6 Examination of the Theory of a Resisting Medium. 
of 1531, and those of two subsequent ones, were all directed op- 
posite to the sun.(15) Pingré subsequently supposed that these tails 
are formed of the most subtile portions of the comet’s atmosphere, 
greatly rarefied by the sun, and driven to the side opposite the 
sun, by the resistance of the ether; aided, perhaps, by the solar 
rays.(16) This direction of comets’ tails, as laid down by Fracastor 
and Apian, seems to have been very universally adopted. Newton 
says the tails of comets arise from their heads, and tend towards 
the parts opposite to the sun.(17) Baz/ly adopts the same opinion, 
in strong language, namely, that the tails are always opposite the 
sun.(18) Delambre is equally unreserved. He says the tail of a 
comet is always opposite the sun, or in prolongation of the radius 
vector of the sun and the comet.(19) Laplace calls them trains of 
vapour, always situated on the other side of the heads of comets, rel- 
atively to the sun.(20) Vince says comets are surrounded by a 
se atmosphere, and from the side opposite the sun they send 
forth a tail.(21) Bonnycastle denominates them fiery tails, which 
continually issue from that side of the comets which is farthest from 
the sun.(22) Brewster states that when a comet is near its peri- 
helion, it is accompanied with a tail or train of light, directly oppo- 
site the sun.(23) Morse avows that comets are usually attended with 
a long train of light, always opposite to the sun.(24) Prof. Farrar, 
of Harvard, describes the trains, and adds, their direction is always 
opposite to the sun.(25) The younger Herschel describes the nu- 
cleus, and adds that from the head, and in a direction opposite to 
that in which the sun is situated from the comet, appear to diverge 
two streams of light, constituting the tail.(26) Sustained by the 
high standing and great numerical force of these authorities, the po- 
sition here assumed has quite regularly found credénce and a place 
(15) Delambre, Histoire de l Astronomie du Moyen Age, p. 390 et 393. 
(16) Delambre, Histoire de YAstronomie du Dix Huitiéme Siecle, p. 680. 
(17) Math. Prin. of Nat. Phil. (vide note 12,) vol. 2, p. 364. 
(18) Histoire de l’Astronomie Moderne, tome 2, p. 549. 
(19) Astronomie Theorique et Practique, tome 3, p. 401. 
(20) Systéme du Monde, p. 128 
(21) popiee System of Astronomy, vol. 1, p, 444. 
(22) An Introduction to Astronomy, p. 44. 
(23) Edinburgh Encyclopedia. 
(24) American Universal Geography, vol. 1, p. 32. 
(25) Cambridge course of Natural Philosophy, fourth part, p. 306. 
(26) A treatise on Astronomy, first published in 1833; American edition, p. 284 
