x88o." The Vehicle of Force. 613 
two particles, at a fixed distance apart, depends upon their 
momentums. If we assume a unit of momentum, then the 
force exerted must depend upon the number of these units 
of moving energy. Consequently a small quantity of matter 
might exert as effective a directive force as a large quantity, 
if the former move with sufficient rapidity to equalise their 
motive energies. In all cases they must exert an equal 
attraction or repulsion upon each other. For if one mass 
be conceived to have ten units ol momentum, and another 
mass one unit, the latter aCts with its unit vigour on each 
of the ten units, giving it ten elements of force. But each 
of the ten units of the larger exerts but one element of force 
on the one unit of the smaller, giving it likewise ten ele- 
ments of force. The fadt is that the force vigour exerted 
upon each other by any two masses in motion is always 
equal, but is not equally effective in speed production unless 
they are of equal momentum. The laigei momentum re- 
sists change in direction more vigorously ; but the quantity 
of effeCt produced upon the inertia of motion is always 
equal, whatever the difference in momentum of the aCting 
masses. . , . 
Thus the utmost vigour of directive force is exerted when 
two bodies move with equal rapidity, and at the highest 
possible speed ; it diminishes in vigour as one or both of the 
bodies lose speed ; it becomes null when one or both attain 
a condition of rest ; and it changes in direction when either 
body reverses its direction of motion. 
It might, however, be deduced from the above argument 
that a vibrating particle, on coming to rest at one extremity 
of its vibration, could not move again, it losing, while in 
that state, all force relations with the neighbouring particles. 
Such might be the case had it no other motions ; but it is 
at rest relatively only, not absolutely. Besides their motions 
of organisation — which we will not here consider all parti- 
cles are in constant general motion, and a relative position 
of rest is only a diminution of some of these common cos- 
mical motions. A particle at rest, in the ordinary accepta- 
tion, is one obeying its cosmical influences only. One 
possessed of special motion is obeying some local influence. 
These common cosmical motions such as the mutual 
movement of particles in the rotation of the earth on its 
axis, and in its revolution around the sun must aCt to 
render the great sum of motions parallel, and the great sum 
of forces attractive. Repulsion is not, in the present con- 
dition of matter, a general affeCtion of nature, at least in so 
far as affairs on the spheres are concerned. It appears only 
