ON THE CONSERVATION OF FORCE 185 



diversified arrangements. We produce by their agency 

 an infinite variety of movements, with the most various 

 degrees of force and rapidity, from powerful steam-ham- 

 mers and rolling-mills, where gigantic masses of iron are 

 cut and shaped like butter, to spinning and weaving-frames, 

 the work of which rivals that of the spider. Modern 

 mechanism has the richest choice of means of transferring 

 the motion of one set of rolling wheels to another with 

 greater or less velocity; of changing the rotating motion 

 of wheels into the up-and-down motion 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 movements of uniform into those of 

 varying velocity, and so forth. Hence this extraordinarily 

 rich utility of machines for so extremely varied branches 

 of industry. But one thing is common to all these differ- 

 ences; 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 mani- 

 fold character of the actions and arrangements which the 

 machines produce; we are only concerned with an expendi- 

 ture of force. 



This very expression which we use so fluently, * expendi- 

 ture of force/ which indicates that the force applied has 

 been expended and lost, leads us to a further characteristic 

 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. We shall see 

 that this peculiarity of becoming exhausted by work is also 

 met with in the moving forces of inorganic nature; indeed, 

 that this capacity of the human arm of being tired is only 



