Examination of the Theory of a Resisting Medium. 15 
planets of the solar system; and revolving as they do in ellipses of 
great eccentricity, many of these bodies having their aphelions at 
such immense distances as are not readily appreciable, by any of 
our methods of computation, their motions are much less subject to 
rigorous demonstration than those of the planets.(54) Still so much 
confidence had Prof. Encke in the conclusions he had been able to 
draw, in the paper we have mentioned, that the movements of all 
these bodies which have been visible since its publication have been 
observed with increased care and assiduity ; while the most rigid 
investigations of their former movements have not been overlooked. 
According to Prof. Encke, the comet which bears his name, in its 
several revolutions, between 1786 and 1819, exhibited a mean de- 
crease in the times of those revolutions. Now, as resistance, from 
an ethereal medium, would have the effect, by diminishing the velo- 
city of the comet, to lessen its centrifugal force, and thus force it 
down nearer the sun, it follows that precisely the result which Encke 
observed, would be the effect of such resistance. ‘To the agency of 
ether, therefore, was this diminution ascribed, though not until after 
all other circumstances which were supposed to have had any agency 
in the result had been carefully considered. SBiela’s comet, or the 
comet of six years.and three quarters, was also observed with refer- 
ence to this action of resisting ether; as was, finally, the comet of 
Halley, whose last disappearance was in 1836. ‘These three are 
the only ones, of all that have been seen, whose recular, periodical 
return is known, at the present day. The acceleration in the mean 
motion of Encke’s comet if not due to the resistance of ether, is still 
unexplained. Biela’s comet, in its return, in 1832, was also retard- 
ed, ‘but it throws new perplexity upon the question of a resisting 
medium. Encke and Gauss find a diminution of nine tenths of a day 
(54) Too many authors of just renown, have overlooked perspicuity, and writ- 
ten vaguely, upon this point. Brewster, (Encyclopedia,) has not wholly escaped 
the charge of sacrificing philosophical accuracy to euphony, in the following: 
“ Traversing unseen the remote portion of its orbit, the comet wheels its ethereal 
course far beyond the limits of our system. What regions it there visits, or upon 
what destination it is sent, the limited powers of man are unable to discover. ~ Af- 
ter the lapse of years, we perceive it again returning to our system, and tracing a 
portion of the same orbit round the sun, which it had formerly described.” If it 
leave the sphere of our sun’s attraction must it not of necessity, gravitate to some 
other body, and be thus prevented from ever returning? Laplace, (Systeme du 
Monde,) has been more careful. ‘ Innumerable comets, after having approached | 
the sun, are elongated from it to such distances as to prove that its empire extends 
much beyond the known limits of our planetary system.” 
