Chapter XII: The Origin of the Constellation-Figures 332 (7)

This evidence seems to me to suggest that the intense heat which suddenly affected this star had its origin from without. At the same time, I cannot agree with Meyer and Klein in considering that the cause of the heat was either the downfall of a planetary mass on the star, or the collision of the star with a star-cloudlet, or nebula, traversing space in one direction while the star swept 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 its sun, at or near the same part of the planet's path. For several circuits this would continue, the grazes not becoming 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 its 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 the successive grazes would be indicated to us by accessions in the star's lustre, the period between each seeming outburst being only a few months at first, and becoming gradually 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 nebulae. There may be small clouds of the same sort scattered much more densely through space; but we have not a particle of evidence that this actually is the case. All we certainly _know_ about star-cloudlets suggest 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.

On the whole, the theory seems more probable than any of these, that enormous flights of large meteoric masses travel around those stars which thus occasionally break forth in conflagration, such flights travelling on exceedingly eccentric paths, and requiring enormously long periods to complete each circuit of their vast orbits. In conceiving this, we are not imagining anything new. Such a meteoric flight would differ only in degree not kind from meteoric flights which are known to circle around our own sun. I am not sure, indeed, that it can be definitely asserted that our sun has no meteoric appendages of the same nature as those which, if this theory be true, excite to intense periodic activity the sun round which they circle. We know that comets and meteors are closely connected, every comet being probably (many certainly) attended by flights of meteoric masses. The meteors which produce the celebrated November showers of falling stars follow in the track of a comet invisible to the naked eye. May we not reasonably suppose, then, that those glorious comets which have not only been visible but conspicuous, shining even in the day-time, and brandishing round 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? Now some among 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 J. 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 on 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 the 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.

We have had evidence of the tremendous heat to which the sun's surface would be excited by the downfall of a shower of large meteoric masses. Carrington and Hodgson, on September 1, 1859, observed (independently) the passage of two intensely bright bodies across a small part of the sun's surface--the bodies first increasing in brightness, then diminishing, then fading away. It is generally believed that these were meteoric masses raised to fierce heat by frictional resistance. Now so much brighter did they appear, or rather did that part of the sun's surface appear through which they had rushed, that Carrington supposed the dark glass screen used to protect the eye had broken, and Hodgson described the brightness of this part of the sun as such that the part shone like a brilliant star on the background of the glowing solar surface. Mark, also, the consequences of the downfall of those two bodies only. A magnetic disturbance affected the whole frame of the earth at the very time when the sun had been thus disturbed. Vivid auroras were seen not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. 'By degrees,' says Sir J. Herschel, 'accounts began to pour in of great auroras seen not only in these latitudes, but at Rome, in the West Indies, in the tropics within eighteen degrees of the equator (where they hardly ever appear); nay, what is still more striking, in South America and in Australia--where, at Melbourne, on the night of September 2, the greatest aurora ever seen there made its appearance. These auroras were accompanied with unusually great electro-magnetic disturbances in every part of the world. In many places the telegraph wires struck work. They had too many private messages of their own to convey. At Washington and Philadelphia, in America, the electric signal-men 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.' Seeing that where the two meteors fell the sun's surface glowed thus intensely, and that the effect of this accession of energy upon our earth was thus well marked, can it be doubted that a comet, bearing in its train a flight of many millions of meteoric masses, and falling directly upon the sun, would produce an accession of light and heat whose consequences would be disastrous? 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 train 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 abode of life, would inevitably be destroyed.

The reader must not suppose that this idea has been suggested merely in the attempt to explain outbursts of stars. The following passage from a paper of considerable scientific interest by Professor Kirkwood, of Bloomington, Indiana, a well-known American astronomer, shows that the idea had occurred to him for a very different reason. He speaks here of a probable connection between the comet of 1843 and the great sun-spot which appeared in June 1843. I am not sure, however, but that we may regard the very meteors which seem to have fallen on the sun on September 1, 1859, as bodies travelling in the track of the comet of 1843--just as the November meteors seen in 1867-8, 9, etc., until 1872, were bodies certainly following in the track of the telescopic comet of 1866. 'The opinion has been expressed by more than one astronomer,' he says, speaking of Carrington's observation, 'that this phenomenon was produced by the fall of meteoric matter upon the sun's surface. Now, the fact may be worthy of note that the comet of 1843 actually grazed the sun's atmosphere about three months before the appearance of the great sun-spot of the same year. Had it approached but little nearer, the resistance of the atmosphere would probably have brought its 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.'

