42 THE HUMAN MOTOR 



The kinetic energy of a spring, when it expands is the restora- 

 tion of the potential energy which it accumulated when it was 

 compressed. 



Every body has a reserve of energy, eithei potential or kinetic, 

 stored within it. A body placed at .a, given height possesses the 

 power of performing, in falling, a certain quantity of work. This 

 work is a measure of the kinetic energy which is equal to the 

 potential energy. In all this something is preserved and has 

 merely changed its form.... It is also interesting to note, regarding 

 energy, that it explains the production of heat by friction. Rum- 

 ford, Davy and others showed that friction real mechanical 

 energy was transformed into heat. It was suspected also that 

 light was transformed into heat, and that the latter provided the 

 force of machines and of animals^ 1 ) A poor German doctor, of 

 Heilbronn, Robert -Jules Mayer (1814-1878) attempted to prove 

 this. 



The history of the birth of " energetics " is very curious. The 

 word " energy," in the sense of work was employed by 

 Bernoulli in a letter to Varignon ( 2 ), dated 1717, and by Couplet ( 3 ) 

 in 1726. Young ( 4 ) used it in 1807, but its adoption exclusively 

 in physics took place, thanks to Rankine and William Thomson 

 (later Lord Kelvin) between 1850 and 1870. At the same time a 

 number of natural philosophers, from Huyghens~(1680), Leibnitz 

 (1696) and Lazare Carnot (1803) until Helmholtz (1847) made 

 use of the expression " live force " to designate work, or, 

 more briefly, of the word force ( 5 ). 



These early philosophers were puzzled by the fact that " force " 

 could apparently be destroyed. But Leibnitz solved the diffi- 

 culty by explaining that in impact and in friction the " live force " 

 lost gave birth to heat by the agitation of the particles of the body 

 acted upon. ' The forces," he said, " are not destroyed, but 

 dissipated amongst the component parts of the body. They 

 are not lost ; it is like changing sovereigns into silver ( 6 )." Thus 

 heat was regarded as the movement of invisible parts of bodies, 

 " diversified and very rapid movement," according to Robert 

 Boyle, ( 7 ) Rum ford, ( 8 ) Young (already quoted), Davy,( 9 ) Am- 



( 1 ) Herschell, Outline of Astronomy, 1833. 



( 2 ) Varignon, Nouvelle Mechanique, 2 vols., 1725. 



( 3 ) Couplet, Memoires de I' A cad. Roy. des Sciences, 1726, p. 119. 

 (*) Thomas Young, Lectures on Natural Philosophy, viii., 1807. 



( 6 ) For instance, the famous AlSmoire sur la Conservation de la Force of 

 Helmholtz (1847), French translation by Perard (1869). This publication, 

 which caused a sensation at the time, to-day appears to us as of very 

 mediocre interest. 



( 6 ) Leibnitz, Mem. Acad. Sciences, 1728. 



( 7 ) Robert Boyle, Works, iii., 1744. 



( 8 ) Rumford, from 1798 ; see Memoir de I' Institute, 1804, and Essays. 

 () Davy, Elem. Philos. C him., translated by Van Mons, 2 vols., 1813- 



14, vol.'i., p. 53. 



