30 INDUCTION. 



force which launches a cannon ball into space, produces a 

 motion which would continue for ever unless some other force 

 counteracted it. 



Between the two examples which we have here given, 

 there is a difference worth pointing out. In the former (in 

 which the phenomenon produced is a substance, and not a 

 motion of a substance), since the rust remains for ever and 

 unaltered unless some new cause supervenes, we may speak of 

 the contact of air a hundred years ago as even the proximate 

 cause of the rust which has existed from that time until 

 now. But when the effect is motion, which is itself a change, 

 we must use a different language. The permanency of the 

 effect is now only the permanency of a series of changes. The 

 second foot, or inch, or mile of motion, is not the mere pro- 

 longed duration of the first foot, or inch, or mile, but another 

 fact which succeeds, and which may in some respects be very 

 unlike the former, since it carries the body through a different 

 region of space. Now, the original projectile force which 

 set the body moving is the remote cause of all its motion, how- 

 ever long continued, but the proximate cause of no motion 

 except that which took place at the first instant The motion 

 at any subsequent instant is proximately caused by the motion 

 which took place at the instant preceding. It is on that, 

 and not on the original moving cause, that the motion at any 

 given moment depends. For, suppose that the body passes 

 through some resisting medium, which partially counteracts 

 the effect of the original impulse, and 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 different, being now a com- 

 pound of the effects of two causes acting in contrary directions, 

 instead of the single 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 resisting 

 medium, it continues moving, not with its original, but with 

 its retarded velocity. The motion having once been diminished, 



