether vaporizing its materials, although quite possibly the time of its intensest lustre might precede the completion of the vaporization, yet certainly so soon as the vaporization was complete, the spectrum of the newly vaporized mass would show multitudinous bright lines corresponding to the variety of material existing in the body. No known fact of spectroscopic analysis lends countenance to the belief that a solid or liquid mass, vaporized by intense heat, would shine thenceforth with monochromatic light.

Again, I think we are definitely compelled to abandon Vögel’s explanation of the phenomena by Zöllner’s theory. The reasons which I have urged above are not only strengthened severally by the change which has taken place in the spectrum of the new star since Vögel observed it, but an additional argument of overwhelming force has been introduced. If any one of the suns died out, a crust forming over its surface and this crust being either absolutely dark or only shining with very feeble lustre, the sun would still in one respect resemble all the suns which are spread over the heavens—it would show no visible disc, however great the telescopic power used in observing it. If the nearest of all the stars were as large, or even a hundred times as large, as Sirius, and were observed with a telescope of ten times greater magnifying power than any yet directed to the heavens, it would appear only as a point of light. If it lost the best part of its lustre, it would appear only as a dull point of light. Now the planetary nebulæ show discs, sometimes of considerable breadth. Sir J. Herschel, to whom and to Sir W. Herschel we owe the discovery and observation of nearly all these objects, remarks that “the planetary nebulæ have, as their name imports, a near, in some instances a perfect, resemblance to planets, presenting discs round, or slightly oval, in some quite sharply terminated, in others a little hazy or softened at the borders....” Among the most remarkable may be specified one near the Cross, whose light is about equal to that of a star just visible to the naked eye, “its diameter about twelve seconds, its disc circular or very slightly elliptic, and with a clear, sharp, well-defined outline, having exactly the appearance of a planet, with the exception of its colour, which is a fine and full blue, verging somewhat upon green.” But the largest of these planetary nebulæ, not far from the southernmost of the two stars called the Pointers, has a diameter of 2⅔ minutes of arc, “which, supposing it placed at a distance from us not greater than that of the nearest known star of our northern heavens, would imply a linear diameter seven times greater than that of the orbit of Neptune.” The actual volume of this object, on this assumption, would exceed our sun’s ten million million times. No one supposes that this planetary nebula, shining with a light indicative of gaseity, has a mass exceeding our sun’s in this enormous degree. It probably has so small a mean density as not greatly to exceed, or perhaps barely to equal, our sun in mass. Now though the “new star” in Cygnus presented no measurable disc, and still shines as a mere blue point in the largest telescope, yet inasmuch as its spectrum associated it with the planetary and gaseous nebulæ, which we know to be much larger bodies than the stars, it must be regarded, in its present condition, as a planetary nebula, though a small one; and since we cannot for a moment imagine that the monstrous planetary nebulæ just described are bodies which once were suns, but whose crust has now become non-luminous, while around the crust masses of gas shine with a faint luminosity, so are we precluded from believing that this smaller member of the same family is in that condition.

It is conceivable (and the possibility must be taken into account in any attempt to interpret the phenomena of the new star) that when shining as a star, the new orb, so far as this unusual lustre was concerned, was of sunlike dimensions. For we cannot tell whether the surface which gave the strong light was less or greater than, or equal to, that which is now shining with monochromatic light. Very likely, if we had been placed where we could have seen the full dimensions of the planetary nebula as it at present exists, we should have found only its nuclear part glowing suddenly with increased lustre, which, after very rapidly attaining its maximum, gradually died out again, leaving the nebula as it had been before. But that the mass now shining with monochromatic light is, I will not say enormously large, but of exceedingly small mean density, so that it is enormously large compared with the dimensions it would have if its entire substance were compressed till it had the same mean density as our own sun, must be regarded as, to all intents and purposes, certain.

