Examination of the Theory of a Resisting Medium. 7 
in the numerous works of subordinate authors; msomuch that. we 
have pretty uniformly recognized it in the elementary works upon 
astronomy that we have examined in the English language.(27) 
The cause assigned for this direction of comets’ trains, by Pingreé, 
namely, the resistance of the ether, appears not to have found much 
favour in the minds of his successors ; consequently we find, in gen- 
eral, the expression employed, namely, ‘‘impulston of the sun’s 
rays,” to denote both the agent and the manner of that agent’s ac- 
tion, in producing this result. 
Great additional impulse has, within a few years, been given to 
the theory of a resisting medium by the detailed and able paper of 
Prof. Encke, upon the observed decrease of the times of revolution 
of the comet which bears the name of that astronomer. This pa- 
per has been translated into English, and is more or less extensively 
quoted by almost every writer who has employed his pen upon ce- 
lestial motions, since the date of its appearance. ‘The author says: 
“‘If I may be permitted to express my opinion on a subject whieh, 
for twelve years, has incessantly occupied me, in treating which I 
have avoided no method, however circuitous, no kind of verification, 
in order to reach the truth, as far as it Jay in my power; I can- 
not consider it otherwise than completely established, that an ex- 
traordinary correction is necessary for Pons’ [Eincke’s] comet, and 
equally certain that the principal part of it consists in an increase of 
the mean motion proportionate to the time.”(28) Dr. Bowditch, by 
reference to the memoir of Encke, supposes the existence of a re- 
sisting medium highly probable, as there disclosed, in the motions of 
Encke’s comet, in its successive appearances between the years 1786, 
and 1829.(29) | Arago, of the Royal Observatory, at Paris, in an es- 
say, in 1832, fully recognizes this resisting medium, on the author- 
ity of Encke, and dwells at considerable length upon its effects.(30) 
M. Gautier assumes that, results obtained in 1828, from the move- 
ment of Encke’s comet, accord with those which Encke had previ- 
ously procured, and which induced him, (Encke,) in 1823 to sup- 
pose the existence of a medium or ethereal fluid, in space, of which 
the resistance, acting as a tangential force against the motion of the 
comet, would augment the power of the sun, and shorten the period 
(27) Some of the recent French works, of a similar character, constitute ex- 
ceptions to this rule. 
(28) Prof. Encke’s Memoir, as quoted in the American Almanack, 1834. 
(29) Mécanique Céleste, (Bowditch,) translator’s note, vol. 3, p. 678. 
(30) Tract on Comets, by Arago, translated into English, by Prof. Farrar, Bos- 
ton; 1832. 
