In the year 1600 (the year in which Giordano Bruno was burned at the stake for teaching that there is more than one physical world), a temporary star of the third magnitude broke out in the constellation Cygnus, and curiously enough, considering the rarity of such phenomena, only four years later another surprisingly brilliant one appeared in the constellation Ophiuchus. This is often called "Kepler's star,'' because the great German astronomer devoted to it the same attention that Tycho had given to the earlier phenomenon. It, too, like Tycho's, was at first the brightest object in the stellar heavens, although it seems never to have quite equaled its famous predecessor in splendor. It disappeared after a year, also turning of a red color as it became more faint. We shall see the significance of this as we go on. Some of Kepler's contemporaries suggested that the outburst of this star was due to a meeting of atoms in space, and idea bearing a striking resemblance to the modern theory of "astronomical collisions.''

In 1670, 1848, and 1860 temporary stars made their appearance, but none of them was of great brilliance. In 1866 one of the second magnitude broke forth in the "Northern Crown'' and awoke much interest, because by that time the spectroscope had begun to be employed in studying the composition of the stars, and Huggins demonstrated that the new star consisted largely of incandescent hydrogen. But this star, apparently unlike the others mentioned, was not absolutely new. Before its outburst it had shown as a star of the ninth magnitude (entirely invisible, of course, to the naked eye), and after about six weeks it faded to its original condition in which it has ever since remained. In 1876 a temporary star appeared in the constellation Cygnus, and attained at one time the brightness of the second magnitude. Its spectrum and its behavior resembled those of its immediate predecessor. In 1885, astronomers were surprised to see a sixth-magnitude star glimmering in the midst of the hazy cloud of the great Andromeda Nebula. It soon absolutely disappeared. Its spectrum was remarkable for being "continuous,'' like that of the nebula itself. A continuous spectrum is supposed to represent a body, or a mass, which is either solid or liquid, or composed of gas under great pressure. In January, 1892, a new star was suddenly seen in the constellation Auriga. It never rose much above the fourth magnitude, but it showed a peculiar spectrum containing both bright and dark lines of hydrogen.

But a bewildering surprise was now in store; the world was to behold at the opening of the twentieth century such a celestial spectacle as had not been on view since the times of Tycho and Kepler. Before daylight on the morning of February 22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur astronomer, who had also been the first to see the new star in Auriga, beheld a strange object in the constellation Perseus not far from the celebrated variable star Algol. He recognized its character at once, and immediately telegraphed the news, which awoke the startled attention of astronomers all over the world. When first seen the new star was no brighter than Algol (less than the second magnitude), but within twenty-four hours it was ablaze, outshining even the brilliant Capella, and far surpassing the first magnitude. At the spot in the sky where it appeared nothing whatever was visible on the night before its coming. This is known with certainty because a photograph had been made of that very region on February 21, and this photograph showed everything down to the twelfth magnitude, but not a trace of the stranger which burst into view between the 21st and the 22nd like the explosion of a rocket.

Upon one who knew the stars the apparition of this intruder in a well-known constellation had the effect of a sudden invasion. The new star was not far west of the zenith in the early evening, and in that position showed to the best advantage. To see Capella, the hitherto unchallenged ruler of that quarter of the sky, abased by comparison with this stranger of alien aspect, for there was always an unfamiliar look about the "nova,'' was decidedly disconcerting. It seemed to portend the beginning of a revolution in the heavens. One could understand what the effect of such an apparition must have been in the superstitious times of Tycho. The star of Tycho had burst forth on the northern border of the Milky Way; this one was on its southern border, some forty-five degrees farther east.

Astronomers were well-prepared this time for the scientific study of the new star, both astronomical photography and spectroscopy having been perfected, and the results of their investigations were calculated to increase the wonder with which the phenomenon was regarded. The star remained at its brightest only a few days; then, like a veritable conflagration, it began to languish; and, like the reflection of a dying fire, as it sank it began to glow with the red color of embers. But its changes were spasmodic; once about every three days it flared up only to die away again. During these fluctuations its light varied alternately in the ratio of one to six. Finally it took a permanent downward course, and after a few months the naked eye could no longer perceive it; but it remained visible with telescopes, gradually fading until it had sunk to the ninth magnitude. Then another astonishing change happened: in August photographs taken at the Yerkes Observatory and at Heidelberg showed that the "nova'' was surrounded by a spiral nebula! The nebula had not been there before, and no one could doubt that it represented a phase of the same catastrophe that had produced the outburst of the new star. At one time the star seemed virtually to have disappeared, as if all its substance had been expanded into the nebulous cloud, but always there remained a stellar nucleus about which the misty spiral spread wider and ever wider, like a wave expanding around a center of disturbance. The nebula too showed a variability of brightness, and four condensations which formed in it seemed to have a motion of revolution about the star. As time went on the nebula continued to expand at a rate which was computed to be not less than twenty thousand miles per second! And now the star itself, showing indications of having turned into a nebula, behaved in a most erratic manner, giving rise to the suspicion that it was about to burst out again. But this did not occur, and at length it sunk into a state of lethargy from which it has to the present time not recovered. But the nebulous spiral has disappeared, and the entire phenomena as it now (1909) exists consists of a faint nebulous star of less than the ninth magnitude.

The wonderful transformations just described had been forecast in advance of the discovery of the nebulous spiral encircling the star by the spectroscopic study of the latter. At first there was no suggestion of a nebular constitution, but within a month or two characteristic nebular lines began to appear, and in less than six months the whole spectrum had been transformed to the nebular type. In the mean time the shifting of the spectral lines indicated a complication of rapid motions in several directions simultaneously. These motions were estimated to amount to from one hundred to five hundred miles per second.

The human mind is so constituted that it feels forced to seek an explanation of so marvelous a phenomenon as this, even in the absence of the data needed for a sound conclusion. The most natural hypothesis, perhaps, is that of a collision. Such a catastrophe could certainly happen. It has been shown, for instance, that in infinity of time the earth is sure to be hit by a comet; in the same way it may be asserted that, if no time limit is fixed, the sun is certain to run against some obstacle in space, either another star, or a dense meteor swarm, or one of the dark bodies which there is every reason to believe abound around us. The consequences of such a collision are easy to foretell, provided that we know the masses and the velocities of the colliding bodies. In a preceding chapter we have discussed the motions of the sun and stars, and have seen that they are so swift that an encounter between any two of them could not but be disastrous. But this is not all; for as soon as two stars approached within a few million miles their speed would be enormously increased by their reciprocal attractions and, if their motion was directed radially with respect to their centers, they would come together with a crash that would reduce them both to nebulous clouds. It is true that the chances of such a "head-on'' collision are relatively very small; two stars approaching each other would most probably fall into closed orbits around their common center of gravity. If there were a collision it would most likely be a grazing one instead of a direct front-to-front encounter. But even a close approach, without any actual collision, would probably prove disastrous, owing to the tidal influence of each of the bodies on the other. Suns, in consequence of their enormous masses and dimensions and the peculiarities of their constitution, are exceedingly dangerous to one another at close quarters. Propinquity awakes in them a mutually destructive tendency. Consisting of matter in the gaseous, or perhaps, in some cases, liquid, state, their tidal pull upon each other if brought close together might burst them asunder, and the photospheric envelope being destroyed the internal incandescent mass would gush out, bringing fiery death to any planets that were revolving near. Without regard to the resulting disturbance of the earth's orbit, the close approach of a great star to the sun would be in the highest degree perilous to us. But this is a danger which may properly be regarded as indefinitely remote, since, at our present location in space, we are certainly far from every star except the sun, and we may feel confident that no great invisible body is near, for if there were one we should be aware of its presence from the effects of its attraction. As to dark nebul which may possibly lie in the track that the solar system is pursuing at the rate of 375,000,000 miles per year, that is another question — and they, too, could be dangerous!