We certainly have not here, then, the case of a sun which has grown old and dead and dark save at the surface, but within whose interior fire has still remained, only waiting some disturbing cause to enable it for a while to rush forth. If we could suppose that in such a case there could be such changes as the spectroscope has indicated—that the bright lines of the gaseous outbursting matter would, during the earlier period of the outburst, show on a bright continuous background, due to the glowing lips of the opening through which the matter had rushed, but later would shine alone, becoming also fewer in number, till at last only one was left,—we should find ourselves confronted with the stupendous difficulty that that single remaining line is the bright line of the planetary and other gaseous nebulæ. Any hypothesis accounting for its existence in the spectrum of the faint blue starlike object into which the star in Cygnus has faded ought to be competent to explain its existence in the spectrum of those nebulæ. But this hypothesis certainly does not so explain its existence in the nebular spectrum. The nebulæ cannot be suns which have died out save for the light of gaseous matter surrounding them, for they are millions, or rather millions of millions, of times too large. If, for instance, a nebula, like the one above described as lying near the southernmost Pointer, were a mass of this kind, having the same mean density as the sun, and lying only at the distance of the nearest of the stars from us, then not only would it have the utterly monstrous dimensions stated by Sir J. Herschel, but it would in the most effective way perturb the whole solar system. With a diameter exceeding seven times that of the orbit of Neptune, it would have a volume, and therefore a mass, exceeding our sun’s volume and mass more than eleven millions of millions of times. But its distance on this assumption would be only about two hundred thousand times the sun’s, and its attraction reduced, as compared with his, on this account only forty thousand millions of times. So that its attraction on the sun and on the earth would be greater than his attraction on the earth, in the same degree that eleven millions are greater than forty thousand—or two hundred and seventy-five times. The sun, despite his enormous distance from such a mass, would be compelled to fall very quickly into it, unless he circuited (with all his family) around it in about one-sixteenth of a year, which most certainly he does not do. Nor would increasing the distance at which we assume the star to lie have any effect to save the sun from being thus perturbed, but the reverse. If we double for instance our estimate of the nebula’s distance, we increase eightfold our estimate of its mass, while we only diminish its attraction on our sun fourfold on account of increased distance; so that now its attraction on our sun would be one-fourth its former attraction multiplied by eight, or twice our former estimate. We cannot suppose the nebula to be much nearer than the nearest star. Again, we cannot suppose that the light of these gaseous nebulæ comes from some bright orb within them of only starlike apparent dimensions, for in that case we should constantly recognize such starlike nucleus, which is not the case. Moreover, the bright-line spectrum from one of these nebulæ comes from the whole nebula, as is proved by the fact that if the slit of the spectroscope be opened it becomes possible to see three spectroscopic images of the nebula itself, not merely the three bright lines.

So that, if we assume the so-called star in Cygnus to be now like other objects giving the same monochromatic spectrum—and this seems the only legitimate assumption—we are compelled to believe that the light now reaching us comes from a nebulous mass, not from the faintly luminous envelope of a dead sun. Yet, remembering that when at its brightest this orb gave a spectrum resembling in general characteristics that of other stars or suns, and closely resembling even in details that of stars like Gamma Cassiopeiæ, we are compelled by parity of reasoning to infer that when the so-called new star was so shining, the greater part of its light came from a sunlike mass. Thus, then, we are led to the conclusion that in the case of this body we have a nucleus or central mass, and that around this central mass there is a quantity of gaseous matter, resembling in constitution that which forms the bulk of the other gaseous nebulæ. The denser nucleus ordinarily shines with so faint a lustre that the continuous spectrum from its light is too faint to be discerned with the same spectroscopic means by which the bright lines of the gaseous portion are shown; and the gaseous portion ordinarily shines with so faint a lustre that its bright lines would not be discernible on the continuous background of a stellar spectrum. Through some cause unknown—possibly (as suggested in an article on the earlier history of this same star in my “Myths and Marvels of Astronomy”) the rush of a rich and dense flight of meteors upon the central mass—the nucleus was roused to a degree of heat far surpassing its ordinary temperature. Thus for a time it glowed as a sun. At the same time the denser central portions of the nebulous matter were also aroused to intenser heat, and the bright lines which ordinarily (and certainly at present) would not stand out bright against the rainbow-tinted background of a stellar spectrum, showed brightly upon the continuous spectrum of the new star. Then as the rush of meteors upon the nucleus and on the surrounding nebulous matter ceased—if that be the true explanation of the orb’s accession of lustre—or as the cause of the increase of brightness, whatever that cause may have been, ceased to act, the central orb slowly returned to its usual temperature, the nebulous matter also cooling, the continuous spectrum slowly fading out, the denser parts of the nebulous matter exercising also a selective absorption (explaining the bands seen in the spectrum at this stage) which gradually became a continuous absorption—that is, affected the entire spectrum. Those component gases, also, of the nebulous portion which had for a while been excited to sufficient heat to show their bright lines, cooled until their lines disappeared, and none remained visible except for a while the three usual nebular lines, and latterly (owing to still further cooling) only the single line corresponding to the monochromatic light of the fainter gaseous nebulæ.

