314 



ON THE CONSERVATION OF FORCE. 



A raised weight can produce work, but in doing so it must 

 necessarily sink from its height, and, when it has fallen as deep 

 as it can fall, its gravity remains as before, but it can no longer 

 do work. 



A stretched spring can do work, but in so doing it becomes 

 loose. The velocity of a moving mass can do work, but in doing 

 so it comes to rest. Heat can perform work ; it is destroyed in 



FIG. 51. 



the operation. Chemical forces can perform work, but they ex- 

 haust themselves in the effort. 



Electrical currents can perform work, but to keep them up 

 we must consume either chemical or mechanical forces, or heat. 



We may express this generally. It is a universal character of 

 all known natural forces that their capacity for work is ex- 

 hausted in the degree in which they actually perform work. 



We have seen, further, that when a weight fell without per- 

 forming any work, it either acquired velocity or produced heat. 