This brings us directly back to "Nova Persei,'' for among the many suggestions offered to explain its outburst, as well as those of other temporary stars, one of the most fruitful is that of a collision between a star and a vast invisible nebula. Professor Seeliger, of Munich, first proposed this theory, but it afterward underwent some modifications from others. Stated in a general form, the idea is that a huge dark body, perhaps an extinguished sun, encountered in its progress through space a widespread flock of small meteors forming a dark nebula. As it plunged into the swarm the friction of the innumerable collisions with the meteors heated its surface to incandescence, and being of vast size it then became visible to us as a new star. Meanwhile the motion of the body through the nebula, and its rotation upon itself, set up a gyration in the blazing atmosphere formed around it by the vaporized meteors; and as this atmosphere spread wider, under the laws of gyratory motion a rotation in the opposite direction began in the inflamed meteoric cloud outside the central part of the vortex. Thus the spectral lines were caused to show motion in opposite directions, a part of the incandescent mass approaching the earth simultaneously with the retreat of another part. So the curious spectroscopic observations before mentioned were explained. This theory might also account for the appearance of the nebulous spiral first seen some six months after the original outburst. The sequent changes in the spectrum of the "nova'' are accounted for by this theory on the assumption, reasonable enough in itself, that at first the invading body would be enveloped in a vaporized atmosphere of relatively slight depth, producing by its absorption the fine dark lines first observed; but that as time went on and the incessant collisions continued, the blazing atmosphere would become very deep and extensive, whereupon the appearance of the spectral lines would change, and bright lines due to the light of the incandescent meteors surrounding the nucleus at a great distance would take the place of the original dark ones. The vortex of meteors once formed would protect the flying body within from further immediate collisions, the latter now occurring mainly among the meteors themselves, and then the central blaze would die down, and the original splendor of the phenomenon would fade.

But the theories about Nova Persei have been almost as numerous as the astronomers who have speculated about it. One of the most startling of them assumed that the outburst was caused by the running amuck of a dark star which had encountered another star surrounded with planets, the renewed outbreaks of light after the principal one had faded being due to the successive running down of the unfortunate planets! Yet another hypothesis is based on what we have already said of the tidal influence that two close approaching suns would have upon each other. Supposing two such bodies which had become encrusted, but remained incandescent and fluid within, to approach within almost striking distance; they would whirl each other about their common center of gravity, and at the same time their shells would burst under the tidal strain, and their glowing nuclei being disclosed would produce a great outburst of light. Applying this theory to a "nova,'' like that of 1866 in the "Northern Crown,'' which had been visible as a small star before the outbreak, and which afterward resumed its former aspect, we should have to assume that a yet shining sun had been approached by a dark body whose attraction temporarily burst open its photosphere. It might be supposed that in this case the dark body was too far advanced in cooling to suffer the same fate from the tidal pull of its victim. But a close approach of that kind would be expected to result in the formation of a binary system, with orbits of great eccentricity, perhaps, and after the lapse of a certain time the outburst should be renewed by another approximation of the two bodies. A temporary star of that kind would rather be ranked as a variable.

The celebrated French astronomer, Janssen, had a different theory of Nova Persei, and of temporary stars in general. According to his idea, such phenomena might be the result of chemical changes taking place in a sun without interference by, or collision with, another body. Janssen was engaged for many years in trying to discover evidence of the existence of oxygen in the sun, and he constructed his observatory on the summit of Mount Blanc specially to pursue that research. He believed that oxygen must surely exist in the sun since we find so many other familiar elements included in the constitution of the solar globe, and as he was unable to discover satisfactory evidence of its presence he assumed that it existed in a form unknown on the earth. If it were normally in the sun's chromosphere, or coronal atmosphere, he said, it would combine with the hydrogen which we know is there and form an obscuring envelope of water vapor. It exists, then, in a special state, uncombined with hydrogen; but let the temperature of the sun sink to a critical point and the oxygen will assume its normal properties and combine with the hydrogen, producing a mighty outburst of light and heat. This, Janssen thought, might explain the phenomena of the temporary stars. It would also, he suggested, account for their brief career, because the combination of the elements would be quickly accomplished, and then the resulting water vapor would form an atmosphere cutting off the radiation from the star within.

This theory may be said to have a livelier human interest than some of the others, since, according to it, the sun may carry in its very constitution a menace to mankind; one does not like to think of it being suddenly transformed into a gigantic laboratory for the explosive combination of oxygen and hydrogen! But while Janssen's theory might do for some temporary stars, it is inadequate to explain all the phenomena of Nova Persei, and particularly the appearance of the great spiral nebula that seemed to exhale from the heart of the star. Upon the whole, the theory of an encounter between a star and a dark nebula seems best to fit the observations. By that hypothesis the expanding billow of light surrounding the core of the conflagration is very well accounted for, and the spectroscopic peculiarities are also explained.

Dr Gustov Le Bon offers a yet more alarming theory, suggesting that temporary stars are the result of atomic explosion; but we shall touch upon this more fully in Chapter 14.

Twice in the course of this discussion we have called attention to the change of color invariably undergone by temporary stars in the later stages of their career. This was conspicuous with Nova Persei which glowed more and more redly as it faded, until the nebulous light began to overpower that of the stellar nucleus. Nothing could be more suggestive of the dying out of a great fire. Moreover, change of color from white to red is characteristic of all variable stars of long period, such as "Mira'' in Cetus. It is also characteristic of stars believed to be in the later stages of evolution, and consequently approaching extinction, like Antares and Betelgeuse, and still more notably certain small stars which "gleam like rubies in the field of the telescope.'' These last appear to be suns in the closing period of existence as self-luminous bodies. Between the white stars, such as Sirius and Rigel, and the red stars, such as Aldebaran and Alpha Herculis, there is a progressive series of colors from golden yellow through orange to deep red. The change is believed to be due to the increase of absorbing vapors in the stellar atmosphere as the body cools down. In the case of ordinary stars these changes no doubt occupy many millions of years, which represent the average duration of solar life; but the temporary stars run through similar changes in a few months: they resemble ephemeral insects — born in the morning and doomed to perish with the going down of the sun.

Explosive and Whirling Nebul

One of the most surprising triumphs of celestial photography was Professor Keeler's discovery, in 1899, that the great majority of the nebul have a distinctly spiral form. This form, previously known in Lord Rosse's great "Whirlpool Nebula,'' had been supposed to be exceptional; now the photographs, far excelling telescopic views in the revelation of nebular forms, showed the spiral to be the typical shape. Indeed, it is a question whether all nebul are not to some extent spiral. The extreme importance of this discovery is shown in the effect that it has had upon hitherto prevailing views of solar and planetary evolution. For more than three-quarters of a century Laplace's celebrated hypothesis of the manner of origin of the solar system from a rotating and contracting nebula surrounding the sun had guided speculation on that subject, and had been tentatively extended to cover the evolution of systems in general. The apparent forms of some of the nebul which the telescope had revealed were regarded, and by some are still regarded, as giving visual evidence in favor of this theory. There is a "ring nebula'' in Lyra with a central star, and a "planetary nebula'' in Gemini bearing no little resemblance to the planet Saturn with its rings, both of which appear to be practical realizations of Laplace's idea, and the elliptical rings surrounding the central condensation of the Andromeda Nebula may be cited for the same kind of proof.