STAR-GROUPING, STAR-DRIFT, AND STAR-MIST.
A Lecture delivered at the Royal Institution on May 6, 1870.

Nearly a century has passed since the greatest astronomer the world has ever known—the Newton of observational astronomy, as he has justly been called by Arago—conceived the daring thought that he would gauge the celestial depths. And because in his day, as indeed in our own, very little was certainly known respecting the distribution of the stars, he was forced to found his researches upon a guess. He supposed that the stars, not only those visible to the naked eye, but all that are seen in the most powerful telescopes, are suns, distributed with a certain general uniformity throughout space. It is my purpose to attempt to prove that—as Sir Wm. Herschel was himself led to suspect during the progress of his researches—this guess was a mistaken one; that but a small proportion of the stars can be regarded as real suns; and that in place of the uniformity of distribution conceived by Sir Wm. Herschel, the chief characteristic of the sidereal system is infinite variety.

In order that the arguments on which these views are based may be clearly apprehended, it will be necessary to recall the main results of Sir Wm. Herschel’s system of star-grouping.

Directing one of his 20-feet reflectors to different parts of the heavens, he counted the stars seen in the field of view. Assuming that the telescope really reached the limits of the sidereal system, it is clear that the number of stars seen in any direction affords a means of estimating the relative extension of the system in that direction, provided always that the stars are really distributed throughout the system with a certain approach to uniformity. Where many stars are seen, there the system has its greatest extension; where few, there the limits of the system must be nearest to us.

Sir Wm. Herschel was led by this process of star-grouping to the conclusion that the sidereal system has the figure of a cloven disc. The stars visible to the naked eye lie far within the limits of this disc. Stars outside the relatively narrow limits of the sphere including all the visible stars, are separately invisible. But where the system has its greatest extension these orbs produce collectively the diffused light which forms the Milky Way.

Sir John Herschel, applying a similar series of researches to the southern heavens, was led to a very similar conclusion. His view of the sidereal system differs chiefly in this respect from his father’s, that he considered the stars within certain limits of distance from the sun to be spread less richly through space than those whose united lustre produces the milky light of the galaxy.

Now it is clear that if the supposition on which these views are based is just, the three following results are to be looked for.

In the first place, the stars visible to the naked eye would be distributed with a certain general uniformity over the celestial sphere; so that if on the contrary we find certain extensive regions over which such stars are strewn much more richly than over the rest of the heavens, we must abandon Sir Wm. Herschel’s fundamental hypothesis and all the conclusions which have been based upon it.

In the second place, we ought to find no signs of the aggregation of lucid stars into streams or clustering groups. If we should find such associated groups, we must abandon the hypothesis of uniform distribution and all the conclusions founded on it.

