54 Remarks on Inertia. 



proof of the position is still weaker than this : Avhat is meant 

 then by the body's resisting us ? not surely that when we 

 impel it in one direction, it makes an effort in the opposite ; 

 all therefore that can be intended is that the body does not 

 assist us; — that is, it is entirely passive in our hands. Of course 

 the gravity must be overcome by the force impressed ; but ab- 

 stracting from this, matter is wholly without action, or a ten- 

 dency to motion. This being so, when it is said that we can- 

 not conceive how motion could be communicated unless mat- 

 ter resisted us, — the meaning is, unless matter were passive. 

 The proposition then is equivalent to this, that, if matter 

 were active or in motion, we do not know how motion could 

 be communicated; and certainly, if by communicating mo- 

 tion, be intended the commencing of it, the remark would 

 be just, as the body is supposed to be in motion already. If 

 by not being passive, or by being active, it be designed to 

 express some active power in matter, to be exerted at pleas- 

 ure, even though this matter should not be in motion, the 

 argument is still unfounded : since it is unreasonable to sup- 

 pose that all bodies would at all times be so perverse as to 

 exert their efforts in opposition to ours :* perhaps they might 

 co-operate with us, and thus our labor would be very much 

 diminished. We think then that no author has any right to 

 assert gravely, as if it were a decided point, that inertia is the 

 means by which motion is communicated : to do this is to 

 wander from ascertained fact and experiment into the field 

 of learned scholastic trifling. 



The last defence which we have seen of the phrase " force 

 d'inertie," is given by Brisson : To the objection that, after a 

 body, suspended by a string, has assumed a vertical position, 

 "it resists a change of place by its gravity, and therefore that 

 what is called " force d'inertie, 1 ' is the same thing as gravity, 

 he answers, that when the line that supports the body is ver- 

 tical, the gravity is wholly counteracted by it ; and for this 

 reason, cannot resist a force until the line of support is mov- 

 ed from the vertical direction : " Son deplacement doit done 

 preceder l'effort de sa pesanteur. Mais pour operer ce de- 

 placement, il faut employer une force, reelle, qui, si elle 

 est trop petite pour deplacer la boule n'en est pas moins une 

 force reelle, et cependant n'a point d'effet. Dans ce cas-la, 



* We are not Gnostics, nor Manicheans. 