But since Keeler's discovery there has been a decided turning away of speculation another way. The form of the spiral nebul seems to be entirely inconsistent with the theory of an originally globular or disk-shaped nebula condensing around a sun and throwing or leaving off rings, to be subsequently shaped into planets. Some astronomers, indeed, now reject Laplace's hypothesis in toto, preferring to think that even our solar system originated from a spiral nebula. Since the spiral type prevails among the existing nebul, we must make any mechanical theory of the development of stars and planetary systems from them accord with the requirements which that form imposes. A glance at the extraordinary variations upon the spiral which Professor Keeler's photographs reveal is sufficient to convince one of the difficulty of the task of basing a general theory upon them. In truth, it is much easier to criticize Laplace's hypothesis than to invent a satisfactory substitute for it. If the spiral nebul seem to oppose it there are other nebul which appear to support it, and it may be that no one fixed theory can account for all the forms of stellar evolution in the universe. Our particular planetary system may have originated very much as the great French mathematician supposed, while others have undergone, or are now undergoing, a different process of development. There is always a too strong tendency to regard an important new discovery and the theories and speculations based upon it as revolutionizing knowledge, and displacing or overthrowing everything that went before. Upon the plea that "Laplace only made a guess'' more recent guesses have been driven to extremes and treated by injudicious exponents as "the solid facts at last.''

Before considering more recent theories than Laplace's, let us see what the nature of the photographic revelations is. The vast celestial maelstrom discovered by Lord Rosse in the "Hunting Dogs'' may be taken as the leading type of the spiral nebul, although there are less conspicuous objects of the kind which, perhaps, better illustrate some of their peculiarities. Lord Rosse's nebula appears far more wonderful in the photographs than in his drawings made with the aid of his giant reflecting telescope at Parsonstown, for the photographic plate records details that no telescope is capable of showing. Suppose we look at the photograph of this object as any person of common sense would look at any great and strange natural phenomenon. What is the first thing that strikes the mind? It is certainly the appearance of violent whirling motion. One would say that the whole glowing mass had been spun about with tremendous velocity, or that it had been set rotating so rapidly that it had become the victim of "centrifugal force,'' one huge fragment having broken loose and started to gyrate off into space. Closer inspection shows that in addition to the principal focus there are various smaller condensations scattered through the mass. These are conspicuous in the spirals. Some of them are stellar points, and but for the significance of their location we might suppose them to be stars which happen to lie in a line between us and the nebula. But when we observe how many of them follow most faithfully the curves of the spirals we cannot but conclude that they form an essential part of the phenomenon; it is not possible to believe that their presence in such situations is merely fortuitous. One of the outer spirals has at least a dozen of these star-like points strung upon it; some of them sharp, small, and distinct, others more blurred and nebulous, suggesting different stages of condensation. Even the part which seems to have been flung loose from the main mass has, in addition to its central condensation, at least one stellar point gleaming in the half-vanished spire attached to it. Some of the more distant stars scattered around the "whirlpool'' look as if they too had been shot out of the mighty vortex, afterward condensing into unmistakable solar bodies. There are at least two curved rows of minute stars a little beyond the periphery of the luminous whirl which clearly follow lines concentric with those of the nebulous spirals. Such facts are simply dumbfounding for anyone who will bestow sufficient thought upon them, for these are suns, though they may be small ones; and what a birth is that for a sun!

Look now again at the glowing spirals. We observe that hardly have they left the central mass before they begin to coagulate. In some places they have a "ropy'' aspect; or they are like peascods filled with growing seeds, which eventually will become stars. The great focus itself shows a similar tendency, especially around its circumference. The sense that it imparts of a tremendous shattering force at work is overwhelming. There is probably more matter in that whirling and bursting nebula than would suffice to make a hundred solar systems! It must be confessed at once that there is no confirmation of the Laplacean hypothesis here; but what hypothesis will fit the facts? There is one which it has been claimed does so, but we shall come to that later. In the meanwhile, as a preparation, fix in the memory the appearance of that second spiral mass spinning beside its master which seems to have spurned it away.

For a second example of the spiral nebul look at the one in the constellation Triangulum. God, how hath the imagination of puny man failed to comprehend Thee! Here is creation through destruction with a vengeance! The spiral form of the nebula is unmistakable, but it is half obliterated amid the turmoil of flying masses hurled away on all sides with tornadic fury. The focus itself is splitting asunder under the intolerable strain, and in a little while, as time is reckoned in the Cosmos, it will be gyrating into stars. And then look at the cyclonic rain of already finished stars whirling round the outskirts of the storm. Observe how scores of them are yet involved in the fading streams of the nebulous spirals; see how they have been thrown into vast loops and curves, of a beauty that half redeems the terror of the spectacle enclosed within their lines — like iridescent cirri hovering about the edges of a hurricane. And so again are suns born!

Let us turn to the exquisite spiral in Ursa Major; how different its aspect from that of the other! One would say that if the terrific coil in Triangulum has all but destroyed itself in its fury, this one on the contrary has just begun its self-demolition. As one gazes one seems to see in it the smooth, swift, accelerating motion that precedes catastrophe. The central part is still intact, dense, and uniform in texture. How graceful are the spirals that smoothly rise from its oval rim and, gemmed with little stars, wind off into the darkness until they have become as delicate as threads of gossamer! But at bottom the story told here is the same — creation by gyration!

Compare with the above the curious mass in Cetus. Here the plane of the whirling nebula nearly coincides with our line of sight and we see the object at a low angle. It is far advanced and torn to shreds, and if we could look at it perpendicularly to its plane it is evident that it would closely resemble the spectacle in Triangulum.

Then take the famous Andromeda Nebula (see Frontispiece), which is so vast that notwithstanding its immense distance even the naked eye perceives it as an enigmatical wisp in the sky. Its image on the sensitive plate is the masterpiece of astronomical photography; for wild, incomprehensible beauty there is nothing that can be compared with it. Here, if anywhere, we look upon the spectacle of creation in one of its earliest stages. The Andromeda Nebula is apparently less advanced toward transformation into stellar bodies than is that in Triangulum. The immense crowd of stars sprinkled over it and its neighborhood seem in the main to lie this side of the nebula, and consequently to have no connection with it. But incipient stars (in some places clusters of them) are seen in the nebulous rings, while one or two huge masses seem to give promise of transformation into stellar bodies of unusual magnitude. I say "rings'' because although the loops encompassing the Andromeda Nebula have been called spirals by those who wish utterly to demolish Laplace's hypothesis, yet they are not manifestly such, as can be seen on comparing them with the undoubted spirals of the Lord Rosse Nebula. They look quite as much like circles or ellipses seen at an angle of, say, fifteen or twenty degrees to their plane. If they are truly elliptical they accord fairly well with Laplace's idea, except that the scale of magnitude is stupendous, and if the Andromeda Nebula is to become a solar system it will surpass ours in grandeur beyond all possibility of comparison.

There is one circumstance connected with the spiral nebul, and conspicuous in the Andromeda Nebula on account of its brightness, which makes the question of their origin still more puzzling; they all show continuous spectra, which, as we have before remarked, indicate that the mass from which the light comes is either solid or liquid, or a gas under heavy pressure. Thus nebul fall into two classes: the "white'' nebul, giving a continuous spectrum; and the "green'' nebul whose spectra are distinctly gaseous. The Andromeda Nebula is the great representative of the former class and the Orion Nebula of the latter. The spectrum of the Andromeda Nebula has been interpreted to mean that it consists not of luminous gas, but of a flock of stars so distant that they are separately indistinguishable even with powerful telescopes, just as the component stars of the Milky Way are indistinguishable with the naked eye; and upon this has been based the suggestion that what we see in Andromeda is an outer universe whose stars form a series of elliptical garlands surrounding a central mass of amazing richness. But this idea is unacceptable if for no other reason than that, as just said, all the spiral nebul possess the same kind of spectrum, and probably no one would be disposed to regard them all as outer universes. As we shall see later, the peculiarity of the spectra of the spiral nebul is appealed to in support of a modern substitute for Laplace's hypothesis.