Thirdly, and most obviously of all, the lucid stars ought not to be associated in a marked manner with the figure of the Milky Way. To take an illustrative instance. When we look through a glass window at a distant landscape we do not find that the specks in the substance of the glass seem to follow the outline of valleys, hills, trees, or whatever features the landscape may present. In like manner, regarding the sphere of the lucid stars as in a sense the window through which we view the Milky Way, we ought not to find these stars, which are so near to us, associated with the figure of the Milky Way, whose light comes from distances so enormously exceeding those which separate us from the lucid stars. Here again, then, if there should appear signs of such association, we must abandon the theory that the sidereal system is constituted as Sir Wm. Herschel supposed.

It should further be remarked that the three arguments derived from these relations are independent of each other. They are not as three links of a chain, any one of which being broken the chain is broken. They are as three strands of a triple cord. If one strand holds, the cord holds. It may be shown that all three are to be trusted.

It is not to be expected, however, that the stars as actually seen should exhibit these relations, since far the larger number are but faintly visible; so that the eye would look in vain for the signs of law among them, even though law may be there. What is necessary is that maps should be constructed on a uniform and intelligible plan, and that in these maps the faint stars should be made bright, and the bright stars brighter.

The maps exhibited during this discourse [since published as my “Library Atlas”] have been devised for this purpose amongst others. There are twelve of them, but they overlap, so that in effect each covers a tenth part of the heavens. There is first a north-polar map, then five maps symmetrically placed around it; again, there is a south-polar map, and five maps symmetrically placed round that map; and these five so fit in with the first five as to complete the enclosure of the whole sphere. In effect, every map of the twelve has five maps symmetrically placed around it and overlapping it.

Since the whole heavens contain but 5932 stars visible to the naked eye, each of the maps should contain on the average about 593 stars. But instead of this being the case, some of the maps contain many more than their just proportion of stars, while in others the number as greatly falls short of the average. One recognizes, by combining these indications, the existence of a roughly circular region, rich in stars, in the northern heavens, and of another, larger and richer, in the southern hemisphere.

To show the influence of these rich regions, it is only necessary to exhibit the numerical relations presented by the maps.

The north-polar map, in which the largest part of the northern rich region falls, contains no less than 693 lucid stars, of which upwards of 400 fall within the half corresponding to the rich region. Of the adjacent maps, two contain upwards of 500 stars, while the remaining three contain about 400 each. Passing to the southern hemisphere, we find that the south-polar map, which falls wholly within a rich region, contains no less than 1132 stars! One of the adjacent maps contains 834 stars, and the four others exhibit numbers ranging from 527 to 595.

It is wholly impossible not to recognize so unequal a distribution as exhibiting the existence of special laws of stellar aggregation.

It is noteworthy, too, that the greater Magellanic cloud falls in the heart of the southern rich region. Were there not other signs that this wonderful object is really associated with the sidereal system, it might be rash to recognize this relation as indicating the existence of a physical connection between the Nubecula Major and the southern region rich in stars. Astronomers have indeed so long regarded the Nubeculæ as belonging neither to the sidereal nor to the nebular systems, that they are not likely to recognize very readily the existence of any such connection. Yet how strangely perverse is the reasoning which has led astronomers so to regard these amazing objects. Presented fairly, that reasoning amounts simply to this: The Magellanic clouds contain stars and they contain nebulæ; therefore they are neither nebular nor stellar. Can perversity of reasoning be pushed further? Is not the obvious conclusion this, that since nebulæ and stars are seen to be intermixed in the Nubeculæ, the nebular and stellar systems form in reality but one complex system?

As to the existence of star-streams and clustering aggregations, we have also evidence of a decisive character. There is a well-marked stream of stars running from near Capella towards Monoceros. Beyond this lies a long dark rift altogether bare of lucid orbs, beyond which again lies an extensive range of stars, covering Gemini, Cancer, and the southern parts of Leo. This vast system of stars resembles a gigantic sidereal billow flowing towards the Milky Way as towards some mighty shore-line. Nor is this description altogether fanciful; since one of the most marked instances of star-drift presently to be adduced refers to this very region. These associated stars are urging their way towards the galaxy, and that at a rate which, though seemingly slow when viewed from beyond so enormous a gap as separates us from this system, must in reality be estimated by millions of miles in every year.

