viii PREFACE TO THE FIFTH EDITION. 



others, that similar ideas have independently presented 

 themselves to different minds about the same period. 

 In * Liebig and Wohler's Annalen ' for May 1842,* a 

 paper appeared by M. Mayer, which I had not read 

 when my third edition was published, but which I 

 have now read in the translation by Mr. Youmans, of 

 New York. It deduces very much the same conclu- 

 sions to which I had been led, the author starting 

 partly from d priori reasoning and partly from an ex- 

 periment by which water was heated by agitation ; and 

 from another, which had, however, 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 heat appeared, which, though not in 

 terms touching on the mutual and necessary depen- 

 dence of alt the Physical Forces, yet bears most im- 

 portantly upon the doctrine. 



While my third edition was going through the press 

 I had the good fortune to make the acquaintance of 

 M. Seguin, who informed me that his uncle, the emi- 

 nent Montgolfier, had long entertained the idea that 

 force was indestructible, though, with the exception of 

 one sentence in his paper on the hydraulic ram, and 

 where he is apparently speaking of mechanical force, 

 he has left nothing in print on the subject. Not so, 

 however, M. Seguin himself, who in 1839, in a work 

 on the ' Influence of Railroads/ has distinctly expressed 



* I am informed that these papers are, in fact, published some time after the 

 date which they bear in the magazine. 



