ON THE CONSERVATION OF FORCE. 281 



Just so is it with machines : they are used for the most 

 diversified arrangements. We produce by their agency an infinite 

 variety of movements, with the most various degrees of force 

 and rapidity, from powerful steam-hammers 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 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, 'expenditure 

 of force,' which indicates that the force applied has been ex- 

 pended and lost, leads us to a further characteristic analogy be- 

 tween 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 



