THE THEORY OF ENERGY AND THE LIVING WORLD. 519 



It is that, for instance, which in the collision of bodies remains to effect 

 the rebound. We say that the energy is conserved invariable amid all 

 mechanical transformations. 



In the history of mechanics we learn with what difficulty the ideas 

 of force and work (now known as mechanical energy) have been dis- 

 tinguished. Force has no objective existence, no duration, no perma- 

 nence. It is measured by its effect, the motion which it produces. 

 When, for example, an hydraulic press is put in operation, there is 

 exerted upon the platform exactly the same work which is expended 

 at the piston. The machine only produces a change in the manner of 

 doing work. But, on the other hand, the force is multiplied indefinitely. 

 The whole surface at the platform may be considered as made up of 

 small areas each equal to that of the small piston and, by Pascal's 

 principle, each acted upon by the same pressure applied at the piston. 

 The moment this pressure ceases, the relatively infinite pressure at the 

 platform falls to zero. What real thing can fall instantly from infinity 

 to zero? Work and force belong to different orders of things ; they 

 can not have the same expression. Force is a vector quantity; that is, 

 it includes the idea of direction. Work is a scaler quantity, which 

 admits of the opposition of senses involved in the terms plus and 

 minus. Energy, and in this only it differs from work, is a quantity 

 admitting not even opposition of sigu. We shall see a little further on, 

 however, that a very eminent physiologist, M. Chauveau, has proposed 

 the same term, " the energy of contraction," for the two phenomena of 

 effort and of work. It might seem from the point of view of the ex- 

 penditure of the organism that these two modes of activity, the con- 

 traction static and the contraction dynamic, are really comparable. 

 But although this way of regarding the matter may be perfectly 

 exact and of value, the author's persistence in using nomenclature 

 contrary to the received usage has prevented the acceptance of very 

 useful facts by physicists and even by some physiologists. 



The idea of mechanical power differs from either that of force or of 

 work. It includes the idea of time. In describing a mechanical oper- 

 ation it is not sufficient to give the amount of work done, for the time 

 occupied is an important factor. This is especially the case when the 

 conditions of accomplishment are being considered, as in comparing 

 machines. The one which does the work in the shortest time is called 

 the most powerful. The unit of power is that of a machine which exe- 

 cutes 1 kilogram-meter in a second. For industrial purposes a unit 75 

 times as great as this, called the cheval-vapeur, is frequently employed. 

 It is the power of a machine which does 75 kilogram-meters per second. 

 In electrical industries power is reckoned in kilowatts (equal to 36 

 cheval-vapeur) or in watts, a unit one-thousandth as great. 



It is useless to attempt to determine the power of the human machine 

 relative to industrial machines; for experiment has shown that the 

 mechanical power of living beings depends upon the nature of the work 