Finally, without having by any means exhausted the variety exhibited by the spiral nebul, let us turn to the great representative of the other species, the Orion Nebula. In some ways this is even more marvelous than the others. The early drawings with the telescope failed to convey an adequate conception either of its sublimity or of its complication of structure. It exists in a nebulous region of space, since photographs show that nearly the whole constellation is interwoven with faintly luminous coils. To behold the entry of the great nebula into the field even of a small telescope is a startling experience which never loses its novelty. As shown by the photographs, it is an inscrutable chaos of perfectly amazing extent, where spiral bands, radiating streaks, dense masses, and dark yawning gaps are strangely intermingled without apparent order. In one place four conspicuous little stars, better seen in a telescope than in the photograph on account of the blurring produced by over-exposure, are suggestively situated in the midst of a dark opening, and no observer has ever felt any doubt that these stars have been formed from the substance of the surrounding nebula. There are many other stars scattered over its expanse which manifestly owe their origin to the same source. But compare the general appearance of this nebula with the others that we have studied, and remark the difference. If the unmistakably spiral nebul resemble bursting fly-wheels or grindstones from whose perimeters torrents of sparks are flying, the Orion Nebula rather recalls the aspect of a cloud of smoke and fragments produced by the explosion of a shell. This idea is enforced by the look of the outer portion farthest from the bright half of the nebula, where sharply edged clouds with dark spaces behind seem to be billowing away as if driven by a wind blowing from the center.

Next let us consider what scientific speculation has done in the effort to explain these mysteries. Laplace's hypothesis can certainly find no standing ground either in the Orion Nebula or in those of a spiral configuration, whatever may be its situation with respect to the grand Nebula of Andromeda, or the "ring'' and "planetary'' nebul. Some other hypothesis more consonant with the appearances must be found. Among the many that have been proposed the most elaborate is the "Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. It is to be remarked that it applies to the spiral nebul distinctively, and not to an apparently chaotic mass of gas like the vast luminous cloud in Orion. The gist of the theory is that these curious objects are probably the result of close approaches to each other of two independent suns, reminding us of what was said on this subject when we were dealing with temporary stars. Of the previous history of these appulsing suns the theory gives us no account; they are simply supposed to arrive within what may be called an effective tide-producing distance, and then the drama begins. Some of the probable consequences of such an approach have been noticed in Chapter 5; let us now consider them a little more in detail.

Tides always go in couples; if there is a tide on one side of a globe there will be a corresponding tide on the other side. The cause is to be found in the law that the force of gravitation varies inversely as the square of the distance; the attraction on the nearest surface of the body exercised by another body is greater than on its center, and greater yet than on its opposite surface. If two great globes attract each other, each tends to draw the other out into an ellipsoidal figure; they must be more rigid than steel to resist this — and even then they cannot altogether resist. If they are liquid or gaseous they will yield readily to the force of distortion, the amount of which will depend upon their distance apart, for the nearer they are the greater becomes the tidal strain. If they are encrusted without and liquid or gaseous in the interior, the internal mass will strive to assume the figure demanded by the tidal force, and will, if it can, burst the restraining envelope. Now this is virtually the predicament of the body we call a sun when in the immediate presence of another body of similarly great mass. Such a body is presumably gaseous throughout, the component gases being held in a state of rigidity by the compression produced by the tremendous gravitational force of their own aggregate mass. At the surface such a body is enveloped in a shell of relatively cool matter. Now suppose a great attracting body, such as another sun, to approach near enough for the difference in its attraction on the two opposite sides of the body and on its center to become very great; the consequence will be a tidal deformation of the whole body, and it will lengthen out along the line of the gravitational pull and draw in at the sides, and if its shell offers considerable resistance, but not enough to exercise a complete restraint, it will be violently burst apart, or blown to atoms, and the internal mass will leap out on the two opposite sides in great fiery spouts. In the case of a sun further advanced in cooling than ours the interior might be composed of molten matter while the exterior crust had become rigid like the shell of an egg; then the force of the "tidal explosion'' produced by the appulse of another sun would be more violent in consequence of the greater resistance overcome. Such, then, is the mechanism of the first phase in the history of a spiral nebula according to the Planetesimal Hypothesis. Two suns, perhaps extinguished ones, have drawn near together, and an explosive outburst has occured in one or both. The second phase calls for a more agile exercise of the imagination.

To simplify the case, let us suppose that only one of the tugging suns is seriously affected by the strain. Its vast wings produced by the outburst are twisted into spirals by their rotation and the contending attractions exercised upon them, as the two suns, like battleships in desperate conflict, curve round each other, concentrating their destructive energies. Then immense quantities of dbris are scattered about in which eddies are created, and finally, as the sun that caused the damage goes on its way, leaving its victim to repair its injuries as it may, the dispersed matter cools, condenses, and turns into streams of solid particles circling in elliptical paths about their parent sun. These particles, or fragments, are the "planetesimals'' of the theory. In consequence of the inevitable intersection of the orbits of the planetesimals, nodes are formed where the flying particles meet, and at these nodes large masses are gradually accumulated. The larger the mass the greater its attraction, and at last the nodal points become the nuclei of great aggregations from which planets are shaped.

This, in very brief form, is the Planetesimal Hypothesis which we are asked to substitute for that based on Laplace's suggestion as an explanation of the mode of origin of the solar system; and the phenomena of the spiral nebul are appealed to as offering evident support to the new hypothesis. We are reminded that they are elliptical in outline, which accords with the hypothesis; that their spectra are not gaseous, which shows that they may be composed of solid particles like the planetesimals; and that their central masses present an oval form, which is what would result from the tidal effects, as just described. We also remember that some of them, like the Lord Rosse and the Andromeda nebul, are visually double, and in these cases we might suppose that the two masses represent the tide-burst suns that ventured into too close proximity. It may be added that the authors of the theory do not insist upon the appulse of two suns as the only way in which the planetesimals may have originated, but it is the only supposition that has been worked out.

But serious questions remain. It needs, for instance, but a glance at the Triangulum monster to convince the observer that it cannot be a solar system which is being evolved there, but rather a swarm of stars. Many of the detached masses are too vast to admit of the supposition that they are to be transformed into planets, in our sense of planets, and the distances of the stars which appear to have been originally ejected from the focal masses are too great to allow us to liken the assemblage that they form to a solar system. Then, too, no nodes such as the hypothesis calls for are visible. Moreover, in most of the spiral nebul the appearances favor the view that the supposititious encountering suns have not separated and gone each rejoicing on its way, after having inflicted the maximum possible damage on its opponent, but that, on the contrary, they remain in close association like two wrestlers who cannot escape from each other's grasp. And this is exactly what the law of gravitation demands; stars cannot approach one another with impunity, with regard either to their physical make-up or their future independence of movement. The theory undertakes to avoid this difficulty by assuming that in the case of our system the approach of the foreign body to the sun was not a close one — just close enough to produce the tidal extrusion of the relatively insignificant quantity of matter needed to form the planets. But even then the effect of the appulse would be to change the direction of flight, both of the sun and of its visitor, and there is no known star in the sky which can be selected as the sun's probable partner in their ancient pas deux. That there are unconquered difficulties in Laplace's hypothesis no one would deny, but in simplicity of conception it is incomparably more satisfactory, and with proper modifications could probably be made more consonant with existing facts in our solar system than that which is offered to replace it. Even as an explanation of the spiral nebul, not as solar systems in process of formation, but as the birthplaces of stellar clusters, the Planetesimal Hypothesis would be open to many objections. Granting its assumptions, it has undoubtedly a strong mathematical framework, but the trouble is not with the mathematics but with the assumptions. Laplace was one of the ablest mathematicians that ever lived, but he had never seen a spiral nebula; if he had, he might have invented a hypothesis to suit its phenomena. His actual hypothesis was intended only for our solar system, and he left it in the form of a "note'' for the consideration of his successors, with the hope that they might be able to discover the full truth, which he confessed was hidden from him. It cannot be said that that truth has yet been found, and when it is found the chances are that intuition and not logic will have led to it.

