xvi PREFACE TO THE FIFTH EDITION. 



sion permits me to devote to science so prevent the 

 continuity of attention necessary for the proper evolu- 

 tion 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 

 pardonable, 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 unin- 

 telligible to many for whom it is intended. I have 

 endeavoured so to arrange the subjects 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. 



For this fifth edition I have revised the whole of 

 the text ; and several correspondents having suggested 

 that I should add to it my address as President of the 

 British Association, 1866, it seemed to me on consi- 



