PREFACE TO THE FIFTH EDITION. xv 



necessarily adhered to the form and matter which I 

 had orally communicated. In preparing subsequent 

 editions I found that, without destroying the identity 

 of the work, I could not alter the style ; although it 

 would have been less difficult and more satisfactory to 

 me to have done so, the work would not then have been 

 a republication ; and I was for obvious reasons anxious 

 to preserve as far as I could the original text, which, 

 though added to, is but little altered. 



The form of lectures has necessarily continued the 

 use of the 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, the reason is that 

 when opinions are always hedged in by qualifications, 

 the style becomes embarrassed and the meaning 

 frequently unintelligible. 



As the main object of a course of lectures is to 

 induce the auditor to think, and 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 criticism 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 mathema- 

 tical labours of Sir W. 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 profes- 



