142 Observations on the Tails of Halley’s Comet. 
Arr. XII.—Observations on the Tails of Halley’s Comet, as they 
appeared at Union College, Schenectady, N. Y., in Oct. 1835; 
by Prof. B. F. Josuiy. 
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 1839, 
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 terrestrial 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, a 
to pursue its regular and prescribed (and now calculated) course 
through the heavens, at least during a period in which fifteen oF 
twenty generations of men have been swept in succession from the 
face of the earth. 
Salient 
* See American Almanac, for 1835. 
