CHAPTER XV. 



OF PROGRESSIVE EFFECTS; AND OF THE CONTINUED 

 ACTION OF CAUSES. 



1. IN the last four chapters we have traced the general 

 outlines of the theory of the generation of derivative laws 

 from ultimate ones. In the present chapter our attention 

 will be directed to a particular case of the derivation of 

 laws from other laws, but a case so general, and so im 

 portant, as not only to repay, but to require, a separate 

 examination. This is, the case of a complex phenomenon 

 resulting from one simple law, by the continual addition of 

 an effect to itself. 



There are some phenomena, some bodily sensations for 

 example, which are essentially instantaneous, and whose exis 

 tence can only be prolonged by the prolongation of the exis 

 tence of the cause by which they are produced. But most 

 phenomena are in their own nature permanent ; having begun 

 to exist, they would exist for ever unless some cause inter 

 vened having a tendency to alter or destroy them. Such, for 

 example, are all the facts or phenomena which we call bodies. 

 Water, once produced, will not of itself relapse into a state of 

 hydrogen and oxygen ; such a change requires some agent 

 having the power of decomposing the compound. Such, 

 again, are the positions in space, and the movements, of 

 bodies. No object at rest alters its position without the 

 intervention of some conditions extraneous to itself; and 

 when once in motion, no object returns to a state of rest, or 

 alters either its direction or its velocity, unless some new ex 

 ternal conditions are superinduced. It, therefore, perpetually 

 happens that a temporary cause gives rise to a permanent 

 effect. The contact of iron with moist air for a few hours, pro 

 duces a rust which may endure for centuries ; or a projectile 