The spiral nebul, then, remain among the greatest riddles of the universe, while the gaseous nebul, like that of Orion, are no less mysterious, although it seems impossible to doubt that both forms give birth to stars. It is but natural to look to them for light on the question of the origin of our planetary system; but we should not forget that the scale of the phenomena in the two cases is vastly different, and the forces in operation may be equally different. A hill may have been built up by a glacier, while a mountain may be the product of volcanic forces or of the upheaval of the strata of the planet.

The Banners of the Sun

As all the world knows, the sun, a blinding globe pouring forth an inconceivable quantity of light and heat, whose daily passage through the sky is caused by the earth's rotation on its axis, constitutes the most important phenomenon of terrestial existence. Viewed with a dark glass to take off the glare, or with a telescope, its rim is seen to be a sharp and smooth circle, and nothing but dark sky is visible around it. Except for the interference of the moon, we should probably never have known that there is any more of the sun than our eyes ordinarily see.

But when an eclipse of the sun occurs, caused by the interposition of the opaque globe of the moon, we see its immediate surroundings, which in some respects are more wonderful than the glowing central orb. These surroundings, although not in the sense in which we apply the term to the gaseous envelope of the earth, may be called the sun's atmosphere. They consist of two very different parts — first, the red "prominences,'' which resemble tongues of flame ascending thousands of miles above the sun's surface; and, second, the "corona,'' which extends to distances of millions of miles from the sun, and shines with a soft, glowing light. The two combined, when well seen, make a spectacle without parallel among the marvels of the sky. Although many attempts have been made to render the corona visible when there is no eclipse, all have failed, and it is to the moon alone that we owe its revelation. To cover the sun's disk with a circular screen will not answer the purpose because of the illumination of the air all about the observer. When the moon hides the sun, on the other hand, the sunlight is withdrawn from a great cylinder of air extending to the top of the atmosphere and spreading many miles around the observer. There is then no glare to interfere with the spectacle, and the corona appears in all its surprising beauty. The prominences, however, although they were discovered during an eclipse, can now, with the aid of the spectroscope, be seen at any time. But the prominences are rarely large enough to be noticed by the naked eye, while the streamers of the corona, stretching far away in space, like ghostly banners blown out from the black circle of the obscuring moon, attract every eye, and to this weird apparition much of the fear inspired by eclipses has been due. But if the corona has been a cause of terror in the past it has become a source of growing knowledge in our time.

The story of the first scientific observation of the corona and the prominences is thrillingly interesting, and in fact dramatic. The observation was made during the eclipse of 1842, which fortunately was visible all over Central and Southern Europe so that scores of astronomers saw it. The interest centers in what happened at Pavia in Northern Italy, where the English astronomer Francis Baily had set up his telescope. The eclipse had begun and Bailey was busy at his telescope when, to quote his own words in the account which he wrote for the Memoirs of the Royal Astronomical Society:

I was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints...

Pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which "made the welkin ring,'' and for the moment withdrew my attention from the object with which I was immediately occupied. I had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but I did not expect, from any of the accounts of preceding eclipses that I had read, to witness so magnificent an exhibition as that which took place...

Splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet I must confess that there was at the same time something in its singular and wonderful appearance that was appalling...

But the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. They had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. They somewhat resembled the tops of the snowy Alpine mountains when colored by the rising or the setting sun. They resembled the Alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona...

The whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored.

I have quoted nearly all of this remarkable description not alone for its intrinsic interest, but because it is the best depiction that can be found of the general phenomena of a total solar eclipse. Still, not every such eclipse offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for instance, which were seen by the writer, the first in South Carolina and the second in Spain, fell far short of that described by Bailey in splendor and impressiveness. Of course, something must be allowed for the effect of surprise; Bailey had not expected to see what was so suddenly disclosed to him. But both in 1900 and 1905 the amount of scattered light in the sky was sufficient in itself to make the corona appear faint, and there were no very conspicuous prominences visible. Yet on both occasions there was manifest among the spectators that mingling of admiration and awe of which Bailey speaks. The South Carolinians gave a cheer and the ladies waved their handkerchiefs when the corona, ineffably delicate of form and texture, melted into sight and then in two minutes melted away again. The Spaniards, crowded on the citadel hill of Burgos, with their king and his royal retinue in their midst, broke out with a great clapping of hands as the awaited spectacle unfolded itself in the sky; and on both occasions, before the applause began, after an awed silence a low murmur ran through the crowds. At Burgos it is said many made the sign of the cross.

It was not long before Bailey's idea that the prominences were a part of the corona was abandoned, and it was perceived that the two phenomena were to a great extent independent. At the eclipse of 1868, which the astronomers, aroused by the wonderful scene of 1842, and eager to test the powers of the newly invented spectroscope, flocked to India to witness, Janssen conceived the idea of employing the spectroscope to render the prominences visible when there was no eclipse. He succeeded the very next day, and these phenomena have been studied in that way ever since.

There are recognized two kinds of prominences — the "erruptive'' and the "quiescent.'' The latter, which are cloud-like in form, may be seen almost anywhere along the edge of the sun; but the former, which often shoot up as if hurled from mighty volcanoes, appear to be associated with sun-spots, and appear only above the zones where spots abound. Either of them, when seen in projection against the brilliant solar disk, appears white, not red, as against a background of sky. The quiescent prominences, whose elevation is often from forty thousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. The latter, it will be remembered, is an element which was known to be in the sun many years before the discovery that it also exists in small quantities on the earth. A fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. The eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors — i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. The velocity of their ascent occasionally amounts to three hundred or four hundred miles per second. It is known from mathematical considerations that the gravitation of the sun would not be able to bring back any body that started from its surface with a velocity exceeding three hundred and eighty-three miles per second; so it is evident that some of the matter hurled forth in eruptive prominences may escape from solar control and go speeding out into space, cooling and condensing into solid masses. There seems to be no reason why some of the projectiles from the sun might not reach the planets. Here, then, we have on a relatively small scale, explosions recalling those which it has been imagined may be the originating cause of some of the sudden phenomena of the stellar heavens.

Of the sun-spots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptive prominences, as well as some relation, not yet fully understood, with the corona. Of the real cause of sun-spots we know virtually nothing, but recent studies by Professor Hale and others have revealed a strange state of things in the clouds of metallic vapors floating above them and their surroundings. Evidences of a cyclonic tendency have been found, and Professor Hale has proved that sun-spots are strong magnetic fields, and consist of columns of ionized vapors rotating in opposite directions in the two hemispheres. A fact which may have the greatest significance is that titanium and vanadium have been found both in sun-spots and in the remarkable variable Mira Ceti, a star which every eleven months, or thereabout, flames up with great brilliancy and then sinks back to invisibility with the naked eye. It has been suggested that sun-spots are indications of the beginning of a process in the sun which will be intensified until it falls into the state of such a star as Mira. Stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so that there is much reason for regarding sunspots as emblems of advancing age.

