Examination of the Theory of a Resisting Medium. 19° 
dicate, portions of the small masses of these bodies are occasionally 
removed from the nucleus or its vicinity to form the tails, which 
are sometimes extended to enormous lengths, while at others these 
portions of matter are reassembled around the nucleus, in whole or 
in part, these changes, by shifting the centre of gravity of the com- 
etary body, must effect the action of foreign bodies thereon, and con- 
sequently influence the comet’s motions. One other source of un- 
certainty, and one too which it would seem must forever remain such, 
in the movements of comets, is their action upon each other. ‘To 
remove this source of errour no less would seem to be required than 
to identify every comet belonging to our solar system ; to know the 
mass of each, the elements of the orbit it describes, as well as the 
elements of all those which perturbations may cause it hereafter to 
assume; and to weigh all its disturbing forces with such accuracy 
as to be able to determine its place, relatively to the sun and to 
every other body, at any given point of time. May not these nu- 
merous and active causes very well account, not only for the ine- 
qualities we have observed in the motions of comets, but even for 
much greater and more numerous ones, without the aid of a resisting 
medium? Some of these taken singly would, indeed, produce only ~ 
slight results ; but when it is considered that ‘‘in the immense ellipse 
described by a comet, the imperfection of analysis obliges the ge- 
ometrician to folldw that body step by step, as it were, without once 
losing sight of it for a single.moment” throughout its revolutions, 
they may readily enough be supposed to cause greater deviations 
from calculated periods than ‘three one hundredths of a day,” or 
less than forty-four minutes in a term of six years and three quarters. 
Pontecoulant deems it impossible, in the present state of science, to 
determine within one or two days, the instant of the passage of a 
comet through its perihelion; so very uncertain are the elements 
which astronomy furnishes for calculating their perturbations.(65) 
Having thus submitted the leading positions and arguments favour- 
able to the theory of a resisting medium in the celestial regions, to 
detailed examination, the whole, according to the views we have 
taken, may be resolved into the following heads : 
Ist. That in periods of the most remote antiquity there prevailed 
a belief in the presence of ether in the celestial regions; but the 
proof, if any, upon which this belief was founded has not been pre- 
(65) Connaissance des Tems, pour l’an 1838, p. 119. 
