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LXXI. Notices respecting New Books. 



An Elementary Exposition of the Doctrine of Energy. By J). D. Heath, 

 M.A., formerly Fellow of Trinity College, Cambridge. London : 

 Longmans, Green, and Co. 1874 (post 8vo, pp. 129). 



VI/'E have here a very clear and exact " Elementary Exposition of 

 * * the Doctrine of Energy." The author presupposes on the part 

 of the reader an acquaintance with the elements of Mechanical 

 Science (which is, indeed, an indispensable preliminary), and pro- 

 ceeds, after a brief introduction/to expound the Doctrine of Energy 

 of Motion and Energy of Position, i. e. the principle of vis viva 

 embodied in the formula 



2(?nu 2 ) = C -f- 2 j* 2m(Xda7 -|- Ydy + Zdz), 



understood with the usual well-known restrictions. This occupies 

 mere than half of the volume (pp. 1-69). The author begins from 

 the most elementary principles, and takes great care to notice the 

 conditions presupposed in the various statements as they follow 

 one after another — thus giving an account of the subject which 

 might be read with advantage even by an advanced student. There 

 is, of course, nothing new in the leading facts ; but the author has 

 thoroughly mastered the subject for himself, and gives a lucid 

 account of it as it looks from his own point of view ; and in this 

 sense it may be justly spoken of as an original exposition of the 

 matter. 



From the dynamical doctrine the author passes to the considera- 

 tion of the subjects of impact, friction, and heat, of the energy de- 

 rived from heat, and of the Energy of Elastic Fluids ; these he 

 treats at some length, and then notices more briefly Electric Energy, 

 Chemical Energy, Animal Energy, Energy of Vegetation, and Dissi- 

 pation of Energy. Of these, Chemical Energy is the only form of 

 Energy which the author notices with any thing approaching to 

 detail — though, in this as in other cases, strictly under the view of 

 the connexion between Chemical Change and Energy considered as a 

 " numerically measurable thing." This, properly speaking, com- 

 pletes the subject ; but a sort of Appendix of sixteen pages is added 

 on " Molecular Theories." The whole subject is treated with the 

 same sort of originality which we noted above as exemplified in 

 the account of the principle of vis viva ; and the work (for this 

 reason) might be read with advantage by almost any student ; but 

 it will be no more than right to let the author speak for himself as 

 to the class of readers for which it is specially designed. He says : — 

 " My aim was to lay before them [i. e. the boys of the sixth form 

 in the Surrey County School], lads of from sixteen to eighteen, 

 soon about to quit school for active life, not a popular account of 

 recent discoveries or theories, .... but such an exhibition of the 

 principles and methods of modern science, and of some of the re- 

 sults obtained therefrom, as might show them the use of the mathe- 

 matical groundwork in which they had been carefully instructed. 