Other streams and clustering aggregations there are which need not here be specially described. But it is worth noticing that all the well-marked streams recognized by the ancients seem closely associated with the southern rich region already referred to. This is true of the stars forming the River Eridanus, the serpent Hydra, and the streams from the water-can of Aquarius. It is also noteworthy that in each instance a portion of the stream lies outside the rich region, the rest within it; while all the streams which lie on the same side of the galaxy tend towards the two Magellanic clouds.

Most intimate signs of association between lucid stars and the galaxy can be recognized—(i.) in the part extending from Cygnus to Aquila; (ii.) in the part from Perseus to Monoceros; (iii.) over the ship Argo; and (iv.) near Crux and the feet of Centaurus.

Before proceeding to the subject of Star-drift, three broad facts may be stated. They are, I believe, now recognized for the first time, and seem decisive of the existence of special laws of distribution among the stars:—

First, the rich southern region, though covering but a sixth part of the heavens, contains one-third of all the lucid stars, leaving only two-thirds for the remaining five-sixths of the heavens.

Secondly, if the two rich regions and the Milky Way be considered as one part of the heavens, the rest as another, then the former part is three times as richly strewn with lucid stars as the second.

Thirdly, the southern hemisphere contains one thousand more lucid stars than the northern, a fact which cannot but be regarded as most striking when it is remembered that the total number of stars visible to ordinary eyesight in both hemispheres falls short of 6000.

Two or three years ago, the idea suggested itself to me that if the proper motions of the stars were examined, they would be found to convey clear information respecting the existence of variety of structure, and special laws of distribution within the sidereal system.

In the first place, the mere amount of a star’s apparent motion must be regarded as affording a means of estimating the star’s distance. The nearer a moving object is, the faster it will seem to move, and vice versâ. Of course in individual instances little reliance can be placed on this indication; but by taking the average proper motions of a set of stars, a trustworthy measure may be obtained of their average distance, as compared with the average distance of another set.

For example, we have in this process the means of settling the question whether the apparent brightness of a star is indeed a test of relative nearness. According to accepted theories the sixth-magnitude stars are ten or twelve times as far off as those of the first magnitude. Hence their motions should, on the average, be correspondingly small. Now, to make assurance doubly sure, I divided the stars into two sets, the first including the stars of the 1st, 2nd, and 3rd, the second including those of the 4th, 5th, and 6th magnitude. According to accepted views, the average proper motion for the first set should be about five times as great as that for the second. I was prepared to find it about three times as great; that is, not so much greater as the accepted theories require, but still considerably greater. To my surprise, I found that the average proper motion of the brighter orders of stars is barely equal to that of the three lower orders.

This proves beyond all possibility of question that by far the greater number of the fainter orders of stars (I refer here throughout to lucid stars) owe their faintness not to vastness of distance, but to real relative minuteness.

To pass over a number of other modes of research, the actual mapping of the stellar motions, and the discovery of the peculiarity to which I have given the name of star-drift, remain to be considered.

In catalogues it is not easy to recognize any instances of community of motion which may exist among the stars, owing to the method in which the stars are arranged. What is wanted in this case (as in many others which yet remain to be dealt with) is the adoption of a plan by which such relations may be rendered obvious to the eye. The plan I adopted was to attach to each star in my maps a small arrow, indicating the amount and direction of that star’s apparent motion in 36,000 years (the time-interval being purposely lengthened, as otherwise most of the arrows would have been too small to be recognized). When this was done, several well-marked instances of community of motion could immediately be recognized.

