February 1, 1896.] 



KNOWLEDGE 



41 



that had elapsed since 1878, the predicted place -was 

 very close to the actual. This happy recovery makes one 

 hesitate before pronouncing a comet lost, and necessitates 

 a close examination of the circumstances under which the 

 return takes place. 



The third comet on the list is not likely to cause any 

 confusion, because a second name is attached to that of 

 Tempel, -nho first found the comet in 1809. This addition 

 of Swift's name marks a particular fact in its history, 

 which serves to identify the comet. No definite orbit was 

 derived from the observations of 1869, although it was 

 known that parabolic elements would not represent the 

 path. Consequently, no indication of its future career 

 was made. The comet made its approach to the sun in 

 1875 and was not seen, but in 1880 it was independently 

 discovered by Prof. Swift, whose name is therefore linked 

 in the description. One peculiarity in its history arises 

 from the fact that its period is almost exactly five and a 

 half years, so that for a long time the returns will be 

 alternately favourable and unfavourable for observation. In 

 1886 it passed unnoticed ; but in 1891, when its theoretical 

 brilliancy was one hundred times greater, it was well 

 observed, and is apparently as much a permanent member 

 of our system as a comet can be. 



'Winnecke, D'Arrest, and Faye call for little remark. 

 They have been imder regular observation for approxi- 

 mately half a century, and are well iinder control. 

 Winnecke dates back to 1819 with certainty, and probably 

 to 17C(>, though it was not till 1858 that its peculiar 

 motion was recognized. D'Arrest was first observed in 

 1851, and has been repeatedly seen since, though the 

 circumstances of its return are occasionally unfavourable. 

 Faye, a comet which is now under observation in our larger 

 telescopes, has run an uniform course since 1813, and has, 

 thanks to Prof. Axel. Mdller's admirable discussion, perhaps 

 proved itself the best timekeeper of the periodic comets. 



The two remaining comets, those of Finlay and Wolf, 

 are modern introductions, and are entitled to more 

 consideration on account of the many interesting problems 

 they ofl'er for solution in regard to the capture of comets 

 by Jupiter, or the connection that may exist with older 

 comets since disappeared. In the case of Finlaj% the 

 position of the orbit with reference to the ecliptic is very 

 similar to that of the comet 1841 I. (known as De Yico's), 

 a comet which will appear in the second list, for though 

 undoubtedly periodic it has never been seen since that 

 date. The dimensions of the orbit do not accord so 

 well, but the disagreement is not so great but that the 

 plausible hypothesis was raised at the time that the 1844 

 comet had encountered a small planet and sufl'ered con- 

 siderable perturbation. But Finlay's comet requires 

 more than a year longer for its revolution about the sun 

 than does De Vice's, a circumstance which evidently tells 

 against the probability of the two objects being identical, 

 since it is difficult to understand how one element of the 

 orbit should be so materially affected and the others 

 remain comparatively undisturbed. If two objects, seen 

 to be moving in similar orbits after a considerable interval, 

 are concluded to be identical because of this similarity, it 

 must be assumed that perturbation in the interval has 

 been slight, and therefore cannot be invoked to explain 

 one point of difi-erence. Another supposition which has 

 been raised is that the comet is a reappearance of that 

 known by the name of Lexell — seen once, and once only, 

 in 1770. This point can be more conveniently considered 

 in treating of the second catalogue, since another comet 

 known as Brooks', 18S9 V., also has claims to be con- 

 sidered the lost LexeU. Fortunately, Finlay's comet was 

 seen in ls92 (though somewhat fainter than could have 



been wished), and its past history can therefore be well 

 determined ; but it will remain in our catalogues distin- 

 guished by the name of the discoverer. There is another 

 and a much larger question. Is the present condition of 

 cometary astronomy insufficient to decide with certainty 

 the identity of two comets '? That is to say, is it 

 possible for astronomers to accumulate observations of the 

 same body and believe them to refer to two distinct 

 objects '? That is a question which it is hardly safe to 

 answer categorically, but it is one to which we shall 

 return. 



Wolf's comet, the last on the list, has a history as 

 interesting as any. It was first seen in 1884, and the 

 earliest published elements showed similarity with those of 

 a comet discovered by Coggia in 1874, and whose probable 

 period is three hundred years. More trustworthy elliptic 

 elements proved, as Dr. Hind pointed out, that the path of 

 the comet approached very closely to that of Jupiter, and 

 that in May, 1875, the comet was actually so near to that 

 planet as to be brought within its influence. This point 

 has been examined very closely by the lately deceased 

 ^I. Lehmann Filhi^-s ; and he has derived, with very close 

 approximation, not only the elements in which the comet 

 moved when under the influence of Jupiter's attraction, 

 but also the character of the orbit before undergoing that 

 violent alteration. This inquiry explains why the comet 

 had not been previously seen, though it may have regularly 

 revolved about the sun for ages before. The original 

 elements, as we may call them — that is, previous to 1875 — 

 possessed a much smaller eccentricity with a longer period. 

 Consequently the perihelion distance had been materially 

 reduced by -Jupiter's attraction (as a matter of fact to about 

 one half that which it previously had) , and the practical eflect 

 for us was to bring the comet within the range of our tele- 

 scopes. The investigation will probably be made again more 

 rigorously, and with more correct data ; but, as it stands, 

 it ofl'ers a very satisfactory explanation of the reason why 

 the comet had not been seen earlier, and was seen then. 

 It must have passed us in 1878, but it is known that the 

 position in the sky was then imfavourable. 



We have now to consider three comets, all of which 

 there is grave reason to believe have escaped beyond our 

 cognizance, and are no longer to be recognized as comets. 

 They are given in the foUowhig table in the order of their 

 period, together with some additional information which 

 may assist us in forming an opinion of their possible 

 recovery : — 



It is, perhaps, a little premature to consider the first — 

 Brorsen's — hopelessly lost, though (the table shows that it 

 has not been seen since 1879, to which mast be added the 

 significant fact that though the comet passed through its 

 perihelion this autumn, no serious efl'ort appears to have 

 been made to recover it. In some respects the history of 

 the comet resembles that of Wolf, last mentioned, inasmuch 

 as it was brought within our range by the disturbing 

 action of Jupiter a few years before it was discovered. In 

 1H46, Brorsen first observed it, and in 1842 the comet 

 made a close approach to that planet, with the result that 

 the perihelion distance was much red.iced, being brought 

 down from 1-") to 0(i, an alteration of the same character 

 as was instanced in the case of Wolf, and one which, of 