The association of the corona with sun-spots is less evident than that of the eruptive prominences; still such an association exists, for the form and extent of the corona vary with the sun-spot period of which we shall presently speak. The constitution of the corona remains to be discovered. It is evidently in part gaseous, but it also probably contains matter in the form of dust and small meteors. It includes one substance altogether mysterious — "coronium.'' There are reasons for thinking that this may be the lightest of all the elements, and Professor Young, its discoverer, said that it was "absolutely unique in nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical.'' The enormous extent of the corona is one of its riddles. Since the development of the curious subject of the "pressure of light'' it has been proposed to account for the sustentation of the corona by supposing that it is borne upon the billows of light continually poured out from the sun. Experiment has proved, what mathematical considerations had previously pointed out as probable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon is sufficiently small. In that case the light pressure will prevail over the attraction of gravitation, and propel the attenuated matter away from the sun in the teeth of its attraction. The earth itself would be driven away if, instead of consisting of a solid globe of immense aggregate mass, it were a cloud of microscopic particles. The reason is that the pressure varies in proportion to the surface of the body acted upon, while the gravitational attraction is proportional to the volume, or the total amount of matter in the body. But the surface of any body depends upon the square of its diameter, while the volume depends upon the cube of the diameter. If, for instance, the diameter is represented by 4, the surface will be proportional to 4 4, or 16, and the volume to 4 4 4, or 64; but if the diameter is taken as 2, the surface will be 2 2, or 4, and the volume 2 2 2, or 8. Now, the ratio of 4 to 8 is twice as great as that of 16 to 64. If the diameter is still further decreased, the ratio of the surface to the volume will proportionally grow larger; in other words, the pressure will gain upon the attraction, and whatever their original ratio may have been, a time will come, if the diminution of size continues, when the pressure will become more effective than the attraction, and the body will be driven away. Supposing the particles of the corona to be below the critical size for the attraction of a mass like that of the sun to control them, they would be driven off into the surrounding space and appear around the sun like the clouds of dust around a mill. We shall return to this subject in connection with the Zodiacal Light, the Aurora, and Comets.

On the other hand, there are parts of the corona which suggest by their forms the play of electric or magnetic forces. This is beautifully shown in some of the photographs that have been made of the corona during recent eclipses. Take, for instance, that of the eclipse of 1900. The sheaves of light emanating from the poles look precisely like the "lines of force'' surrounding the poles of a magnet. It will be noticed in this photograph that the corona appears to consist of two portions: one comprising the polar rays just spoken of, and the other consisting of the broader, longer, and less-defined masses of light extending out from the equatorial and middle-latitude zones. Yet even in this more diffuse part of the phenomenon one can detect the presence of submerged curves bearing more or less resemblance to those about the poles. Just what part electricity or electro-magnetism plays in the mechanism of the solar radiation it is impossible to say, but on the assumption that it is a very important part is based the hypothesis that there exists a direct solar influence not only upon the magnetism, but upon the weather of the earth. This hypothesis has been under discussion for half a century, and still we do not know just how much truth it represents. It is certain that the outbreak of great disturbances on the sun, accompanied by the formation of sun-spots and the upshooting of eruptive prominences (phenomena which we should naturally expect to be attended by action), have been instantly followed by corresponding "magnetic storms'' on the earth and brilliant displays of the auroral lights. There have been occasions when the influence has manifested itself in the most startling ways, a great solar outburst being followed by a mysterious gripping of the cable and telegraph systems of the world, as if an invisible and irresistible hand had seized them. Messages are abruptly cut off, sparks leap from the telegraph instruments, and the entire earth seems to have been thrown into a magnetic flurry. These occurrences affect the mind with a deep impression of the dependence of our planet on the sun, such as we do not derive from the more familiar action of the sunlight on the growth of plants and other phenomena of life depending on solar influences.

Perhaps the theory of solar magnetic influence upon the weather is best known in connection with the "sun-spot cycle.'' This, at any rate, is, as already remarked, closely associated with the corona. Its existence was discovered in 1843 by the German astronomer Schwabe. It is a period of variable length, averaging about eleven years, during which the number of spots visible on the sun first increases to a maximum, then diminishes to a minimum, and finally increases again to a maximum. For unknown reasons the period is sometimes two or three years longer than the average and sometimes as much shorter. Nevertheless, the phenomena always recur in the same order. Starting, for instance, with a time when the observer can find few or no spots, they gradually increase in number and size until a maximum, in both senses, is reached, during which the spots are often of enormous size and exceedingly active. After two or three years they begin to diminish in number, magnitude, and activity until they almost or quite disappear. A strange fact is that when a new period opens, the spots appear first in high northern and southern latitudes, far from the solar equator, and as the period advances they not only increase in number and size, but break out nearer and nearer to the equator, the last spots of a vanishing period sometimes lingering in the equatorial region after the advance-guard of its successor has made its appearance in the high latitudes. Spots are never seen on the equator nor near the poles. It was not very long after the discovery of the sun-spot cycle that the curious observation was made that a striking coincidence existed between the period of the sun-spots and another period affecting the general magnetic condition of the earth. When a curved line representing the varying number of sun-spots was compared with another curve showing the variations in the magnetic state of the earth the two were seen to be in almost exact accord, a rise in one curve corresponding to a rise in the other, and a fall to a fall. Continued observation has proved that this is a real coincidence and not an accidental one, so that the connection, although as yet unexplained, is accepted as established. But does the influence extend further, and directly affect the weather and the seasons as well as the magnetic elements of the earth? A final answer to this question cannot yet be given, for the evidence is contradictory, and the interpretations put upon it depend largely on the predilections of the judges.

But, in a broad sense, the sun-spots and the phenomena connected with them must have a relation to terrestial meteorology, for they prove the sun to be a variable star. Reference was made, a few lines above, to the resemblance of the spectra of sun-spots to those of certain stars which seem to be failing through age. This in itself is extremely suggestive; but if this resemblance had never been discovered, we should have been justified in regarding the sun as variable in its output of energy; and not only variable, but probably increasingly so. The very inequalities in the sun-spot cycle are suspicious. When the sun is most spotted its total light may be reduced by one-thousandth part, although it is by no means certain that its outgiving of thermal radiations is then reduced. A loss of one-thousandth of its luminosity would correspond to a decrease of .0025 of a stellar magnitude, considering the sun as a star viewed from distant space. So slight a change would not be perceptible; but it is not alone sun-spots which obscure the solar surface, its entire globe is enveloped with an obscuring veil. When studied with a powerful telescope the sun's surface is seen to be thickly mottled with relatively obscure specks, so numerous that it has been estimated that they cut off from one-tenth to one-twentieth of the light that we should receive from it if the whole surface were as brilliant as its brightest parts. The condition of other stars warrants the conclusion that this obscuring envelope is the product of a process of refrigeration which will gradually make the sun more and more variable until its history ends in extinction. Looking backward, we see a time when the sun must have been more brilliant than it is now. At that time it probably shone with the blinding white splendor of such stars as Sirius, Spica, and Vega; now it resembles the relatively dull Procyon; in time it will turn ruddy and fall into the closing cycle represented by Antares. Considering that once it must have been more radiantly powerful than at present, one is tempted to wonder if that could have been the time when tropical life flourished within the earth's polar circles, sustained by a vivific energy in the sun which it has now lost.

The corona, as we have said, varies with the sun-spot cycle. When the spots are abundant and active the corona rises strong above the spotted zones, forming immense beams or streamers, which on one occasion, at least, had an observed length of ten million miles. At the time of a spot minimum the corona is less brilliant and has a different outline. It is then that the curved polar rays are most conspicuous. Thus the vast banners of the sun, shaken out in the eclipse, are signals to tell of its varying state, but it will probably be long before we can read correctly their messages.

