12 Examination of the Theory of a Resisting Medium. 
is not so. Indeed they form so many different angles, both in re- 
gard to the comet’s line of motion, and to the relative position of 
the sun, that no settled fact seems deducible from the circumstance 
of their direction. Flamsted, in his account of a comet which he 
observed at Greenwich, in May, 1677, is at the pains to state that its 
tail was not directed in a line opposite the sun, but deviated there- 
from at an angle of ten degrees.(44) Hevelius, of a comet he ob- 
served, in 1682, says, ‘‘sometimes its tail was directed pretty ex- 
actly in opposition to the sun, as August 30, in the morning; but 
often with a considerable deviation, as is usual in most comets.”(45) 
The great comet of 1744 had, at one time, no less than six distinct 
tails, spread out like a fan. ‘They were each about 4° broad; and 
the space between these several tails was as dark as the rest of the 
heavens. ‘There exist. other examples of the tails of comets which 
have separated into several branches.(46) Newton cites two com- 
ets, the tails of which deviated from a right line joining the sun and 
comet, one ten, and the other no less than twenty one degrees.(47) 
The comet which appeared in January, 1824, besides the usual tail, 
opposite the sun, had another directed from the nucleus of the comet 
towards the sun. ‘‘'The singular form of this comet,” says the nar- 
rator, ‘‘adds new difficulties to the problem by which it has been 
explained, in a manner quite satisfactory, that the impulsion of the 
sun’s rays is the principal cause of comets’ tails always taking a di- 
rection opposite to the sun.”’(48) Much that has been written upon 
the cause, nature and character of these peculiar appendages of com- 
ets, appears to have been based entirely upon assumed-data. Such 
authority is alike unsafe and detrimental. ‘The views of Arago are 
more sane, and therefore more valuable. ‘‘ Kepler supposed the 
formation of the tails of comets was the result of the impulsion of the 
solar rays, which detached from the head of the comet the lighter 
portions of that body, and removed them to a distance beyond it. 
To render this explanation admissible it is necessary to prove that 
the solar rays are endowed with an impulsive force; for the most 
(44) Philos. Trans. of the Royal Society, (vide note 39;) vol. 2, p. 394. 
(45) Ibid. p. 559. 
(46) Delambre, l’Astronomie au dix-Huitiéme Siecle, p. 680; et Delambre, 
Astronomie Théorique et Practique, tome 3, planche. Also, Arago, tract on 
Comets, Farrar’s translation. 
(47) Math. Prin. (vide note 12,) vol. 2, pp. 360 and 364. 
(48) Jambon, Nouveau cours démonstratif et elémentaire d’Astronomie, p. 330, 
et 331. 
