Principles of Energetics. 277 
some obscurity of thought to speak of “intrinsic or absolute 
inertia,” 
The law of the Composition of Motions is but an extension of 
that of Inertia*. For the compounding of a motion is but the 
beginning of another motion; and the change in velocity and 
line of motion of the particle due to each force (difference of 
pressure) is the same as if the others did not act. 
There seems to be a clearer conception afforded of uniform 
and accelerated motion by referring these phenomena, as by this 
principle, to their actual physical conditions. 
9. The application of this principle leads to the following 
theorem suggestive of an explanation of the apparent effect and 
non-effect of the resisting medium on the comets and planets 
respectively. 
According as the resultant of a resisting medium passes or 
not through the centre of gravity of a revolving body is it an 
accelerating force of revolution, or a partially neutralized accele- 
rating force of rotation. 
If the medium is uniform, or if—though it varies in density, 
according to some such law as that with so great probability 
assumed for the solar medium, viz. inversely as the square of the 
distance from the central body t—the face of the revolving body 
is so small that the resultant of the resisting pressures thereon 
passes infinitesimally near the centre of gravity of the whole 
body, it may be easily proved that such a resultant of resistance 
will act as an accelerating force, which, did the body move on a 
solid surface, would retard its revolution, but which, as it moves 
through'a fluid medium, will, by the progressive decrease of its 
major axis and excentricicy, cause its orbit to approach more and 
more to the circular form ; and there will hence result, as in the 
case of Encke’s comet, a secular inequality in the expression of 
the mean longitude, and consequently in the period. 
But if, with the above law of decreasing density, the re- 
sultant of resistance to revolution falls at a finite distance below 
the centre of gravity of the body, it is clear that an unbalanced 
pressure thus applied will affect, not the revolution, but the rota- 
tion of the body ; and that the tendency either to cause or acce- 
lerate rotation will be partly at least neutralized by the resistance 
of the medium to this new motion. 
For let a be the direction of revolution, @’ the resultant of the 
resistance thereto of a medium varying in density. It is evi- 
* Price, ‘ Treatise on Infinitesimal Calculus,’ vol. ii. p. 370. 
Tt See Encke, Ueber die Existenz eines widerstehenden Mittels im 
Weltraume; and Pontécoulant, Théorie Analytique du Systeme du Monde, 
vol. tii. book 4, chap. 5, 