There are those, myself among the number, who consider the periodicity of the solar spots, that tide of spots which flows to its maximum and then ebbs to its minimum in a little more than eleven years, as only explicable on the theory that a small comet having this period, and followed by a meteor train, has a path intersecting the sun's surface. In an article entitled 'The Sun a Bubble,' which appeared in the 'Cornhill Magazine' for October 1874, I remarked that from the observed phenomena of sun-spots we might be led to suspect the existence of some as yet undetected comet with a train of exceptionally large meteoric masses, travelling in a period of about eleven years round the sun, and having its place of nearest approach to that orb so close to the solar surface that, when the main flight is passing, the stragglers fall upon the sun's surface. In this case, we could readily understand that, as this small comet unquestionably causes our sun to be variable to some slight degree in brilliancy, in a period of about eleven years, so some much larger comet circling around Mira, in a period of about 331 days, may occasion those alternations of brightness which have been described above. It may be noticed in passing, that it is by no means certain that the time when the sun is most spotted is the time when he gives out least light. Though at such times his surface is dark where the spots are, yet elsewhere it is probably brighter than usual; at any rate, all the evidence we have tends to show that when the sun is most spotted, his energies are most active. It is then that the coloured flames leap to their greatest height and show their greatest brilliancy, then also that they show the most rapid and remarkable changes of shape.

Supposing there really is, I will not say danger, but a possibility, that our sun may one day, through the arrival of some very large comet travelling directly towards him, share the fate of the suns whose outbursts I have described above, we might be destroyed unawares, or we might be aware for several weeks of the approach of the destroying comet. Suppose, for example, the comet, which might arrive from any part of the heavens, came from out that part of the star-depths which is occupied by the constellation Taurus--then, if the arrival were so timed that the comet, which might reach the sun at any time, fell upon him in May or June, we should know nothing of that comet's approach: for it would approach in that part of the heavens which was occupied by the sun, and his splendour would hide as with a veil the destroying enemy. On the other hand, if the comet, arriving from the same region of the heavens, so approached as to fall upon the sun in November or December, we should see it for several weeks. For it would then approach from the part of the heavens high above the southern horizon at midnight. Astronomers would be able in a few days after it was discovered to determine its path and predict its downfall upon the sun, precisely as Newton calculated the path of _his_ comet and predicted its near approach to the sun. It would be known for weeks then that the event which Newton contemplated as likely to cause a tremendous outburst of solar heat, competent to destroy all life upon the surface of our earth, was about to take place; and, doubtless, the minds of many students of science would be exercised during that interval in determining whether Newton was right or wrong. For my own part, I have very little doubt that, though the change in the sun's condition in consequence of the direct downfall upon his surface of a very large comet would be but temporary, and in that sense slight--for what are a few weeks in the history of an orb which has already existed during thousands of millions of years?--yet the effect upon the inhabitants of the earth would be by no means slight. I do not think, however, that any students of science would remain, after the catastrophe, to estimate or to record its effects.

Fortunately, all that we have learned hitherto from the stars favours the belief that, while a catastrophe of this sort may be possible, it is exceedingly unlikely. We may estimate the probabilities precisely in the same way that an insurance company estimates the chance of a railway accident. Such a company considers the number of accidents which occur among a given number of railway journeys, and from the smallness of the number of accidents compared with the largeness of the number of journeys estimates the safety of railway travelling. Our sun is one among many millions of suns, any one of which (though all but a few thousands are actually invisible) would become visible to the naked eye, if exposed to the same conditions as have affected the suns in flames described in the preceding pages. Seeing, then, that during the last two thousand years or thereabouts, only a few instances of the kind, certainly not so many as twenty, have been recorded, while there is reason to believe that some of these relate to the same star which has blazed out more than once, we may fairly consider the chance exceedingly small that during the next two thousand, or even the next twenty thousand years, our sun will be exposed to a catastrophe of the kind.

We might arrive at this conclusion independently of any considerations tending to show that our sun belongs to a safe class of system-rulers, and that all, or nearly all, the great solar catastrophes have occurred among suns of a particular class. There are, however, several considerations of the kind which are worth noting.

