11.] 



TRANSFORMATIONS OF ENERGY. 31 



come consists in friction, we now know that the work 



done is represented by heat or electricity. But when the 



work done is of such a nature as to alter the state of the No exeep- 



body on which work is done, as for instance in sawing Jeappea?^ 



wood or grinding corn, it might be supposed that the ance of the 



energy that has done the work is represented by the work that^has 



done, and will not reappear as energy. This, however, is "lone work. 



not the case ; it is transformed into heat or electricity, just 



as when the work done consists simply in overcoming 



friction. 



In speaking of the potential energy of gravitation, I 

 have remarked that a force cannot produce energy, if, like 

 the pressure of the ocean on its bed, it is so placed that it 

 cannot cause motion. But the force that is overcome in 

 sawing wood or grinding corn — namely, the force of solid 

 cohesion — is a force that cannot produce energy, not for Forces that 

 want of room to act, but because it is not its nature to do so. |',™'J°*g 

 It is mere resistance ; and it would be an improvement in energy. 

 our scientific language if the word force were confined to 

 forces that can produce motion, and the word strength 

 always applied to forces of mere resistance. 



Strength is measured by the force required to overcome Measure of 

 it : the strength of materials, for instance, is measured by ° 

 the weights needed to break or crush the materials. 



