PROGRESSIVE EFFECTS. 33 



manent. Whatever effect has been produced up 

 to a given time, would (unless prevented by the 

 intervention of some new cause) subsist perma- 

 nently, 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 con- 

 verted 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 intercepted 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 perma- 

 nence of the cause, although without increase, gives 

 rise to a constant progressive increase of the effect, 



VOL. II. D 