In the first place, we may dismiss as altogether unlikely the visit of a comet from the star-depths to our sun, on a course carrying the comet directly upon the sun's surface. But if, among the comets travelling in regular attendance upon the sun, there be one whose orbit intersects the sun's globe, then that comet must several times ere this have struck the sun, raising him temporarily to a destructive degree of heat. Now, such a comet must have a period of enormous length, for the races of animals now existing upon the earth must all have been formed since that comet's last visit--on the assumption, be it remembered, that the fall of a large comet upon the sun, or rather the direct passage of the sun through the meteoric nucleus of a large comet, would excite the sun to destructive heat. If all living creatures on the earth are to be destroyed when some comet belonging to the solar system makes its next return to the sun, that same comet at its last visit must have raised the sun to an equal, or even greater intensity of heat, so that either no such races as at present exist had then come into being, or, if any such existed, they must at that time have been utterly destroyed. We may fairly believe that all comets of the destructive sort have been eliminated. Judging from the evidence we have on the subject, the process of the formation of the solar system was one which involved the utilisation of cometic and meteoric matter; and it fortunately so chanced that the comets likely otherwise to have been most mischievous--those, namely, which crossed the track of planets, and still more those whose paths intersected the globe of the sun--were precisely those which would be earliest and most thoroughly used up in this way.

Secondly, it is noteworthy that all the stars which have blazed out suddenly, except one, have appeared in a particular region of the heavens--the zone of the Milky Way (all, too, on one half of that zone). The single exception is the star in the Northern Crown, and that star appeared in a region which I have found to be connected with the Milky Way by a well-marked stream of stars, not a stream of a few stars scattered here and there, but a stream where thousands of stars are closely aggregated together, though not quite so closely as to form a visible extension of the Milky Way. In my map of 324,000 stars this stream can be quite clearly recognised; but, indeed, the brighter stars scattered along it form a stream recognisable with the naked eye, and have long since been regarded by astronomers as such, forming the stars of the Serpent and the Crown, or a serpentine streak followed by a loop of stars shaped like a coronet. Now the Milky Way, and the outlying streams of stars connected with it, seem to form a region of the stellar universe where fashioning processes are still at work. As Sir W. Herschel long since pointed out, we can recognise in various parts of the heavens various stages of development, and chief among the regions where as yet Nature's work seems incomplete, is the Galactic zone--especially that half of it where the Milky Way consists of irregular streams and clouds of stellar light. As there is no reason for believing that our sun belongs to this part of the galaxy, but on the contrary good ground for considering that he belongs to the class of insulated stars, few of which have shown signs of irregular variation, while none have ever blazed suddenly out with many hundred times their former lustre, we may fairly infer a very high degree of probability in favour of the belief that, for many ages still to come, the sun will continue steadily to discharge his duties as fire, light, and life of the solar system.

VII.

_THE RINGS OF SATURN._

The rings of Saturn, always among the most interesting objects of astronomical research, have recently been subjected to close scrutiny under high telescopic powers by Mr. Trouvelot, of the Harvard Observatory, Cambridge, U.S. The results which he has obtained afford very significant evidence respecting these strange appendages, and even throw some degree of light on the subject of cosmical evolution. The present time, when Saturn is the ruling planet of the night, seems favourable for giving a brief account of recent speculations respecting the Saturnian ring-system, especially as the observations of Mr. Trouvelot appear to remove all doubt as to the true nature of the rings, if indeed any doubt could reasonably be entertained after the investigations made by European and American astronomers when the dark inner ring had but recently been recognised.

It may be well to give a brief account of the progress of observation from the time when the rings were first discovered.

