ENCKE'S COMET.
W ITH an opera-glass—possibly with the naked eye—there can now be seen each evening towards the west one of the most remarkable Comets known to astronomers. It is not, indeed, one of those amazing objects whose appearance used of old to
inspire fear among the nations ; nor does it present a very striking picture when viewed with oven the most powerful telescope. We have before us, as we write, a sketch of this comet taken by Dr.
Huggins with the splendid telescope lately placed in his hands by the Royal Society, and this drawing presents only a faint un- symmetrical cloud slightly condensed on the aide where a larger comet would have a tail. Yet not oven the splendid comets of 1843, 1858, and 1861 outvio this little cloud in real interest, or convey to the thoughtful mind a more instructive lesson. For the faint nebulous gleam is the famous comet named after Encke,—the first comet ever shown to travel in a short period, and on a com-
paratively settled course around the orb which rules our Earth and her fellow-planets. Halley's comet had long before been proved to be a member of the Sun's family ; but the course on which it travels is unplanetary to a very high degree ; it passes from little more than one-half the earth's distance from the sun into those far- distant depths where Neptune pursues his dismal career ; it ranges far away from that general level near which the planets travel ; and it pursues its course, not as the planets do, from west to east, but from east to west. So that though Halley's comet belongs to the sun's family, he is but an unruly member of it.. Encke's comet, on the other hand, is little less staid than some of the planets belonging to that strange ring which circles between the paths of Mars and Jupiter. It travels the right way round the sun, and in its widest excursion keeps far within the path of giant Jupiter, while the slope or tilt of its path is even less than that of some planetary orbits. Its very aspect suggests the settled nature of this comet, for though it changes visibly in shape as it approaches the sun, and has even been detected on one or two occasions attempting, as it would seem, to throw out a tail, yet these efforts have never resulted in any marked success. Halley's comet, on the other hand, has be- haved most strangely in this respect. On most of the occasions when it has been in our neighbourhood it has had a magnificent tail ; but Sir John Herschel tells us that the fine new tail which it threw out in the autumn of 1835 had disappeared on January 21, 1836, taking the head (all but the nucleus) along with it ; and this when the comet was just in that pert of its course where one would have expected to see the tail most fully developed. Then it swelled out so rapidly that in seventeen days its bulk had increased more than seventy-fold, and as the comet passed away towards the cold regions it had come from it changed in shape "as if it intended to develop a new tail."
Eucke's comet is remarkable, then, among comets because, unlike most comets, it varies little in aspect, and moves on a course little distinguished from the paths of the planets. But there is much else to attract us to the study of this little body. We speak of it, indeed, as little ; but perhaps this description will be thought inexact, when we mention that the comet sometimes occupies a much larger space than this earth. It is little, however, as respects its mass or weight ; for it has been known to approach very close to Mercury, the least of all the planets ; aud though Mercury swayed the comet notably from its track, the comet in no sensible way disturbed Mercury. It is far from being beyond belief,—on the contrary, it is highly probable,—that if the whole mass of this object could be collected into a convenient hulk and placed upon the earth, a man of average strength could readily lift it. Nay, there are those who think that the whole mass of many
far larger comets would scarcely fill a wine-glass, if compressed to the density of water. Be this as it may, there can be little question that Encke's comet is " a more bunch of vapours," to use the words applied by Sir John Herschel to another comet. It is worthy of notice, in the next place, that this cornet was detected several times before its real nature was recognized. Mechain detected it on January 17, 1786, and was not a little gratified to find that he had anticipated Messier, the most laborious comet-seeker of those days. It may have been on this very occasion, indeed, that Messier, as the story runs, missed a new comet through the interruption caused by his wife's fatal illness,— a circumstance which led him some time after to mistake con- dolences addressed to him as a widower, and to inveigh against the unhappy chance which had enabled another "to discover one of his comets." //is comets! On the next occasion, November 7, 1795, this comet was discovered by a lady, no other than Miss Caroline Herschel, sister of the greatest astronomer the world has known. Thuile, of Marseilles, saw the comet again in October, 1805. It was not until November, 1818, that the last discovery (properly so called) took place. For then the comet's path was determined, and at every return since then astronomers have known precisely when and where the comet would come into view. They are as confident, almost, respecting its periodical return (three times in ten years) as about the motions of Mars or Mercury ; and they would even be quite as confident, but for a contingency to which the planets are fortunately not exposed. It happened a dew years since that a comet of much the same nature as Encke's, and supposed like it to be a settled member of the Solar family, -divided itself into two distinct comets ; nor did its vagaries end 'with this achievement, for though the double comet came back yet once again along the assigned track, it has never since been seen, so that some astronomers suppose that it has divided into parts too small to be separately discerned.
Encke's comet, however, must not suffer in reputation because a brother comet has been thus dissipated. Astronomers have -seen no signs, either in the motions or in the changes of appear- ance of the comet, which would lead them to entertain any doubts respecting its remaining for many centuries a member of the Solar family. One peculiarity, however, its motions manifest, which suggests the conclusion that at no indefinitely distant epoch it will be destroyed by the Sun himself. It is circling in a continu- ally diminishing orbit, and always in a shorter period. At the time of its first discovery it accomplished the circuit of its path in about 1,213 days (taking one circuit with another) ; at present its period is rather less than 1,210 days. It is the generally received opinion (though Sir J. Herschel suggested another explanation) that this constant hastening—which implies a constant tendency in the comet's path to draw nearer towards the sun, is due to the resistance of some very rare medium, through which the solid planets pass without appreciable check. If so (and it is difficult to resist the conclusion) thei comet is doomed to destruction. The nearer its path draws to the sun the greater the resistance which the comet will experience,—as well because the interplanetary atmosphere must be denser near the sun than elsewhere, as because the cornet's motion must grow continually more rapid. This last circumstance, indeed, will seem to many a strange result of the resistance experienced by the comet, but it is none the less certain that such a result must follow. The time will at length come, then, when this comet at each return to the sun will graze his mighty orb, giving up gradually more and more of its substance, until finally it is completely absorbed by him, and so vanishes for over from the planetary system.
It remains only to be noticed that Dr. Huggins has already succeeded in testing the light of this comet with the spectroscope. Like those few of its fellows which have been similarly examined, it consists in the main of self-luminous vapour,—or at least (for one must not be too positive about these abnormal bodies) it shines with light having the same qualities under spectroscopic analysis as the light of glowing gases. It seems to be, as we have already said for another reason, a " mere bunch of vapours."
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