It is necessary to premise, however, that before the experiment was tried, there were reasons for feeling very doubtful whether it would succeed. A system of stars might really be drifting athwart the heavens, and yet the drift might be rendered unrecognizable through the intermixture of more distant or nearer systems having motions of another sort and seen accidentally in the same general direction.

This was found to be the case, indeed, in several instances. Thus the stars in the constellation Ursa Major, and neighbouring stars in Draco, exhibit two well-marked directions of drift. The stars β, γ, δ, ε, and ζ of the Great Bear, besides two companions of the last-named star, are travelling in one direction, with equal velocity, and clearly form one system. The remaining stars in the neighbourhood are travelling in a direction almost exactly the reverse. But even this relation, thus recognized in a region of diverse motions, is full of interest. Baron Mädler, the well-known German astronomer, recognizing the community of motion between ζ Ursæ and its companions, calculated the cyclic revolution of the system to be certainly not less than 7000 years. But when the complete system of stars showing this motion is considered, we get a cyclic period so enormous, that not only the life of man, but the life of the human race, the existence of our earth, nay, even the existence of the solar system, must be regarded as a mere day in comparison with that tremendous cycle.

Then there are other instances of star-drift where, though two directions of motion are not intermixed, the drifting nature of the motion is not at once recognized, because of the various distances at which the associated stars lie from the eye.

A case of this kind is to be met with in the stars forming the constellation Taurus. It was here that Mädler recognized a community of motion among the stars, but he did not interpret this as I do. He had formed the idea that the whole of the sidereal system must be in motion around some central point; and for reasons which need not here be considered, he was led to believe that in whatever direction the centre of motion may lie, the stars seen in that general direction would exhibit a community of motion. Then, that he might not have to examine the proper motions all over the heavens, he inquired in what direction (in all probability) the centre of motion may be supposed to lie. Coming to the conclusion that it must lie towards Taurus, he examined the proper motions in that constellation, and found a community of motion which led him to regard Alcyone, the chief star of the Pleiades, as the centre around which the sidereal system is moving. Had he examined further he would have found more marked instances of community of motion in other parts of the heavens, a circumstance which would have at once compelled him to abandon his hypothesis of a central sun in the Pleiades, or at least to lay no stress on the evidence derivable from the community of motion in Taurus.

Perhaps the most remarkable instance of star-drift is that observed in the constellations Gemini and Cancer. Here the stars seem to set bodily towards the neighbouring part of the Milky Way. The general drift in that direction is too marked, and affects too many stars, to be regarded as by any possibility referable to accidental coincidence.

It is worthy of note that if the community of star-drift should be recognized (or I prefer to say, when it is recognized), astronomers will have the means of determining the relative distances of the stars of a drifting system. For differences in the apparent direction and amount of motion can be due but to differences of distance and position, and the determination of these differences becomes merely a question of perspective.18

Before long it is likely that the theory of star-drift will be subjected to a crucial test, since spectroscopic analysis affords the means of determining the stellar motions of recess or approach. The task is a very difficult one, but astronomers have full confidence that in the able hands of Mr. Huggins it will be successfully accomplished. I await the result with full confidence that it will confirm my views. (See pages 92–94 for the result.)

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Turning to the subject of Star-mist, under which head I include all orders of nebulæ, I propose to deal with but a small proportion of the evidence I have collected to prove that none of the nebulæ are external galaxies. That evidence has indeed become exceedingly voluminous.

I shall dwell, therefore, on three points only.

First, as to the distribution of the nebulæ:—They are not spread with any approach to uniformity over the heavens, but are gathered into streams and clusters. The one great law which characterizes their distribution is an avoidance of the Milky Way and its neighbourhood. This peculiarity has, strangely enough, been regarded by astronomers as showing that there is no association between the nebulæ and the sidereal system. They have forgotten that marked contrast is as clear a sign of association as marked resemblance, and has always been so regarded by logicians.

