84 THE ADVANCE OF SCIENCE 



to get rid of the dual conception of the 

 causes of motion which has hitherto pre- 

 vailed. On this hypothesis, the hammer- 

 ing of the ultra-mundane corpuscles on 

 the bob confers its kinetic energy, on the 

 one hand, and takes it away on the other ; 

 and the state of potential energy means 

 the condition of the bob during the instant 

 at which the energy, conferred by the ham- 

 mering during the one half -arc, has just 

 been exhausted by the hammering during 

 the other half -arc. It seems safe to look 

 forward to the time when the conception 

 of attractive and repulsive forces, having 

 served its purpose as a useful piece of 

 scientific scaffolding, will be replaced by 

 the deduction of the phenomena known 

 as attraction and repulsion, from the 

 general laws of motion. 



The doctrine of the conservation of en- 

 ergy which I have endeavored to illustrate 

 is thus defined by the late Clerk Max- 

 well : 



