EQUILIBRATION. 529 



on their cohesion, or their inertia, or both. In other words, 

 the denser or rarer medium which occupies the places from 

 moment to moment passed through by such moving body, 

 having to be expelled from them, as much motion is ab- 

 stracted from the moving body as is given to the medium in 

 expelling it from these places. This being the condition 

 under which all motion occurs, two corollaries result. The 

 first is, that the deductions perpetually made by the com- 

 munication of motion to the resisting medium, cannot but 

 bring the motion of the body to an end in a longer or shorter 

 time. The second is, that the motion of the body cannot 

 cease until these deductions destroy it. In other words, 

 movement must continue till equilibration takes place; and 

 equilibration must eventually take place. Both these are 

 manifest deductions from the persistence of force. To say 

 that the whole or part of a body's motion can disappear, save 

 by transfer to something which resists its motion, is to say 

 that the whole or part of its motion can disappear without 

 effect; which is to deny the persistence of force. Con- 

 versely, to say that the medium traversed can be moved out 

 of the body's path, without deducting from the body's mo- 

 tion, is to say that motion of the medium can arise out of 

 nothing; which is to deny the persistence of force. Hence 

 this primordial truth is our immediate warrant for the con- 

 clusions, that the changes which Evolution presents, cannot 

 end until equilibrium is reached ; and that equilibrium must 

 at last be reached. 



Equally necessary, because equally deducible from this 

 same truth that transcends proof, are the foregoing proposi- 

 tions respecting the establishment and maintenance of mov- 

 ing equilibria, under their several aspects. It follows from 

 the persistence of force, that the various motions possessed 

 by any aggregate, either as a whole or among its parts, must 

 be severally dissipated by the resistances they severally en- 

 counter ; and that thus, such of them as are least in amount, 

 or meet with greatest opposition, or both, will be brought to 



