322 ON THE CONSERVATION OF FORCE. 



of the piston-rod, of the shuttle, of falling hammers and 

 stamps ; or, conversely, of changing the latter into the 

 former ; or it can, on the other hand, change move- 

 ments of uniform into those of varying velocity, and so 

 forth. Hence this extraordinarily rich utility of ma- 

 chines for so extremely varied branches of industry. But 

 one thing is common to all these differences; they all 

 need a 'moving force, which sets and keeps them in 

 motion, just as the works of the human hand all need the 

 moving force of the muscles. 



Now, the work of the smith requires a far greater and 

 more intense exertion of the muscles than that of the 

 violin-player; and there are in machines corresponding 

 differences in the power and duration of the moving 

 force required. These differences, which correspond to 

 the different degree of exertion of the muscles in human 

 labour, are alone what we have to think of when we 

 speak of the amount of work of a machine. We have 

 nothing to do here with the manifold character of the 

 actions and arrangements which the machines produce ; 

 we are only concerned with an expenditure of force. 



This very expression which we use so fluently, ' expen- 

 diture of force,' which indicates that the force applied 

 has been expended and lost, leads us to a further charac- 

 teristic analogy between the effects of the human arm and 

 those of machines. The greater the exertion, and the 

 longer it lasts, the more is the arm tired, and the more 

 is the store of its moving force for the time exhausted. 

 \Ve shall see that this peculiarity of becoming exhausted 

 by work is also met with in the moving forces of inor- 

 ganic nature ; indeed, that this capacity of the human 

 arm of being tired is only one of the consequences of the 

 law with which we are now concerned. When fatigue 

 sets in, recovery is needed, and this can only be effected 

 by rest and nourishment. We shall find that also in the 