In passing, I may remark that the failure of Galileo to ascertain the real shape of these appendages has always seemed to me to afford striking evidence of the importance of careful reasoning upon all observations whose actual significance is not at once apparent. If Galileo had been thus careful to analyse his observations of Saturn, he could not have failed to ascertain their real meaning. He had seen the planet apparently attended by two large satellites, one on either side, 'as though supporting the aged Saturn upon his slow course around the sun.' Night after night he had seen these attendants, always similarly placed, one on either side of the planet, and at equal distances from it. Then in 1612 he had again examined the planet, and lo, the attendants had vanished, 'as though Saturn had been at his old tricks, and had devoured his children.' But after a while the attendant orbs had reappeared in their former positions, had seemed slowly to grow larger, until at length they had presented the appearance of two pairs of mighty arms encompassing the planet. If Galileo had reasoned upon these changes of appearance, he could not have failed, as it seems to me, to interpret their true meaning. The three forms under which the rings had been seen by him sufficed to indicate the true shape of the appendage. Because Saturn was seen with two attendants of apparently equal size and always equi-distant from him, it was certain that there must be some appendage surrounding him, and extending to that distance from his globe. Because this appendage disappeared, it was certain that it must be thin and flat. Because it appeared at another time with a dark space between the arms and the planet, it was certain that the appendage is separated by a wide gap from the body of the planet. So that Galileo might have concluded--not doubtfully, but with assured confidence--that the appendage is a thin flat ring nowhere attached to the planet, or, as Huyghens said some forty years later, Saturn '_annulo cingitur tenui, plano, nusquam cohaerente_.' Whether such reasoning would have been accepted by the contemporaries of Galileo may be doubtful. The generality of men are not content with reasoning which is logically sound, but require evidence which they can easily understand. Very likely Huyghens' proof from direct observation, though in reality not a whit more complete and far rougher, would have been regarded as the first true proof of the existence of Saturn's ring, just as Sir W. Herschel's observation of one star actually moving round another was regarded as the first true proof of the physical association of certain stars, a fact which Michell had proved as completely and far more neatly half a century earlier, by a method, however, which was 'caviare to the general.'

However, as matters chanced, the scientific world was not called upon to decide between the merits of a discovery made by direct observation and one effected by means of abstract reasoning. It was not until Saturn had been examined with much higher telescopic power than Galileo could employ, that the appendage which had so perplexed the Florentine astronomer was seen to be a thin flat ring, nowhere touching the planet, and considerably inclined to the plane in which Saturn travels. We cannot wonder that the discovery was regarded as a most interesting one. Astronomers had heretofore had to deal with solid masses, either known to be spheroidal, like the earth, the sun, the moon, Jupiter, and Venus, or presumed to be so, like the stars. The comets might be judged to be vaporous masses of various forms; but even these were supposed to surround or to attend upon globe-shaped nuclear masses. Here, however, in the case of Saturn's ring, was a quoit-shaped body travelling around the sun in continual attendance upon Saturn, whose motions, no matter how they varied in velocity or direction, were so closely followed by this strange attendant that the planet remained always centrally poised within the span of its ring-girdle. To appreciate the interest with which this strange phenomenon was regarded, we must remember that as yet the law of gravity had not been recognised. Huyghens discovered the ring (or rather perceived its nature) in 1659, but it was not till 1666 that Newton first entertained the idea that the moon is retained in its orbit about the earth by the attractive energy which causes unsupported bodies to fall earthwards; and he was unable to demonstrate the law of gravity before 1684. Now, in a general sense, we can readily understand in these days how a ring around a planet continues to travel along with the planet despite all changes of velocity or direction of motion. For the law of gravity teaches that the same causes which tend to change the direction and velocity of the planet's motion tend in precisely the same degree to change the direction and velocity of the ring's motion. But when Huyghens made his discovery it must have appeared a most mysterious circumstance that a ring and planet should be thus constantly associated--that during thousands of years no collision should have occurred whereby the relatively delicate structure of the ring had been destroyed.

Only six years later a discovery was made by two English observers, William and Thomas Ball, which enhanced the mystery. Observing the northern face of the ring, which was at that time turned earthwards, they perceived a black stripe of considerable breadth dividing the ring into two concentric portions. The discovery did not attract so much attention as it deserved, insomuch that when Cassini, ten years later, announced the discovery of a corresponding dark division on the southern surface, none recalled the observation made by the brothers Ball. Cassini expressed the opinion that the ring is really divided into two, not merely marked by a dark stripe on its southern face. This conclusion would, of course, have been an assured one, had the previous observation of a dark division on the northern face been remembered. With the knowledge which we now possess, indeed, the darkness of the seeming stripe would be sufficient evidence that there must be a real division there between the rings; for we know that no mere darkness of the ring's substance could account for the apparent darkness of the stripe. It has been well remarked by Professor Tyndall, that if the moon's whole surface could be covered with black velvet, she would yet appear white when seen on the dark background of the sky. And it may be doubted whether a circular strip of black velvet 2000 miles wide, placed where we see the dark division between the rings, would appear nearly as dark as that division. Since we could only admit the possibility of some substance resembling our darker rocks occupying this position (for we know of nothing to justify the supposition that a substance as dark as lampblack or black velvet could be there), we are manifestly precluded from supposing that the dark space is other than a division between two distinct rings.

