8 PEEFACE. 



first person, and I would beg my readers not to attribute to me, 

 from the modes of expression used, a dogmatism which, is far from 

 my thought. If my opinions are expressed broadly, it is that, if 

 opinions are always hedged in by qualifications, the style becomes 

 embarrassed and the meaning frequently unintelligible. 



As a course of lectures can only be useful by inducing the 

 auditor to consult works on the subject he hears treated, so the 

 object of this Essay is more to induce a particular train of thought 

 on the known facts of physical science than to enter with minute 

 criticisms into each separate branch. 



In one or two of the reviews of previous editions the general 

 idea of the work was objected to. I believe, however, that will 

 not now be the case ; the mathematical labours of Mr. Thompson, 

 Clausius, and others, though not suitable for insertion in an 

 Essay such as this, have awakened an interest for many portions 

 of the subject, which promises much for its future progress. . 



The short and irregular intervals which my profession permits 

 me to devote to science so prevent the continuity of attention 

 necessary for the proper evolution of a train of thought, that I 

 certainly should not now have courage to publish for the first 

 time such an Essay ; and it is only the favour it has received 

 from those whose opinions I highly value, and the, I trust pardon 

 able, wish not to let some favourite thoughts of my youth lose all 

 connection with my name, that have induced me to reprint it. 



My scientific readers will, I hope, excuse the very short notices 

 of certain branches of science which are introduced, as without 

 them the work would be unintelligible to many for whom it is 

 intended. I have endeavoured so to arrange my matter that each 

 division should form an introduction to those which follow, and 

 to assume no more preliminary knowledge to be possessed by my 

 readers than would be expected from persons acquainted with the 

 elements of physical science. 



The notes contain references to the original memoirs in which 

 the branches of science alluded to are to be found, as well as to 

 those which bear on the main arguments ; where these memoirs 

 are numerous, or not easy of access, I have referred to treatises in 

 which they are collated. To prevent the reader s attention being 

 interrupted, I have in the notes referred to the pages of the text, 

 instead of to interpolated letters. 



