EQUILIBRATION. 501 



density, undergo no sensible diminution in such periods of 

 time as we can measure. 



All these kinds of equilibration may, however, from the 

 highest point of view, be regarded as different modes of one 

 kind. For in every case the balance arrived at is relative, 

 and not absolute — is a cessation of the motion of some par- 

 ticular body in relation to a certain point or points, in- 

 volving neither the disappearance of the relative motion lost, 

 which is simply transformed into other motions, nor a dimi- 

 nution of the body's motions with respect to other points. 

 Thus understanding equilibration, it manifestly includes 

 that equilibrium mobile, which at first sight seems of an- 

 other nature. For any system of bodies exhibiting, like 

 those of the Solar System, a combination of balanced 

 rhythms, has this peculiarity: — that though the constitu- 

 ents of the system have relative movements, the system as a 

 whole has no movement. The centre of gravity of the entire 

 group remains fixed. Whatever quantity of motion any 

 member of it has in any direction, is from moment to mo- 

 ment counter-balanced by an equivalent motion in some 

 other part of the group in an opposite direction ; and so the 

 aggregate matter of the group is in a state of rest. Whence 

 it follows that the arrival at a state of moving equilibrium, 

 is the disappearance of some movement which the aggre- 

 gate had in relation to external things, and a continuance 

 of those movements only which the different parts of the 

 aggregate have in relation to each other. Thus generaliz- 

 ing the process, it becomes clear that all forms of equilibra- 

 tion are intrinsically the same; since in every aggregate, 

 it is the centre of gravity only that loses its motion: the 

 constituents always retaining some motion with respect to 

 each other — the motion of molecules if none else. Every 

 equilibrium commonly regarded as absolute, is in one sense 

 a moving equilibrium; because along with a motionless 

 state of the whole there is always some relative movement 

 of its insensible parts. And, conversely, every moving 



