ENERGY IN GENERAL. 73 



It will be remarked that the idea of time does not 

 intervene in our estimation of work. The notion of 

 work is independent of the ideas of velocity and 

 time. " The greater or less time that we take to do 

 a piece of work is of no more assistance in measur- 

 ing its magnitude than the number of years that 

 a man may have taken to grow rich or to ruin him- 

 self can help to estimate the present amount of his 

 fortune." 



Going back to Carnot's comparison, an employer 

 who employed his workmen only on piece-work, — that 

 is to say, who would only care about the amount of 

 work done, and would be indifferent to the time that 

 they took over it, — would be at the same point of view 

 as the advocates of the mechanical theory. M. 

 Bouasse, whom we follow here, has remarked that this 

 idea of mechanical work may be traced back to 

 Descartes. His predecessors, and Galileo in par- 

 ticular, had quite a different idea of the way in which 

 mechanical activity should be measured ; and so, 

 among the mathematicians of the eighteenth century, 

 Leibniz and, later, John Bernoulli were almost alone 

 in looking at it from this point of view. 



Energy. — Work thus understood is mechanical 

 energy. It represents the lasting and objective effect 

 of the mechanical activity independent of all the cir- 

 cumstances under which it was carried out. The 

 same work may be done under very different con- 

 ditions of time, velocity, force, and displacement. 

 It is therefore the permanent element in the variety of 

 mechanical aspects. Work, for example, in the 

 collision of bodies when the motion of a body appears 

 to be destroyed on impact with another, reappears as 

 indestructible vis viva. This, then, is exactly what 



