PREFACE 



TO 



THE FIFTH EDITION. 



THE PHRASE * Correlation of Physical Forces/ in the 

 sense in which I have used it, having become re- 

 cognised 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 

 metaphysical sense somewhat differing from that which 

 I attached to the substantive correlation. The passage 

 in the text (p. 165) explains the meaning I have given 

 to the term. 



Twenty-five years having elapsed since I promul- 

 gated the views contained in this Essay, which were 

 first advanced in a lecture at the London Institution, 

 in January 1842, printed by the Proprietors, and subse- 

 quently more fully developed in a course of lectures in 

 1843, published in abstract in the 'Literary Gazette/ 

 I 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 



