138 SCIENTIFIC THOUGHT. 



underlying unity and correspondence of all natural phe- 

 nomena, inasmuch as they all depend on the trans- 

 formation of a quantity, termed energy, which is in 

 many cases measurable in its best-known form i.e., as 

 energy of motion and, where this is not possible, in 

 the form of heat. 



Helmholtz had already, in 1847, summarily reviewed 

 the whole field, beginning with a restatement of the 

 fundamental formulae of dynamics in the light of the 

 new principle, and ending with a reference to the trans- 

 formation of energy in living vegetable and animal 

 organisms. The key to his explanations is to be found 

 in the introduction of a term to denote what becomes 

 of energy if it ceases to exist as energy of motion or as 

 a velocity, when it is changed to energy of mere position. 

 28. To this end he introduces the idea of stress or tension. 



Helm- 

 holtz on The conception is already contained in older books on 



"tension. * 



mechanics as latent force (Carnot), 1 and the purely 

 mathematical treatment of dynamics by Lagrange and 

 Hamilton had prepared the ground by showing how all 

 dynamical problems could be reduced to the knowledge 

 of two quantities, the vis viva and the force function. 



1 L. N. M. Carnot (1753-1823), 

 usually termed the great Carnot, 

 father of Sadi Carnot, member of the 

 Directory, War Minister, and one 

 of the most celebrated generals of 

 France, has a name in science 

 through his ' Essai sur les Machines 

 en general ' (Dijon, 1784), his 'Prin- 

 cipes fondamentaux de 1'Equilibre 

 et du Mouvement' (Paris, 1803), as 

 well as through his ' Reflexions sur 

 la Metaphysique du Calcul infi- 

 nite'simal ' (Paris, 1797) and his 

 ' The'orie des Transversales ' (Paris, 

 1806), by which he became, to- 



gether with Monge, one of the 

 founders of modern geometry, of 

 which more in a subsequent chapter. 

 He introduced the principle of the 

 ' Correlation des Figures de Geo- 

 metric ' (Paris, 1801). His books 

 were translated in Germany, where 

 they had a great influence. On his 

 connection with the history of the 

 conception of energy, see Bohn in 

 ' Phil. Mag.,' iv. 300, vol. xxix. ; 

 also Helm, 'Energetik,' p. 13 ; and 

 the Eloge by Arago of the year 

 1837. 



