An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

2.6G:co's Ns.
derg(o by rcfrring theml to the agoncies of leat and cold.'ellc
intense helcat to whiclh tley are sbjecct in approaclhitng so netar
the sun as some of them do, is alleged as a sufficient reason for*
the great expalsion of the thin lnebulous;atmlosltheres forming
tlitir tails s; and the inconceivable cold to whliCl thlly arl'C subject inlcccedilng to such a distance fiom the sulln, is supposed to
account for tlhe condensation of the same Iatter until it returns
to its original dlimensions. The teml)crature eX)Criellccd by
tilm coillcts of G680 and 1-843: at tlciv perilhelion, would b1 sufficient to volatiliz% te le ost obdurate stul)tanclcs and to expand tl;e va'por to vast dilnensions; and tle opl)osite o.eftcts
of the extrem cotl d to whlich it would be subjecot in the regions remoto from tlhe sun, would le a dequate to condense it
into it;s blt'ner volumlle.
Tlsl'l cxypxlanation, however, does not account for the direction
of the tail, extending, as it usually does, only in a line opposite
to the slun. Sonle writers, tllerfore, as I)Celambre, Su1)pose
tlhat the nebulouts matter of tlle comet, after being expanded
to such a. volulme thla thle I)particles are no lonIger attracted to
thle nucleus unless by the sliglhtst conceivable force, is carried
off ill a, direction ftiom the sun by thle imlpulse of tle solar rays
t.lclmselves. )tlers conceive of a force of or}2ulsml), independent of any mecclt nical imipulse tlulanating fi'om tlhe sun. BIu1t
to assign such.a po\wer of communicating,1; motion to tlhe slun's
I'afrys while they iav never beeln provted to 1have any lno1lntUilil or to a r'ptulsive force whichl has no independenl( t proof
of its existcnce, is unphilosophiceal; and we aie Coml)elled to
place tlhe pl!cnomenl(a of comets' tails among thle poilts of astrolnomy yet to be explaiined.t{'
105. Sifice those colmets whlich 1ve thlir perihelion very
1iear the sun,111 like the0 conet of 1680, cross t1he olrbits of all tlhe
planets, t/e possibtilt tiat one of thenm mipa'y stbike tAe e(aO'tA,
hras frecqulnty been suggtt sted. Still, it Xmay quiet ourll apprehellnsion118s ii thtis slulject, to rtIlee; on thle vast extent of the plttletary spaces, in wltiel] these bodies are not crowded togetfle as
])eltambre's Astrolloy, t. iii., p.'401..:t Prof. Norton onl tOhe:'Jormation of Colmets' Tails," (Amer Jourial,
xlvii., p. 104).