Yet Sir W. Herschel, in examining the rings of Saturn with his powerful telescopes, for a long time favoured the theory that there is no real division. He called it the 'broad black mark,' and argued that it can neither indicate the existence of a zone of hills upon the ring, nor of a vast cavernous groove, for in either case it would present changes of appearance (according to the ring's changes of position) such as he was unable to detect. It was not until the year 1790, eleven years after his observations had commenced, that, perceiving a corresponding broad black mark upon the ring's southern face, Herschel expressed a 'suspicion' that the ring is divided into two concentric portions by a circular gap nearly 2000 miles in width. He expressed at the same time, very strongly, his belief that this division was the only one in Saturn's ring-system.

A special interest attached at that time to the question whether the ring is divided or not, for Laplace had then recently published the results of his mathematical inquiry into the movements of such a ring as Saturn's, and, having _proved_ that a single solid ring of such enormous width could not continue to move around the planet, had expressed the _opinion_ that Saturn's ring consists in reality of many concentric rings, each turning, with its own proper rotation rate, around the central planet. It is singular that Herschel, who, though not versed in the methods of the higher mathematics, had considerable native power as a mathematician, was unable to perceive the force of Laplace's reasoning. Indeed, this is one of those cases where clearness of perception rather than profundity of mathematical insight was required. Laplace's equations of motion did not express all the relations involved, nor was it possible to judge, from the results he deduced, how far the stability of the Saturnian rings depended on the real structure of these appendages. One who was well acquainted with mechanical matters, and sufficiently versed in mathematics to understand how to estimate generally the forces acting upon the ring-system, could have perceived as readily the general conditions of the problem as the most profound mathematician. One may compare the case to the problem of determining whether the action of the moon in causing the tidal wave modifies in any manner the earth's motion of rotation. We know that as a mathematical question this is a very difficult one. The Astronomer Royal, for example, not long ago dealt with it analytically, and deduced the conclusion that there is no effect on the earth's rotation, presently however, discovering by a lucky chance a term in the result which indicates an effect of that kind. But if we look at the matter in its mechanical aspect, we perceive at once, without any profound mathematical research, that the retardation so hard to detect mathematically must necessarily take place. As Sir E. Beckett says in his masterly work, _Astronomy without Mathematics_, 'the conclusion is as evident without mathematics as with them, when once it has been suggested.' So when we consider the case of a wide flat ring surrounding a mighty planet like Saturn, we perceive that nothing could possibly save such a ring from destruction if it were really one solid structure.

To recognise this the more clearly, let us first notice the dimensions of the planet and rings.

We have in Saturn a globe about 70,000 miles in mean diameter, an equatorial diameter being about 73,000 miles, the polar diameter 66,000 miles. The attractive force of this mighty mass upon bodies placed on its surface is equal to about one-fifth more than terrestrial gravity if the body is near the pole of Saturn, and is almost exactly the same as terrestrial gravity if the body is at the planet's equator. Its action on the matter of the ring is, of course, very much less, because of the increased distance, but still a force is exerted on every part of the ring which is comparable with the familiar force of terrestrial gravity. The outer edge of the outer ring lies about 83,500 miles from the planet's centre, the inner edge of the inner ring (I speak throughout of the ring-system as known to Sir W. Herschel and Laplace) about 54,500 miles from the centre, the breadth of the system of bright rings being about 29,000 miles. Between the planet's equator and the inner edge of the innermost bright ring there intervenes a space of about 20,000 miles. Roughly speaking, it may be said that the attraction of the planet on the substance of the ring's inner edge is less than gravity at Saturn's equator (or, which is almost exactly the same thing, is less than terrestrial gravity) in about the proportion of 9 to 20; or, still more roughly, the inner edge of Saturn's inner bright ring is drawn inwards by about half the force of gravity at the earth's surface. The outer edge is drawn towards Saturn by a force less than terrestrial gravity in the proportion of about 3 to 16--say roughly that the force thus exerted by Saturn on the matter of the outer edge of the ring-system is equivalent to about one-fifth of the force of gravity at the earth's surface.

