January 1, 1894.] 



KNOWLEDGE 



it is usual to call it Swift's comet. The body known as 

 Tempel's first periodical comet was discovered by him at 

 Marseilles on the 3rd of April, 1867 ; the period was 

 found to be about sis years, and it duly returned in 1873 

 and 1870, but has not since been seen. His second 

 periodical comet, which was discovered at ]\lilan on the 

 3rd of July, 1873, was calculated to have a period about 

 five and a quarter years in duration. It was observed 

 again in 1878, but. not afterwards, so that it seems rash to 

 speak of a return as due in the present year, though three 

 times the length of a period will in the spring have elapsed 

 since 1878, when the comet passed its perihelion on the 

 19th of July. 



But we now have to refer to a comet which has returned 

 so regidarly during more than a century, that it has come 

 to be looked upon as a friend whose periodical visits may 

 be confidently expected. 1 now allude to Encke's comet, 

 which, first discovered by Mechain in 1786, only acquired its 

 name after the return in 1818, when its orbit was accu- 

 rately determined by the illustrious Seeberg astronomer, 

 who became director of the Berlin Observatory in 1825. 

 He found that the period was much shorter than that of 

 any other known comet, and that its duration amounted 

 to only twelve hundred and twelve days, or about three 

 and one-third years. Extending his investigations, he 

 showed that this was slightly diminishing by a little more 

 than a tenth of a day at each return, and suggested, as the 

 probable cause of this, the action of a resisting medium in 

 the inner portion of the comet's orbit (when in perihelion 

 it passes within the orbit of Mercury) checking the 

 tangential motion of the comet and causing it to be drawn 

 somewhat nearer to the sun, so as to shorten its length 

 of revolution, and therefore its period. The comet has 

 been observed in some part of the world at every return 

 since 1818 ; and whilst Encke lived he was able to show 

 that the above efl'ect continued to appear and with the same 

 rate of progress, confirming the truth of his theory as to 

 its cause. Nor was it easy to take any other new of the 

 phenomenon, with such a vcm cansK apparently fully able 

 to explain it. But after this had gone on for about fifty 

 years from the time that Encke first called attention to it. 

 Yon Asten and Backlund, who had followed up his inves- 

 tigations, found that in 1868 the observed diminution of 

 the length of period dwindled to one-half its original 

 amount, and it has apparently continued to go on at this 

 diminished rate ever since. It is to be hoiked that more 

 light will, before long, be thrown upon this remarkable 

 circumstance, which imparts additional interest to further 

 observations of Encke's comet. It was last in perihelion on 

 the 18th of October, 1891, and the next return to that posi- 

 tion will be due on the 10th of February, 1895. 



A passing reference may here be made to Brorsen's 

 comet, which was discovered at Kiel in 1846, found to have 

 a period of about five and a half years, and was observed 

 at several subsequent returns, but the last of these was in 

 1879. As we cannot venture to predict its return in 1895, 

 we will pass on to 1896, in which year two periodical comets 

 will be due. One of these is known as Faye's, having been 

 discovered by that astronomer at Paris on the 22nd of 

 November, 1813. Le Verrier calculated its orbit, and 

 found that the length of its period was about seven and a 

 half years ; it was duly observed at every subsequent 

 return, passing its perihelion on the last occasion on the 

 20th of August, 1888, so that another return may be 

 expected in 1896. The other comet due m that year was 

 discovered by Mr. Brooks, at the Smith Observatory, 

 Geneva, N.Y., on the 6th of July, 1889 : great interest 

 was imparted to it by Mr. S. C. Chandler's investigations, 

 which showed that its period when under observation was 



about seven years, but that the orbit had been much 

 disturbed by a near approach to the planet Jupiter in 

 1886, and that the comet was very probably identical with 

 Lexell's comet of 1770, which had not (in consequence also 

 of Jovian attractions) been seen since the year of its dis- 

 covery, though calculated to have a period of only about 

 five and a half years. However plausible this view seemed, 

 later investigations made by Dr. Lane Poor have (as was 

 pointed out by the writer in an article contributed to the 

 number of Knowledge for last November) thrown very 

 considerable doubt upon it, which can only be cleared up 

 by the results of the observations which it is to be hoped 

 will be obtained in 1896. 



In 1897 also two periodical comets will be due. The 

 first of these was discovered by D'Arrest at Leipzig in 

 1851, found to have an elliptic orbit with a period of about 

 six and a half years, observed in 1857 and at every 

 subsequent return except that due m 1884 ( when it was 

 unfavourably placed), passing its perihelion on the last 

 occasion on the 16tli of September, 1890, so that another 

 return will be due early in 1897. The other is Swift's 

 comet, to which we have already alluded, and which since 

 it acquired its name in 1880 has only been seen at the 

 last return in 1891, having been unfavourably placed for 

 observation in 1886. Fiut, as already remarked, it was 

 first discovered by Tempel in 1869, and another return 

 may be expected early in 1897. 



In the summer of 1898 a return will be due of the 

 comet known as Winnecke's, since he detected it in 1858, 

 and showed that it was identical with one discovered by 

 Pons so far back as 1819, with a period of about five and 

 a half years; it escaped observation in 1863 (as it after- 

 wards did in 1880), but was observed near perihelion in 

 1869, 1875, 1886, and 1892, so that another return may 

 be expected in 1898. About the same time a return of 

 Encke's comet will again be due ; and La the autumn of the 

 same year a fine comet, which was discovered by Dr. Max 

 Wolf of Heidelberg on the 17th of September, 1884, found 

 to have a period of nearly seven years, and well observed 

 at the last return in 1891. 



In the last year of the present century a return of 

 Tuttle's comet will be due. This body occupies a 

 unique position in having a period of somewhat less 

 than fourteen years. It acquired its name after Mr. 

 Tuttle had detected it in 1858, but the calculations 

 made of its orbit showed that it was identical with 

 one discovered by iMcchain (also the first discoverer, 

 as we have seen, of Encke's comet four years before) in 

 1790, -and it has since been observed at returns in 1871 

 and 1885, passing its perihelion on the 11th of September 

 in the latter year. The interesting comet discovered by 

 Mr. Holmes at Islington in November, 1892, will also be 

 due in 1899, as its period is less than seven years. Later 

 in the same year another return may be expected of Mr. 

 Finlay's comet, which has a period of about six and a 

 half years, and has, since its discovery by Mr. Finlay in 

 1886, been observed at the return which took place in the 

 summer of last year (1893). But, in addition to all 

 these, a very interesting cometary visitor will be eagerly 

 looked for in 1899 ; we mean the one moving in the 

 same orbit as the Leonids, or mid-November meteors. The 

 comet has only hitherto been seen in the winter of 1865, 

 when Tempel discovered it on the 19th of December, and 

 it passed its perihelion on the 11th of January, 1866. Its 

 period, like that of the meteoroids, is about thirty-three 

 and a quarter years ; and it is the comet of shortest period 

 which moves in a retrograde direction, all those of shorter 

 periods revolving round the sun in the same general 

 direction as the planets. 



