COMETS.
OF all the objects with which astronomers have to deal, Comets are the most mysterious. Their eccentric paths, their
marvellous dimensions, the strange changes to which they are subject, have long been among the most striking of the wonders of astronomy. There is something specially awe-inspiring, too, in the thought of the gloomy domains of space through which the comet that visits our system for a brief time has for countless ages been travelling. Ordinary modes of measuring space and time fail us, indeed, in speaking of these wonders, or at least convey no real meaning to the mind. If the comet, for instance, which is now
a conspicuous object in our northern skies be of this order—if, at our comet-tracker Hind begins to suspect, its path in our neigh- bourhood is parabolic, so that either it has an enormously long period of revolution, or has come to us across the interstellar spaces themselves,—how useless is it to set down the array of numbers representing the extension of its path, or the years during which the comet has been voyaging through desert space ! The comets indeed which come from the star-depths--and obser- vation renders it all but certain that some have done so—cannot in any case have pursued a voyage less than twenty billion* of • miles in length, and cannot have been less than eight million years upon the road. That, too, was but their latest journey. From the last sun they visited to our own sun, such was their voyage ; but who shall ray how many such voyages they had pursued, or how many they will complete after leaving our sun's neighbourhood, before the time comes when Borne chance brings them near enough to a disturbing planet to cause their path to become a closed one? And even those comets which are now known to follow a closed path, returning again and again to the neighbourhood of the sun, need only be studied thoughtfully to present similarly startling con- ceptions. No matter what theory of their origin we adopt, we are brought face to face with the thought of time-intervals so enormous that practically they must be viewed as infinite. If we take the assumption that a comet of this order had been travelling on a path of parabolic or hyperbolic nature towards our sun, had been captured by the disturbing attraction of a planet, an'd oom- pelled thenceforth to circuit on an oval path of greater, or less extent, yet according to all laws of probability, how many time* must it have flitted from star to star before it was thus captured! For the chances are millions to one against so near an approach to a planet as would ensure capture. But if, appalled by the enormous time-intervals thus revealed to us, we turn from that assumption, and find within the solar system itself the origin of the periodic comets, how strange are the theories to which we are led ! Those comets which come very near to the suit may have had a solar origin ; and those which approach very near the path of one of the giant planets may have been propelled from out of such a planet when in its sun-like youth. Even then, how- ever, other comets remain which are not thus to be accounted for, unless we regard them as derived from planets outside Neptune, hitherto undetected, and perhaps detectable in no other way. And when we have taken such theories of cometat7 origin, not, indeed, for acceptance, but to be weighed amongst possibilities, how stupendous are the conceptions to which we are thus introduced ! Suns (for what is true of our sun may be regarded as probable of others) vomiting forth cometic matter, so violently as to corn- numicate velocities capable of bearing such matter to the limita; or beyond the limits of the solar system : planets now passing through later stages of their existence, but presented to according to such theories, as once in a sun-like condition, and
at that time capable of emulating the comet-expelling feats of the great central sun.
Are these thoughts too wild and fanciful to be entertained? They may appear so ; yet where are we to find others less amazing? The comets of the various orders—short-period, long- period, and non-periodic--are there. Their existence has to be in some way accounted for ; or if such explanation is at present impossible, as seems likely, we may yet follow the various lines of reasoning which present themselves. And we have very little choice. Take a comet of long period passing near the orbit, let us say, of Uranus,—even as Tempel's comet, the parent of the November meteors, is known to do. Either that comet has been gathered in from outer space by the sun, and compelled to follow its present path by the disturbing influence of Uranus, or else— what? Only two other theories are 'available. Trace back the cornet's Path in imagination, round and round that oval path, which carries it across the paths of Uranus and the earth but nowhere else brings it within millions of miles of any possible disturbing influences. Rejecting the earth as insufficient in attractive might (or, at least, so inferior to Uranus as to leave us in no doubt in selecting between the two), we have only during the past of the comet, as so traced, the planet Uranus to which we can refer it. We have rejected the attractive influence of Uranus ; but two other in- fluences remain 9 Eruptive action in a former sun-like state, an action corresponding to the eruptive processes known to be taking place in the sun, is one possible origin. The mind of man, unapt though it is to deal with time-intervals so enormous as are re- quired to transmute a giant orb from the sun-like to the planetary condition, May yet accept this interpretation, if no other preient itself which is not stillmore appalling: Only one other, as it seems to us, remains, and this compels us to contemplate time-intervals compared with which those required to change Uranus from sun to planet seem insignificant. If, as we are taught by the nebular hypothesis of the solar system, or, in fact, by any theory of its evolu- tion whatever, the planet Uranus was once in a vaporous condition, extending as a mighty rotating disc far beyond its present sphere, and probably far beyond the path of its outermost satellite, we may conceive a comet arriving from outer space to be captured
• by the resistance of the once vaporous planet, not by its mere attractive force. But to what a result have we thus been led ! If we accepted this view, rather than the theory that Uranus had expelled the comet, we should have first to carry our thoughts back almost to the Very beginning of our solar system, and then to recognise at that inconceivably distant epoch, comets travel- ling from sun to sun, and some of them coming from other suns towards ours, to be captured from time to time by the resistance of the vaporous masses out of which the planets of our system were one day to be evolved.
. We do not know how the questions raised by such thoughts shOttld be answered, although, as has been elsewlrie shown, there is more evidence in favour of the theory of expulsion than of the other two theories just sketched. • But we have reason to feel assured, as we contemplate a comet like that which now adorns our skies, that could we learn its history, a practical infinity of time would be brought before us as the aggregate of the time- intervals we should have to deal with. Nor is the marvel of the comet diminished by what we have learned from observation or from mathematical analysis. We have found that the tracks of comets are followed by countless millions of meteoric bodies, and thus the strangest thoughts—of infinity of space occupied by infinite numbers of cosmical bodies, aggregating towards multi- tudinous centres during infinity of time—are suggested to us. The telescope has shown us wonderful processes taking place during the comet's approach to the sun, and most wonderful process of all, the repulsion of the vaporous matter in the taiVas though to assure us that the expelling power of suns is even more than matched by the repelling power they exert on portions of cometic matter brought in certain conditions under their influence. Analysis by the spectroscope, that wonderful instiument which astronomy owes to Kirchhoff, has taught us much respecting cometic structure, showing that the light of the nucleus is that of a glowing solid or liquid (or of matter reflecting sunlight), the light of the coma that mainly of glowing vapour, while in the tail these two forms of light are combined. And polariscopic analysis speaks with equal clearness of the composite nature of cometic structure. But when all this has been said, we are little nearer to the solution of the mysterious problems which comets present to us. They still teach us, as they have so long taught, that "there are more things in heaven and earth than are dreamed of in our philosophy."
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