It is clear, first, that if the ring-system did not rotate, the forces thus acting on the material of the rings would immediately break them into fragments, and, dragging these down to the planet's equator, would leave them scattered in heaps upon that portion of Saturn's surface. The ring would in fact be in that case like a mighty arch, each portion of which would be drawn towards Saturn's centre by its own weight. This weight would be enormous if Bessel's estimate of the mass of the ring-system is correct. He made the mass of the ring rather greater than the mass of the earth--an estimate which I believe to be greatly in excess of the truth. Probably the rings do not amount in mass to more than a fourth part of the earth's mass. But even that is enormous, and subjected as is the material of the rings to forces varying from one-half to a fifth of terrestrial gravity, the strains and pressures upon the various parts of the system would exceed thousands of times those which even the strongest material built up into their shape could resist. The system would no more be able to resist such strains and pressures than an arch of iron spanning the Atlantic would be able to sustain its own weight against the earth's attraction.

It would be necessary then that the ring-system should rotate around the planet. But it is clear that the proper rate of rotation for the outer portion would be very different from the rate suited for the inner portion. In order that the inner portion should travel around Saturn entirely relieved of its weight, it should complete a revolution in about seven hours twenty-three minutes. The outer portion, however, should revolve in about thirteen hours fifty-eight minutes, or nearly fourteen hours. Thus the inner part should rotate in little more than half the time required by the outer part. The result would necessarily be that the ring-system would be affected by tremendous strains, which it would be quite unable to resist. The existence of the great division would manifestly go far to diminish the strains. It is easily shown that the rate of turning where the division is, would be once in about eleven hours and twenty-five minutes, not differing greatly from the mean between the rotation-periods for the outside and for the inside edges of the system. Even then, however, the strains would be hundreds of times greater than the material of the ring could resist. A mass comparable in weight to our earth, compelled to rotate in (say) nine hours when it ought to rotate in eleven or in seven, would be subjected to strains exceeding many times the resistances which the cohesive power of its substance could afford. That would be the condition of the inner ring. And in like manner the outer ring, if it rotated in about twelve hours and three-quarters, would have its outer portions rotating too fast and its inner portions too slowly, because their proper periods would be fourteen hours and eleven hours and a half respectively. Nothing but the division of the ring into a number of narrow hoops could possibly save it from destruction through the internal strains and pressures to which its material would be subjected.

Even this complicated arrangement, however, would not save the ring-system. If we suppose a fine hoop to turn around a central attracting body as the rings of Saturn rotate around the planet, it may be shown that unless the hoop is so weighted that its centre of gravity is far from the planet, there will be no stability in the resulting motions; the hoop will before long be made to rotate eccentrically, and eventually be brought into destructive collision with the central planet.

It was here that Laplace left the problem. Nothing could have been more unsatisfactory than his result, though it was accepted for nearly half a century unquestioned. He had shown that a weighted fine hoop may possibly turn around a central attracting mass without destructive changes of position, but he had not proved more than the bare possibility of this, while nothing in the appearance of Saturn's rings suggests that any such arrangement exists. Again, manifestly a multitude of narrow hoops, so combined as to form a broad flat system of rings, would be constantly in collision _inter se_. Besides, each one of them would be subjected to destructive strains. For though a fine uniform hoop set rotating at a proper rate around an attracting mass at its centre would be freed from all strains, the case is very different with a hoop so weighted as to have its centre of gravity greatly displaced. Laplace had saved the theoretical stability of the motions of a fine ring at the expense of the ring's power of resisting the strains to which it would be exposed. It seems incredible that such a result (expressed, too, very doubtingly by the distinguished mathematician who had obtained it) should have been accepted so long almost without question. There is nothing in nature in the remotest degree resembling the arrangement imagined by Laplace, which indeed appears on _a priori_ grounds impossible. It was not claimed for it that it removed the original difficulties of the problem; and it introduced others fully as serious. So strong, however, is authority in the scientific world that none ventured to express any doubts except Sir W. Herschel, who simply denied that the two rings were divided into many, as Laplace's theory required. As time went on and the signs of many divisions were at times recognised, it was supposed that Laplace's reasoning had been justified; and despite the utter impossibility of the arrangement he had suggested, that arrangement was ordinarily described as probably existing.

At length, however, a discovery was made which caused the whole question to be reopened.

