PROGRESSIVE EFFECTS. 31 



unless some new external conditions are superinduced. 

 It, therefore, perpetually happens that a temporary 

 cause gives rise to a permanent effect. The contact 

 of iron with moist air for a few hours, produces a rust 

 which may endure for centuries ; or a projectile 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 per- 

 manency 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 prolonged 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, however long continued,, but the 

 proximate cause of no motion except that which took 

 place at the first instant. The motion at any subse- 

 quent 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 



