PROGRESSIVE EFFECTS. 301 



moment depcn<ls. For, suppose that the body passes through some 

 resisting medium, which partially counteracts the effect of the original 

 impuLso, and by so doing retards the motion: thia counteraction (it 

 needs scarcely here be repeated) is as strict an example of obedience 

 to the law of the impulsi-, 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 dij.'iections, 

 instead of the one effect of one cause. Now, what cause does the 

 body obey in its subsequent motion ? The original cause of motion, 

 or tiie 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, 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 recognized 

 by mathematicians when they enumerate among the causes by which 

 the motion of a body is at any instant determined, the force generated 

 by the previous motion; an expression which would be absurd if 

 taken to imply that this "force" was an intermediate 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 tlie 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 Off 

 being instantaneous, is permanent. Wliatever effect has been pro- 

 duced up to a given time, would (unless prevented by the inter^'entiotl 

 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 an uniform effect, we have a progressive series 

 of effects, arising from the accumulated influence 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 would 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 it is all 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 cannot any longer be 

 produced. Again, the earth causes bodies to fall towards it, that is',- 

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

 body to move towards it at the succeeding instant : and if the earth 

 were instantly 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 intercf^pted by some body or deflected 

 by some other force. The earth, however, not being annihilated, goes 

 on producing in the second instant an effect similar and of equal 

 amount to the first, which two effects being added together, there 

 results an accelerated velocity ; and this operation being repeated at 

 each successive instant, the mere permanence of the cause, although 