On November 10, 1850, W. Bond, observing the planet with the telescope of the Harvard Observatory, perceived within the inner bright ring a feeble illumination which he was at a loss to understand. On the next night the faint light was better seen. On the 15th, Tuttle, who was observing with Bond, suggested the idea that the light within the inner bright ring was due to a dusky ring inside the system of bright rings. On November 25, Mr. Dawes in England perceived this dusky ring, and announced the discovery before the news had reached England that Bond had already seen the dark ring. The credit of the discovery is usually shared between Bond and Dawes, though the usual rule in such matters would assign the discovery to Bond alone. It was found that the dark ring had already been seen at Rome so far back as 1828, and again by Galle at Berlin in May 1838. The Roman observations were not satisfactory. Those by Galle, however, were sufficient to have established the fact of the ring's existence; indeed, in 1839 Galle measured the dark ring. But very little attention was attracted to this interesting discovery, insomuch that when Bond and Dawes announced their observation of the dark ring in 1850, the news was received by astronomers with all the interest attaching to the detection of before unnoted phenomena.

It may be well to notice under what conditions the dark ring was detected in 1850. In September 1848 the ring had been turned edgewise towards the sun, and as rather more than seven years are occupied in the apparent gradual opening out of the ring from that edge view to its most open appearance (when the outline of the ring-system is an eclipse whose lesser axis is nearly equal to half the greater), it will be seen that in November 1850 the rings were but slightly opened. Thus the recognition of the dark ring within the bright system was made under unfavourable conditions. For four preceding years--that is, from the year 1846--the rings had been as little or less opened; and again for several years preceding 1846, though the rings had been more open, the planet had been unfavourably placed for observation in northern latitudes, crossing the meridian at low altitudes. Still, in 1838 and 1839, when the rings were most open, although the planet was never seen under favourable conditions, the opening of the rings, then nearly at its greatest, made the recognition of the dark ring possible; and we have seen that Galle then made the discovery. When Bond rediscovered the dark ring, everything promised that before long the appendage would be visible with telescopes far inferior in power to the great Harvard refractor. Year after year the planet was becoming more favourably placed for observation, while all the time the rings were opening out. Accordingly it need not surprise us to learn that in 1853 the dark ring was seen with a telescope less than three inches and a half in aperture. Even so early as 1851, Mr. Hartnup, observing the planet with a telescope eight inches and a half in aperture, found that 'the dark ring could not be overlooked for an instant.'

But while this increase in the distinctness of the dark ring was to be expected, from the mere fact that the ring was discovered under relatively unfavourable conditions, yet the fact that Saturn was thus found to have an appendage of a remarkable character, perfectly obvious even with moderate telescopic power, was manifestly most surprising. The planet had been studied for nearly two centuries with telescopes exceeding in power those with which the dark ring was now perceived. Some among these telescopes were not only of great power, but employed by observers of the utmost skill. The elder Herschel had for a quarter of a century studied Saturn with his great reflectors eighteen inches in aperture, and had at times turned on the planet his monstrous (though not mighty) four-feet mirror. Schroeter had examined the dark space within the inner bright ring for the special purpose of determining whether the ring-system is really disconnected from the globe. He had used a mirror nineteen inches in aperture, and he had observed that the dark space seen on either side of Saturn inside the ring-system not only appeared dark, but actually darker than the surrounding sky. This was presumably (though not quite certainly) an effect of contrast only, the dark space being bounded all round by bright surfaces. If real, the phenomenon signified that whereas the space outside the ring, where the satellites of the planet travel, was occupied by some sort of cosmical dust, the space within the ring-system was, as it were, swept and garnished, as though all the scattered matter which might otherwise have occupied that region had been either attracted to the body of the planet or to the rings.[36] But manifestly the observation was entirely inconsistent with the supposition that there existed in Schroeter's time a dark or dusky ring within the bright system. Again, the elder Struve made the most careful measurement of the whole of the ring-system in 1826, when the system was as well placed for observation as in 1856 (or, in other words, as well placed as it can possibly be); but though he used a telescope nine inches and a half in aperture, and though his attention was specially attracted to the inner edge of the inner bright ring (_which seemed to him indistinct_), he did not detect the dark ring. Yet we have seen that in 1851, under much less favourable conditions, a less practised observer, using a telescope of less aperture, found that the dark ring could not be overlooked for an instant. It is manifest that all these considerations point to the conclusion that the dark ring is a new formation, or, at the least, that it has changed notably in condition during the present century.

