2C8 Biela's Comet. 



BIELA'S COMET. 



BY W. T. LYNN, B.A., F.R.A.S., 



Of the Koyal Observatory, Greenwich. 



(With a Tinted Plate.) 



Aftek performing the extraordinary feat of separating, twenty" 

 years ago, into two portions, this remarkable comet appears 

 now to have vanished entirely; most vigorous and patient 

 searches for it last year, at a time when it should have been 

 conspicuous, having met with no success. The writer has 

 been induced to think that, under these circumstances, an 

 account of all the facts known concerning it, carefully digested 

 into a small compass, would prove interesting, and, acting 

 upon this belief, he has drawn up the following : — 



On the 10th of November, 1805, this comet was dis- 

 covered by M. Pons, at Marseilles, in the constellation of 

 Andromeda. It was then a small comet, with tolerably 

 defined nucleus, discernible by, but not conspicuous to, the 

 naked eye. It afterwards increased in brightness and ap- 

 parent size, and was observed by many astronomers, including 

 Olbers, Schroter, Bouvard, and Maskelyne. According to the 

 measures of Schroter,* the diameter of the nucleus on 

 December 8 amounted to about 125 miles, whilst that of the 

 spherical nebulous shell was more than fifty times as large, or 

 nearly 6400 miles. That astronomer compared this proportion 

 with that derived from his own measures of the comet of 1799. 

 He found that the visible envelope of the latter was more than 

 twice as large as that of the other, when each was at its 

 respective nearest approach to the earth; and as the visibility 

 of the successive layers of the envelope, since they always 

 decrease in density in proportion to their distance from the 

 nucleus, depends greatly upon the comet's distance, he con- 

 cluded that the extent of that of 1799, which never approached 

 the earth nearer than seventy millions of miles, was in reality 

 far greater than that of 1805, which came within four millions 

 of miles. Now, as the diameter of the nucleus of the comet 

 of 1799 amounted to 1,500 miles, the cubical contents of that 

 nucleus was about 1,900 times as great as that of the comet 

 in question; hence the assertion appeared justifiable,. that the 

 mass and attractive force of the nucleus of a cornet* is the 

 principal cause of the extent of its luminous envelope, which 

 may be supposed to consist of attracted particles of ethereal 

 matter. On the 8th of December, when the comet was 

 nearest the earth, the former, winch, from its then considerable 

 southern declination, was observed 'by Schroter at no great 



* Perl iner Aatronomisches Jahrbuch for 1809, p. 140. 