The Zodiacal Light Mystery

There is a singular phenomenon in the sky — one of the most puzzling of all — which has long arrested the attention of astronomers, defying their efforts at explanation, but which probably not one in a hundred, and possibly not one in a thousand, of the readers of this book has ever seen. Yet its name is often spoken, and it is a conspicuous object if one knows when and where to look for it, and when well seen it exhibits a mystical beauty which at the same time charms and awes the beholder. It is called "The Zodiacal Light,'' because it lies within the broad circle of the Zodiac, marking the sun's apparent annual path through the stars. What it is nobody has yet been able to find out with certainty, and books on astronomy usually speak of it with singular reserve. But it has given rise to many remarkable theories, and a true explanation of it would probably throw light on a great many other celestial mysteries. The Milky Way is a more wonderful object to look upon, but its nature can be comprehended, while there is a sort of uncanniness about the Zodiacal Light which immediately impresses one upon seeing it, for its part in the great scheme of extra-terrestrial affairs is not evident.

If you are out-of-doors soon after sunset — say, on an evening late in the month of February — you may perceive, just after the angry flush of the dying winter's day has faded from the sky, a pale ghostly presence rising above the place where the sun went down. The writer remembers from boyhood the first time it was pointed out to him and the unearthly impression that it made, so that he afterward avoided being out alone at night, fearful of seeing the spectral thing again. The phenomenon brightens slowly with the fading of the twilight, and soon distinctly assumes the shape of an elongated pyramid of pearly light, leaning toward the south if the place of observation is in the northern hemisphere. It does not impress the observer at all in the same manner as the Milky Way; that looks far off and is clearly among the stars, but the Zodiacal Light seems closer at hand, as if it were something more intimately concerning the earth. To all it immediately suggests a connection, also, with the sunken sun. If the night is clear and the moon absent (and if you are in the country, for city lights ruin the spectacles of the sky), you will be able to watch the apparition for a long time. You will observe that the light is brightest near the horizon, gradually fading as the pyramidal beam mounts higher, but in favorable circumstances it may be traced nearly to the meridian south of the zenith, where its apex at last vanishes in the starlight. It continues visible during the evenings of March and part of April, after which, ordinarily, it is seen no more, or if seen is relatively faint and unimpressive. But when autumn comes it appears again, this time not like a wraith hovering above the westward tomb of the day-god, but rather like a spirit of the morning announcing his reincarnation in the east.

The reason why the Zodiacal Light is best seen in our latitudes at the periods just mentioned is because at those times the Zodiac is more nearly perpendicular to the horizon, first in the west and then in the east; and, since the phenomenon is confined within the borders of the Zodiac, it cannot be favorably placed for observation when the zodiacal plane is but slightly inclined to the horizon. Its faint light requires the contrast of a background of dark sky in order to be readily perceptible. But within the tropics, where the Zodiac is always at a favorable angle, the mysterious light is more constantly visible. Nearly all observant travelers in the equatorial regions have taken particular note of this phenomenon, for being so much more conspicuous there than in the temperate zones it at once catches the eye and holds the attention as a novelty. Humboldt mentions it many times in his works, for his genius was always attracted by things out of the ordinary and difficult of explanation, and he made many careful observations on its shape, its brilliancy, and its variations; for there can be no doubt that it does vary, and sometimes to an astonishing degree. It is said that it once remained practically invisible in Europe for several years in succession. During a trip to South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a remarkable difference between the appearance of the Zodiacal Light on his going and coming voyages. In fact, when crossing the equator going south he did not see it at all; but on returning he had, on March 6th, when one degree south of the equator, a memorable view of it.

It was a bright, clear night, and the Zodiacal Light was extraordinarily brilliant — brighter than he had ever seen it before. The Milky Way was not to be compared with it. The brightest part extended 75 from the sun. There was a faint and much narrower extension which they could just make out beyond the Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed as a broad truncated column, and it did not appear nearly as conical as he had before seen it.

When out of the brief twilight of intertropical lands, where the sun drops vertically to the horizon and night rushes on like a wave of darkness, the Zodiacal Light shoots to the very zenith, its color is described as a golden tint, entirely different from the silvery sheen of the Milky Way. If I may venture again to refer to personal experiences and impressions, I will recall a view of the Zodiacal Light from the summit of the cone of Mt Etna in the autumn of the year 1896 (more briefly described in Astronomy with the Naked Eye). There are few lofty mountains so favorably placed as Etna for observations of this kind. It was once resorted to by Prof. George E. Hale, in an attempt to see the solar corona without an eclipse. Rising directly from sea-level to an elevation of nearly eleven thousand feet, the observer on its summit at night finds himself, as it were, lost in the midst of the sky. But for the black flanks of the great cone on which he stands he might fancy himself to be in a balloon. On the occasion to which I refer the world beneath was virtually invisible in the moonless night. The blaze of the constellations overhead was astonishingly brilliant, yet amid all their magnificence my attention was immediately drawn to a great tapering light that sprang from the place on the horizon where the sun would rise later, and that seemed to be blown out over the stars like a long, luminous veil. It was the finest view of the Zodiacal light that I had ever enjoyed — thrilling in its strangeness — but I was almost disheartened by the indifference of my guide, to whom it was only a light and nothing more. If he had no science, he had less poetry — rather a remarkable thing, I thought, for a child of his clime. The Light appeared to me to be distinctly brighter than the visible part of the Milky Way which included the brilliant stretches in Auriga and Perseus, and its color, if one may speak of color in connection with such an object, seemed richer than that of the galactic band; but I did not think of it as yellow, although Humboldt has described it as resembling a golden curtain drawn over the stars, and Du Chaillu in Equatorial Africa found it of a bright yellow color. It may vary in color as in conspicuousness. The fascination of that extraordinary sight has never faded from my memory. I turned to regard it again and again, although I had never seen the stellar heavens so brilliant, and it was one of the last things I looked for when the morning glow began softly to mount in the east, and Sicily and the Mediterranean slowly emerged from the profound shadow beneath us.

The Zodiacal Light seems never to have attracted from astronomers in general the amount of careful attention that it deserves; perhaps because so little can really be made of it as far as explanation is concerned. I have referred to the restraint that scientific writers apparently feel in speaking of it. The grounds for speculation that it affords may be too scanty to lead to long discussions, yet it piques curiosity, and as we shall see in a moment has finally led to a most interesting theory. Once it was the subject of an elaborate series of studies which carried the observer all round the world. That was in 1845—46, during the United States Exploring Expedition that visited the then little known Japan. The chaplain of the fleet, the Rev. Mr Jones, went out prepared to study the mysterious light in all its phases. He saw it from many latitudes on both sides of the equator, and the imagination cannot but follow him with keen interest in his world-circling tour, keeping his eyes every night fixed upon the phantasm overhead, whose position shifted with that of the hidden sun. He demonstrated that the flow extends at times completely across the celestial dome, although it is relatively faint directly behind the earth. On his return the government published a large volume of his observations, in which he undertook to show that the phenomenon was due to the reflection of sunlight from a ring of meteoric bodies encircling the earth. But, after all, this elaborate investigation settled nothing.

Prof. E. E. Barnard has more recently devoted much attention to the Zodiacal Light, as well as to a strange attendant phenomenon called the "Gegenschein,'' or Counterglow, because it always appears at that point in the sky which is exactly opposite the sun. The Gegenschein is an extremely elusive phenomenon, suitable only for eyes that have been specially trained to see it. Professor Newcomb has cautiously remarked that

it is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light... This phenomenon is so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside.

It certainly cannot be set aside at all since the observations of Barnard. I recall an attempt to see it under his guidance during a visit to Mount Hamilton, when he was occupied there with the Lick telescope. Of course, both the Gegenschein and the Zodiacal Light are too diffuse to be studied with telescopes, which, so to speak, magnify them out of existence. They can only be successfully studied with the naked eye, since every faintest glimmer that they afford must be utilized. This is especially true of the Gegenschein. At Mount Hamilton, Mr Barnard pointed out to me its location with reference to certain stars, but with all my gazing I could not be sure that I saw it. To him, on the contrary, it was obvious; he had studied it for months, and was able to indicate its shape, its boundaries, its diameter, and the declination of its center with regard to the ecliptic. There is not, of course, the shadow of a doubt of the existence of the Gegenschein, and yet I question if one person in a million has ever seen or ever will see it. The Zodiacal Light, on the other hand, is plain enough, provided that the time and the circumstances of the observation are properly chosen.