I have hitherto only considered the appearance of the dusky ring as seen on either side of the planet's globe within the bright rings. The most remarkable feature of the appendage remains still to be mentioned--the fact, namely, that the bright body of the planet can be seen through this dusky ring. Where the dark ring crosses the planet, it appears as a rather dark belt, which might readily be mistaken for a belt upon the planet's surface; for the outline of the planet can be seen through the ring as through a film of smoke or a crape veil.

Now it is worthy of notice that whereas the dark ring was not detected outside the planet's body until 1838, nor generally recognised by astronomers until 1850, the dark belt across the planet, really caused by the dusky ring, was observed more than a century earlier. In 1715 the younger Cassini saw it, and perceived that it was not curved enough for a belt really belonging to the planet. Hadley again observed that the belt attended the ring as this opened out and closed, or, in other words, that the dark belt belonged to the ring, not to the body of the planet. And in many pictures of Saturn's system a dark band is shown along the inner edge of the inner bright ring where it crosses the body of the planet. It seems to me that we have here a most important piece of evidence respecting the rings. It is clear that the inner part of the inner bright ring has for more than a century and a half (how much more we do not know) been partially transparent, and it is probable that within its inner edge there has been all the time a ring of matter; but this ring has only within the last half-century gathered consistency enough to be discernible. It is manifest that the existence of the dark belt shown in the older pictures would have led directly to the detection of the dark ring, had not this appendage been exceedingly faint. Thus, while the observation of the dark belt across the planet's face proves the dusky ring to have existed in some form long before it was perceived, the same fact only helps to render us certain that the dark ring has changed notably in condition during the present century.

The discovery of this singular appendage, an object unique in the solar system, naturally attracted fresh attention to the question of the stability of the rings. The idea was thrown out by the elder Bond that the new ring may be fluid, or even that the whole ring-system may be fluid, and the dark ring simply thinner than the rest. It was thought possible that the ring-system is of the nature of a vast ocean, whose waves are steadily advancing upon the planet's globe. The mathematical investigation of the subject was also resumed by Professor Benjamin Pierce, of Harvard, and it was satisfactorily demonstrated that the stability of a system of actual rings of solid matter required so nice an adjustment of so many narrow rings as to render the system far more complex than even Laplace had supposed. 'A stable formation can,' he said, 'be nothing other than a very great number of separate narrow rigid rings, each revolving with its proper relative velocity.' As was well remarked by the late Professor Nichol, 'If this arrangement or anything like it were real, how many new conditions of instability do we introduce. Observation tells us that the division between such rings must be extremely narrow, so that the slightest disturbance by external or internal causes would cause one ring to impinge upon another; and we should thus have the seed of perpetual catastrophes.' Nor would such a constitution protect the system against dissolution. 'There is no escape from the difficulties, therefore, but through the final rejection of the idea that Saturn's rings are rigid or in any sense a solid formation.'

The idea that the ring-system may be fluid came naturally next under mathematical scrutiny. Strangely enough, the physical objections to the theory of fluidity appear to have been entirely overlooked. Before we could accept such a theory, we must admit the existence of elements differing entirely from those with which we are familiar. No fluid known to us could retain the form of the rings of Saturn under the conditions to which they are exposed. But the mathematical examination of the subject disposed so thoroughly of the theory that the rings can consist of continuous fluid masses, that we need not now discuss the physical objections to the theory.

There remains only the theory that the Saturnian ring-system consists of discrete masses analogous to the streams of meteors known to exist in great numbers within the solar system. The masses may be solid or fluid, may be strewn in relatively vacant space, or may be surrounded by vaporous envelopes; but that they are discrete, each free to travel on its own course, seemed as completely demonstrated by Pierce's calculations as anything not actually admitting of direct observation could possibly be. The matter was placed beyond dispute by the independent analysis to which Clerk Maxwell subjected the mathematical problem. It had been selected in 1855 as the subject for the Adams Prize Essay at Cambridge, and Clerk Maxwell's essay, which obtained the prize, showed conclusively that only a system of many small bodies, each free to travel upon its course under the varying attractions to which it was subjected by Saturn itself, and by the Saturnian satellites, could possibly continue to girdle a planet as the rings of Saturn girdle him.