362 INDUCTION. 



acts the effect of the original impulse, and retards the motion ; this coun- 

 teraction (it need scarcely here be repeated) is as strict an example of obe- 

 dience 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 compound of the effects of two causes acting in contrary directions, in- 

 stead 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 act- 

 ual motion at the preceding instant? The latter; for when the object 

 issues from the resisting medium, it continues moving, not with its orig- 

 inal, but with its retarded velocity. The motion having once been dimin- 

 ished, 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 change(1. This principle is recognized by mathematicians when they 

 enumerate among the causes by which the motion of a body is at any in- 

 stant determined the force generated by the previous motion ; an expres- 

 sion which would be absurd if taken to imply tliat this " force " was an in- 

 termediate link between 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 precision, consider 

 each link in the succession 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 original impelling force; 

 a permanent effect produced by an instantaneous cause, and possessing the 

 property of self-perpetuation. 



Let us now suppose that the original 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) 

 subsist permanently, even if the cause were to perish. Since, however, the 

 cause does not perish, but continues to exist and to operate, it must go on 

 producing more and more of the effect; and instead of a unifoi-m effect, 

 we have a progressive series of effects, arising from the accumulated influ- 

 ence of a permanent cause. Thus, the contact of iron with the atmosphere 

 causes a portion of it to rust ; and if the cause ceased, the effect already 

 produced Avould be permanent, but no further effect would be added. If, 

 however, the cause, namely, exposure to moist air, continues, more and 

 more of the iron becomes rusted, until all which is exposed is converted 

 into a red powder, when one of the conditions of the production of rust, 

 namely, the presence of unoxidized iron, has ceased, and the effect can not 

 any longer be produced. Again, the earth causes bodies to fall toward it ; 

 that is, the existence of the earth at a given instant causes an unsupported 

 body to move toward it at the succeeding instant ; and if the earth wei'e 

 annihilated, as much of the effect as is already produced would continue ; 

 the object would go on moving in the same direction, with its acquired 

 velocity, until intercepted by some body or deflected by some other force. 

 The earth, however, not being annihilated, goes on producing in the sec- 

 ond instant an effect similar and of equal amount with the first, which two 

 effects being added together, there results an accelerated velocity ; and 

 this operation being repeated at each successive instant, the mere perma- 

 nence of the cause, though without increase, gives rise to a constant pro- 

 gressive increase of the effect, so long as all the conditions, negative and 

 positive, of the production of that effect continue to be realized. 



It is obvious that this state of things is merely a case of the Composi- 

 tion of Causes. A cause which continues in action must on a strict analy- 



