PREFACE. 



THE Phrase ' Correlation of Physical Forces ' in the sense in 

 which I have used it, having become recognized by a large 

 number of scientific writers, it would produce confusion were I 

 now to adopt another title. It would, perhaps, have been better 

 if I had in the first instance used the term Co-relation, as the 

 words ' correlate,' ' correlative,' had acquired a peculiar metaphys- 

 ical sense somewhat differing from that which I attached to the 

 substantive correlation. The passage in the text (p. 183) explains 

 the meaning I have given to the term. 



Twenty years having elapsed since I promulgated the views 

 contained in this Essay, which were first advanced in a lecture at 

 the London Institution in January 1842, and subsequently more 

 fully developed in a course of lectures in 1843, 1 think it advisable 

 to add a little to the Preface with reference to other labourers in 

 the same field. 



It has happened with this subject as with many others, that 

 similar ideas have independently presented themselves to differ- 

 ent minds about the same period. In May 1842 a paper was 

 published by M. Mayer which I had not read when my last edition 

 was published, and indeed only now know imperfectly by the 

 vivd-wce translation of a friend. It deduces very much the same 

 conclusions to which I had been led, the author starting partly 

 from d priori reasoning and partly from an experiment by which 

 water was heated by agitation, and from another, which had, how- 

 ever, previously been made by Davy, viz. that ice can be melted 

 by friction, though kept in a medium which is below the freezing 

 point of water. 



In 1843 a paper by Mr. Joule on the mechanical equivalent of 



