IX THE LAST HALF-CEXTURY. 73 



cal operations, were intimately, and, in 

 various cases, quantitatively related. It 

 was demonstrated that any one could be 

 obtained at the expense of any other; 

 and apparatus was devised which exhib- 

 ited the evolution of all these kinds of 

 action from one source of energy. Hence 

 the idea of the 'correlation of forces' 

 which was the immediate forerunner of the 

 doctrine of the conservation of energy. 



It is a remarkable evidence of the 

 greatness of the progress in this direction 

 which has been effected in our time, that 

 even the second edition of the 'History 

 of the Inductive Sciences,' which was 

 published in 1846, contains no allusion 

 either to the general view of the ' Corre- 

 lation of Forces ' published in England in 

 1842, or to the publication in 1843 of the 

 first of the series of experiments by which 

 the mechanical equivalent of heat was 

 correctly ascertained.* Such a failure on 



* This is the more curious, as Ampere's hypothesis 



