18 Examination of the Theory of a Resisting Medium. 
that the action of these three bodies, neglected as insensible by 
Pontecoulant, was sufficient to produce an acceleration of six days 
and one third in the return of Halley’s comet.(61) With all these . 
uncertainties respecting the larger known planets of our system, we 
must not forget that the masses of the four new planets are in no 
degree known, beyond the fact that, compared with some of the 
older ones, they are very small. But still, small as they are, they 
are probably capable of exercising an influence, according to relative 
position, distance, &c. upon bodies as easily disturbed as comets; 
and yet no sane attempt at a demonstration of the amount of such 
influence can be made, in the present state of our knowlege, for 
want of the necessary data. Brewster, in endeavoring to account 
for the lost comet of 1770, supposed, what indeed the subsequent 
investigations of Laplace have rendered wholly improbable, namely, 
that one of these new planets had arrested that body in its course, 
and added it to its own mass.(62) We have seen that the mass of 
Uranus, as well as of other planets, is unsettled: the number of its 
satellites is equally so. Herschel enumerates six. Laplace says 
powerful telescopes are necessary to perceive the second and the 
fourth, and that the published observations of Herschel upon the 
other four are too few to determine the elements of their orbits, or 
even incontestibly to assure us of their existence.(63) The younger 
Herschel says, of these satellites, “two undoubtedly exist, and four 
more have been suspected.” (6 
The immense periods of time consumed by some comets in per- 
forming their stated revolutions, are sufficient to convince us that the 
space beyond the orbit of the most distant planet now known to us, 
and within which moving bodies gravitate to our sun, is such that its 
extent could not easily be computed by any of our habitual methods. 
Whether planets still undiscovered by us are revolving there, in or- 
bits beyond that of Uranus, is wholly unknown to us, and this igno- 
rance of ours, while it continues, must involve in uncertainty the 
movenients of all such comets as have their aphelions within the re- 
gions in question. The changes in the form and bulk of these bo- 
dies, i in calculations so minute as have been attempted, to establish 
of resistance, deserve attention. If, as appearances in- 
(61) Arago, Annuaire pour l’an 1836. 
_ (62) Edinburgh pos abe article Comets. 
' (63) Systéme du 
(64) Astronomy, p. 289, 
