32 INDUCTION. 



impulse, and by so doing retards the motion : this 

 counteraction (it needs scarcely here be repeated) is 

 as strict an example of obedience to the law of the 

 impulse, as if the body had gone on moving with its 

 original velocity; but the motion which results is dif- 

 ferent, being now a compound of the effects of two 

 causes acting in contrary directions, instead of the one 

 effect of one cause. Now, what cause does the body 

 obey in its subsequent motion? The original cause of 

 motion, or the actual motion at the preceding instant ? 

 The latter : for when the object issues from the resist- 

 ing medium, it continues moving not with its original, 

 but with its retarded, velocity. The motion having 

 once been diminished, all that which follows is dimi- 

 nished. The effect changes, because the cause which 

 it really obeys, the proximate cause, the real cause in 

 fact, has changed. This principle is recognised by 

 mathematicians when they enumerate among the 

 causes by which the motion of a body is at any 

 instant determined, the force generated by the pre- 

 vious motion ; an expression which would be absurd 

 if taken to imply that this "force" was an interme- 

 diate link between the cause and the effect, but which 

 really means only the previous motion itself, consi- 

 dered as a cause of further motion. We must, 

 therefore, if we would speak with perfect precision, 

 consider each link in the succession of motions as the 

 effect of the link preceding it. But if, for the conve- 

 nience of discourse, we speak of the whole series as 

 one effect, it must be as an effect produced by the 

 original impelling force ; a permanent effect produced 

 by an instantaneous cause, and possessing the pro- 

 perty of self-perpetuation. 



Let us now suppose that the original agent 

 or cause, instead of being instantaneous, is per- 



