142 Observations on the Tails ofHalley's Comet. 



Art. XII. — Observations on the Tails ofHalley's Comet, as they 

 appeared at Union College, Schenectady, N. Y., in Oct. 1835; 

 by Prof. B. F. Joslin. 



Preliminary Remarks. — A comparison of the recorded appear- 

 ances of comets with the known period of Halley's, has enabled as- 

 tronomers, by identifying the latter, to trace back its existence for 

 five or six hundred years. This circumstance renders this comet 

 peculiarly interesting, as affording an opportunity of studying the 

 physical changes which this class of bodies may undergo during long 

 periods of time. These changes will be more accurately determin- 

 ed, in proportion as observers shall more particularly note those op- 

 tical and other circumstances which affect their appearance, and es- 

 pecially the length of their trains. 



It would appear from the former history of this comet, that at 

 each of its periodical returns, since these have been recorded, the 

 magnitude of its head, (which consists of the bright central part, 

 called the nucleus, and the surrounding nebulous part, called the en- 

 velope,) and the length of its tail, have been observed to be less 

 than at the preceding return. This has been attributed to a want 

 of sufficient attraction to bring back to the head the material of the 

 tail, and prevent its dissipation. In consequence of these succes- 

 sive degradations, astronomers generally anticipated, that in 1835, 

 its tail, if seen at all, would be far less imposing than at any former 

 period ; and it was doubted by some, whether any part of the comet 

 would be seen with the naked eye, or even without the aid of a pow- 

 erful telescope.* Yet this isolated mass of celestial vapor appears 

 still to be far from being entirely dissipated ; although vapor, under 

 ordinary ^erresfnar circumstances, is proverbial for its transitory 

 character, and strikingly represents the brevity of human life. Yet 

 this body, whose bulk consists chiefly of vapor; this body, at the 

 same time among the lightest and most voluminous in the solar sys- 

 tem, has (notwithstanding its alternate condensation and rarefaction, 

 and its partial dissipation by solar influence) continued to exist, and 

 to pursue its regular and prescribed (and now calculated) course 

 through the heavens, at least during a period in which fifteen or 

 twenty generations of men have been swept in succession from the 

 face of the earth. 



* See American Almanac, for 1835. 