Secondly, there are in the southern heavens two well-marked streams of nebulæ. Each of these streams is associated with an equally well-marked stream of stars. Each intermixed stream directs its course towards a Magellanic Cloud, one towards the Nubecula Minor, the other towards the Nubecula Major. To these great clusters they flow, like rivers towards some mighty lake. And within these clusters, which are doubtless roughly spherical in form, there are found intermixed in wonderful profusion, stars, star-clusters, and all the orders of nebulæ. Can these coincidences be regarded as accidental? And if not accidental, is not the lesson they clearly teach us this, that nebulæ form but portions of the sidereal system, associating themselves with stars on terms of equality (if one may so speak), even if single stars be not more important objects in the scale of creation than these nebulous masses, which have been so long regarded as equalling, if not outvying, the sidereal system itself in extent?

The third point to which I wish to invite attention is the way in which in many nebulæ stars of considerable relative brightness, and belonging obviously to the sidereal system, are so associated with nebulous masses as to leave no doubt whatever that these masses really cling around them. The association is in many instances far too marked to be regarded as the effect of accident.

Among other instances19 may be cited the nebula round the stars c¹ and c² in Orion. In this object two remarkable nebulous nodules centrally surround two double stars. Admitting the association here to be real (and no other explanation can reasonably be admitted), we are led to interesting conclusions respecting the whole of that wonderful nebulous region which surrounds the sword of Orion. We are led to believe that the other nebulæ in that region are really associated with the fixed stars there; that it is not a mere coincidence, for instance, that the middle star in the belt of Orion is involved in nebula, or that the lowest star of the sword is similarly circumstanced. It is a legitimate inference from the evidence that all the nebulæ in this region belong to one great nebulous group, which extends its branches to these stars. As a mighty hand, this nebulous region seems to gather the stars here into close association, showing us, in a way there is no misinterpreting, that these stars form one system.

The nebula around the strange variable star, Eta Argûs, is another remarkable instance of this sort. More than two years ago I ventured to make two predictions about this object. The first was a tolerably safe one. I expressed my belief that the nebula would be found to be gaseous. After Mr. Huggins’s discovery that the great Orion nebula is gaseous, it was not difficult to see that the Argo nebula must be so too. At any rate, this has been established by Captain Herschel’s spectroscopic researches. The other prediction was more venturesome. Sir John Herschel, whose opinion on such points one would always prefer to share, had expressed his belief that the nebula lies far out in space beyond the stars seen in the same field of view. I ventured to express the opinion that those stars are involved in the nebula. Lately there came news from Australia that Mr. Le Sueur, with the great reflector erected at Melbourne, has found that the nebula has changed largely in shape since Sir John Herschel observed it. Mr. Le Sueur accordingly expressed his belief that the nebula lies nearer to us than the fixed stars seen in the same field of view. More lately, however, he has found that the star Eta Argûs is shining with the light of burning hydrogen, and he expresses his belief that the star has consumed the nebulous matter near it. Without agreeing with this view, I recognize in it a proof that Mr. Le Sueur now considers the nebula to be really associated with the stars around it. My belief is that as the star recovers its brilliancy observation will show that the nebula in its immediate neighbourhood becomes brighter (not fainter through being consumed as fuel). In fact, I am disposed to regard the variations of the nebula as systematic, and due to orbital motions among its various portions around neighbouring stars.

As indicative of other laws of association bearing on the relations I have been dealing with, I may mention the circumstance that red stars and variable stars affect the neighbourhood of the Milky Way or of well-marked star-streams. The constellation Orion is singularly rich in objects of this class. It is here that the strange “variable” Betelgeux lies. At present this star shows no sign of variation, but a few years ago it exhibited remarkable changes. One is invited to believe that the star may have been carried by its proper motion into regions where there is a more uniform distribution of the material whence this orb recruits its fires. It may be that in the consideration of such causes of variation affecting our sun in long past ages a more satisfactory explanation than any yet obtained may be found of the problem geologists find so perplexing—the former existence of a tropical climate in places within the temperate zone, or even near the Arctic regions.20

