Principles of the Science of Motion. A5 
the other, it is evident that the mutual pressures of these two 
equal masses will, under such conditions, be unequal, and hence, 
as in the first case, they will approach. It is also evident that 
a body may thus cause the approach to itself of another body, 
whatever the number of interposed bodies. 
9. Thus, if the conception of atoms is applied to the unequal 
and unequally placed bodies of such a world as that presented 
to us, the law of universal attraction follows, and gravity 1s 
mechanically explained, that is, referred to a mechanical con- 
ception. 
But it must be understood that the above proposition is given 
rather to show that, as an actual law, universal attraction may 
be deduced from the theoretical conception of universal repulsion, 
than with any pretension to its being the best attainable form 
of an explanation of the law. It may, however, be remarked 
that such an explanation is in accordance with the chief charac- 
teristics of the force of gravity: it is not polar; and it seems to 
be so far different in kind from other physical forces, that it is 
not interchangeable with them, as they are among each other ; 
for the attraction of gravity is thus referred to difference of 
mass, either between the two attracted bodies, or in the system 
of which they are parts. And other attractions, as will presently 
appear, are referred to differences of tension, or of dynamic equi- 
librium ; and such conceptions do, though that of difference of 
mass does not, involve polarity. 
10. Consider now the second (8) of the above-stated (7) 
general conditions of the development of aforce. And, to fix our 
thoughts, let the system consist of three, and not of an indefinite 
number of bodies. That it must consist of at least three bodies, 
will be evident from the general reasoning of this theory; for 
the fundamental difference between the established and the pro- 
posed conception of a force is this:—a cause of motion is ordi< 
narily defined as “an action between two bodies ;” it is here 
conceived as a differential action between three bodies,—a concep- 
tion, it is submitted, equally defensible by metaphysical reason- 
ing and by physical experiment. 
11. The displacement of a molecule, conceived as an atomic 
centre, may be evidently under such physical and analytically 
expressible conditions as to cause such displacement to be per- 
manent or alternating. The first gives the conception of tension, 
by which electric and magnetic, the second that of vibration 
and undulation, by which optic and acoustic, phenomena are ex- 
plained. 
Let the conditions of the displacement of the molecular centres 
of lines of pressure be such that they are (and consequently, 
the body they constitute) in a state of tension. 