In the attempts to explain the Zodiacal Light, the favorite hypothesis has been that it is an appendage of the sun — perhaps simply an extension of the corona in the plane of the ecliptic, which is not very far from coinciding with that of the sun's equator. This idea is quite a natural one, because of the evident relation of the light to the position of the sun. The vast extension of the equatorial wings of the corona in 1878 gave apparent support to this hypothesis; if the substance of the corona could extend ten million miles from the sun, why might it not extend even one hundred million, gradually fading out beyond the orbit of the earth? A variation of this hypothesis assumes that the reflection is due to swarms of meteors circling about the sun, in the plane of its equator, all the way from its immediate neighborhood to a distance exceeding that of the earth. But in neither form is the hypothesis satisfactory; there is nothing in the appearance of the corona to indicate that it extends even as far as the planet Mercury, while as to meteors, the orbits of the known swarms do not accord with the hypothesis, and we have no reason to believe that clouds of others exist traveling in the part of space where they would have to be in order to answer the requirements of the theory. The extension of the corona in 1878 did not resemble in its texture the Zodiacal Light.

Now, it has so often happened in the history of science that an important discovery in one branch has thrown unexpected but most welcome light upon some pending problem in some other branch, that a strong argument might be based upon that fact alone against the too exclusive devotion of many investigators to the narrow lines of their own particular specialty; and the Zodiacal Light affords a case in point, when it is considered in connection with recent discoveries in chemistry and physics. From the fact that atoms are compound bodies made up of corpuscles at least a thousand times smaller than the smallest known atom — a fact which astounded most men of science when it was announced a few years ago — a new hypothesis has been developed concerning the nature of the Zodiacal Light (as well as other astronomical riddles), and this hypothesis comes not from an astronomer, but from a chemist and physicist, the Swede, Svante Arrhenius. In considering an outline of this new hypothesis we need neither accept nor reject it; it is a case rather for suspension of judgment.

To begin with, it carries us back to the "pressure of light'' mentioned in the preceding chapter. The manner in which this pressure is believed generally to act was there sufficiently explained, and it only remains to see how it is theoretically extended to the particles of matter supposed to constitute the Zodiacal Light. We know that corpuscles, or "fragments of atoms'' negatively electrified, are discharged from hot bodies. Streams of these "ions'' pour from many flames and from molten metals; and the impact of the cathode and ultra-violet rays causes them to gush even from cold bodies. In the vast laboratory of the sun it is but reasonable to suppose that similar processes are taking place. "As a very hot metal emits these corpuscles,'' says Prof. J. J. Thomson, "it does not seem an improbable hypothesis that they are emitted by that very hot body, the sun.'' Let it be assumed, then, that the sun does emit them; what happens next? Negatively charged corpuscles, it is known, serve as nuclei to which particles of matter in the ordinary state are attracted, and it is probable that those emitted from the sun immediately pick up loads in this manner and so grow in bulk. If they grow large enough the gravitation of the sun draws them back, and they produce a negative charge in the solar atmosphere. But it is probable that many of the particles do not attain the critical size which, according to the principles before explained, would enable the gravitation of the sun to retain them in opposition to the pressure of the waves of light, and with these particles the light pressure is dominant. Clouds of them may be supposed to be continually swept away from the sun into surrounding space, moving mostly in or near the plane of the solar equator, where the greatest activity, as indicated by sunspots and related phenomena, is taking place. As they pass outward into space many of them encounter the earth. If the earth, like the moon, had no atmosphere the particles would impinge directly on its surface, giving it a negative electric charge. But the presence of the atmosphere changes all that, for the first of the flying particles that encounter it impart to it their negative electricity, and then, since like electric charges repel like, the storm of particles following will be sheered off from the earth, and will stream around it in a maze of hyperbolic paths. Those that continue on into space beyond the earth may be expected to continue picking up wandering particles of matter until their bulk has become so great that the solar attraction prevails again over the light pressure acting upon them, and they turn again sunward. Passing the earth on their return they will increase the amount of dust-clouds careering round it; and these will be further increased by the action of the ultra-violet rays of the sunlight causing particles to shoot radially away from the earth when the negative charge of the upper atmosphere has reached a certain amount, which particles, although starting sunward, will be swept back to the earth with the oncoming streams. As the final result of all this accumulation of flying and gyrating particles in the earth's neighborhood, we are told that the latter must be transformed into the semblance of a gigantic solid-headed comet provided with streaming tails, the longest of them stretching away from the direction of the sun, while another shorter one extends toward the sun. This shorter tail is due to the particles that we have just spoken of as being driven sunward from the earth by the action of ultra-violet light. No doubt this whole subject is too technical for popular statement; but at any rate the general reader can understand the picturesque side of the theory, for its advocates assure us that if we were on the moon we would doubtless be able to see the comet-like tails of the earth, and then we could appreciate the part that they play in producing the phenomenon of the Zodiacal Light.

That the Light as we see it could be produced by the reflection of sunlight from swarms of particles careering round the earth in the manner supposed by Arrhenius' hypothesis is evident enough; and it will be observed that the new theory, after all, is only another variant of the older one which attributes the Zodiacal Light to an extension of the solar corona. But it differs from the older theory in offering an explanation of the manner in which the extension is effected, and it differentiates between the corona proper and the streams of negative particles shot away from the sun. In its details the hypothesis of Arrhenius also affords an explanation of many peculiarities of the Zodiacal Light, such as that it is confined to the neighborhood of the ecliptic, and that it is stronger on the side of the earth which is just turning away from a position under the sun than on the other side; but it would carry us beyond our limits to go into these particulars. The Gegenschein, according to this theory, is a part of the same phenomenon as the Zodiacal Light, for by the laws of perspective it is evident that the reflection from the streams of particles situated at a point directly opposite to the sun would be at a maximum, and this is the place which the Gegenschein occupies. Apart from its geometrical relations to the position of the sun, the variability of the Zodiacal Light appears to affirm its solar dependence, and this too would be accounted for by Arrhenius' hypothesis better than by the old theory of coronal extension. The amount of corpuscular discharge from the sun must naturally be governed by the state of relative activity or inactivity of the latter, and this could not but be reflected in the varying splendor of the Zodiacal Light. But much more extended study than has yet been given to the subject will be required before we can feel that we know with reasonable certainty what this mysterious phenomenon really is. By the hypothesis of Arrhenius every planet that has an atmosphere must have a Zodiacal Light attending it, but the phenomenon is too faint for us to be able to see it in the case, for instance, of Venus, whose atmosphere is very abundant. The moon has no corresponding "comet's tail'' because, as already explained, of the lack of a lunar atmosphere to repel the streams by becoming itself electrified; but if there were a lunar Zodiacal Light, no doubt we could see it because of the relative nearness of our satellite.

Marvels of the Aurora

One of the most vivid recollections of my early boyhood is that of seeing my father return hastily into the house one evening and call out to the family: "Come outside and look at the sky!'' Ours was a country house situated on a commanding site, and as we all emerged from the doorway we were dumbfounded to see the heavens filled with pale flames which ran licking and quivering over the stars. Instantly there sprang into my terrified mind the recollection of an awful description of "the Day of Judgment'' (the Dies Ir), which I had heard with much perturbation of spirit in the Dutch Reformed church from the lips of a tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned type. My heart literally sank at sight of the spectacle, for it recalled the preacher's very words; it was just as he had said it would be, and it needed the assured bearing of my elders finally to convince me that