34 INDUCTION. 



so long as all the conditions, negative and positive, 

 of the production of that effect continue to be 

 realized. 



It must be obvious that this state of things is 

 merely a case of the Composition of Causes. A cause 

 which continues in action, must on a strict analysis 

 be considered as a number of causes exactly similar, 

 successively introduced, and producing by their com- 

 bination the sum of the effects which they would 

 severally produce if they acted singly. The progres- 

 sive rusting of the iron is in strictness the sum of the 

 effects of many particles of air acting in succession 

 upon corresponding particles of iron. The continued 

 action of the earth upon a falling body is equivalent 

 to a series of forces, applied in successive instants, 

 each tending to produce a certain constant quantity 

 of motion : and the motion at each instant is the sum 

 of the effects of the new force applied at the preceding 

 instant, and of the motion already acquired. In each 

 instant, a fresh effect of which gravity is the proximate 

 cause, is added to the effect of which it was the 

 remote cause : or (to express the same thing in ano- 

 ther manner) the effect produced by the earth's influ- 

 ence at the instant last elapsed, is added to the sum 

 of the effects of which the remote causes were the 

 influences exerted by the earth at all the previous 

 instants since the motion began. The case, therefore, 

 comes under the principle of a concurrence of causes 

 producing an effect equal to the sum of their separate 

 effects. But as the causes come into play not all at 

 once, but successively, and as the effect at each 

 instant is the sum of the effects of those causes only 

 which have come into action up to that instant, the 

 result assumes the form of an ascending series ; a 