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It remains that I should exhibit the general results to which I have been led. It has seemed to many that my views tend largely to diminish our estimate of the extent of the sidereal system. The exact reverse is the case. According to accepted views there lie within the range of our most powerful telescopes millions of millions of suns. According to mine the primary suns within the range of our telescopes must be counted by tens of thousands, or by hundreds of thousands at the outside. What does this diminution of numbers imply but that the space separating sun from sun is enormously greater than accepted theories would permit? And this increase implies an enormous increase in the estimate we are to form of the vital energies of individual suns. For the vitality of a sun, if one may be permitted the expression, is measured not merely by the amount of matter over which it exercises control, but by the extent of space within which that matter is distributed. Take an orb a thousand times vaster than our sun, and spread over its surface an amount of matter exceeding a thousandfold the combined mass of all the planets of the solar system:—So far as living force is concerned, the result is—nil. But distribute that matter throughout a vast space all round the orb:—That orb becomes at once fit to be the centre of a host of dependent worlds. Again, according to accepted theories, when the astronomer has succeeded in resolving the milky light of a portion of the galaxy into stars, he has in that direction, at any rate, reached the limits of the sidereal system. According to my views, what he has really done has been but to analyze a definite aggregation of stars, a mere corner of that great system. Yet once more, according to accepted views, thousands and thousands of galaxies, external to the sidereal system, can be seen with powerful telescopes. If I am right, the external star-systems lie far beyond the reach of the most powerful telescope man has yet been able to construct, insomuch that perchance the nearest of the outlying galaxies may lie a million times beyond the range even of the mighty mirror of the great Rosse telescope.

But this is little. Wonderful as is the extent of the sidereal system as thus viewed, even more wonderful is its infinite variety. We know how largely modern discoveries have increased our estimate of the complexity of the planetary system. Where the ancients recognized but a few planets, we now see, besides the planets, the families of satellites; we see the rings of Saturn, in which minute satellites must be as the sands on the sea-shore for multitude; the wonderful zone of asteroids; myriads on myriads of comets; millions on millions of meteor-systems, gathering more and more richly around the sun, until in his neighbourhood they form the crown of glory which bursts into view when he is totally eclipsed. But wonderful as is the variety seen within the planetary system, the variety within the sidereal system is infinitely more amazing. Besides the single suns, there are groups and systems and streams of primary suns; there are whole galaxies of minor orbs; there are clustering stellar aggregations, showing every variety of richness, of figure, and of distribution; there are all the various forms of nebulæ, resolvable and irresolvable, circular, elliptical, and spiral; and lastly, there are irregular masses of luminous gas, clinging in fantastic convolutions around stars and star-systems. Nor is it unsafe to assert that other forms and variety of structure will yet be discovered, or that hundreds more exist which we may never hope to recognize.

But lastly, even more wonderful than the infinite variety of the sidereal system, is its amazing vitality. Instead of millions of inert masses, we see the whole heavens instinct with energy—astir with busy life. The great masses of luminous vapour, though occupying countless millions of cubic miles of space, are moved by unknown forces like clouds before the summer breeze; star-mist is condensing into clusters; star-clusters are forming into suns; streams and clusters of minor orbs are swayed by unknown attractive energies; and primary suns singly or in systems are pursuing their stately path through space, rejoicing as giants to run their course, extending on all sides the mighty arm of their attraction, gathering from ever-new regions of space supplies of motive energy, to be transformed into the various forms of force—light and heat and electricity—and distributed in lavish abundance to the worlds which circle round them.

Truly may I say, in conclusion, that whether we regard its vast extent, its infinite variety, or the amazing vitality which pervades its every portion, the sidereal system is, of all the subjects man can study, the most imposing and the most stupendous. It is as a book full of mighty problems—of problems which are as yet almost untouched by man, of problems which it might seem hopeless for him to attempt to solve. But those problems are given to him for solution, and he will solve them, whenever he dares attempt to decipher aright the records of that wondrous volume.