334 



INDUCTION. 



are superinduced. It, therefore, per- 

 petually 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 pro- 

 jectile 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 differ- 

 ence 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 un- 

 less some new cause supervenes, we 

 may speak of the contact of air a 

 hundred years ago as even the proxi- 

 mate 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 per- 

 manency of a series of changes. The 

 second foot, or inch, or mile of mo- 

 tion, is not the mere prolonged dura- 

 tion 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 in- 

 stant. The motion at any subsequent 

 instant is proximately caused by the 

 motion which took place at the in- 

 stant preceding. It is on that, and 

 not on the original moving cause, that 

 the motion at any given moment de- 

 pends. 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 

 need scarcely here be repeated) is as 

 strict an example of obedience to the 

 Jaw of th^ impulse as if the body had 



gone on moving with its original velo- 

 city ; but the motion which results is 

 different, being now a compound of 

 the effects of tv/o 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 re- 

 sisting medium, it continues moving, 

 not with its original, but with its re- 

 tarded velocity. The motion having 

 once been diminished, all that which 

 follows is diminished. 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 mathema- 

 ticians when they enumerate among 

 the causes by which the motion of a 

 body is at any instant determined, 

 the ^orce generated by the previous 

 motion ; an expression which would 

 be absurd if taken to imply that this 

 *' force " was an intermediate link be- 

 tween the cause and the effect, but 

 which really means only the previous 

 motion itself, considered as a cause of 

 further motion. We must, therefore, 

 if we would speak with perfect pre- 

 cision, consider each link in the suc- 

 cession of motions as the effect of the 

 link preceding it. But if, for the 

 convenience of discourse, we speak of 

 the whole series as one effect, it must 

 be as an effect produced by the ori- 

 ginal impelling force ; a permanent 

 effect produced by an instantaneous 

 cause, and possessing the property of 

 self- perpetuation . 



Let us now suppose that the origi- 

 nal agent or cause, instead of being in- 

 stantaneous, is permanent. Whatever 

 effect has been produced up to a given 

 time, would (unless prevented by the 

 intervention of some new cause) sub- 

 sist permanently, even if the cause 

 were to perish. Since, however, the 

 oause does not perish, but continues 

 to exist and to operate, it must go 

 on producing more and more of the 

 effect J and instejid pf an uniform 



