Notices respecting New Boohs. 309 



reader to understand the experimental researches, and the book 

 ends with a very interesting account of Prof. Ewing's own modifica- 

 tion of "Weber's theory of magnetism in magnetic metals which has 

 done so much to advance our insight into the subject. An appendix 

 gives an account of the beautiful curve-tracer which Prof. Ewing 

 has invented and which has already contributed to our knowledge 

 of the effects of rapidly alternating magnetic force on the mag- 

 netization of iron, a matter of great interest in connexion with the 

 working of transformers. 



In chapter x., when dealing with the magnetic circuit, Prof. 

 Ewing introduces the notion of a magnetic flux and magnetomotive 

 force. It is to be regretted that the idea was not earlier intro- 

 duced. In the introductory chapter magnetic force and magnetic 

 induction are introduced as more or less similar things, one occurring 

 in a crevasse and the other in a hole. It is to be regretted that 

 Maxwell's notion of induction being analogous to a flux, or dis- 

 placement, produced by the magnetic force was not the foundation 

 on which the book was built instead of this hole-crevasse definition, 

 which gives little or no insight into the way in which energy is 

 stored in the medium nor how it is that the induced electric force 

 depends on the change of induction and not on the magnetic force, 

 and why on earth induction is continuous and magnetic force is 

 not. Students who are brought up on the hole-crevasse definition 

 require to rearrange all their ideas when they try to develop any 

 sensible theory of what is taking place. This is confessedly the 

 case when our author makes this new departure in introducing the 

 important practical applications of the theory to the dynamo and 

 transformer. That Prof. Ewing so determinedly calls attention to 

 magnetization is a valuable protest against the great temptation 

 to theoretical investigators to simplify their symbols as much as 

 possible. It is, however, most important to call attention con- 

 tinually to the part played by the matter present and to the 

 changes in that matter, all of which are represented by the mag- 

 netization thereof. 



No practical application has yet been made of the thermodynamic 

 properties of magnetic metals, and this is probably Prof. Ewing's 

 excuse for having given a very interesting and complete account of 

 the effects of temperature without giving an outline even of the 

 accompanying thermodynamic theory. Some part at least of these 

 effects are reversible, for heat-engines can be made to transform 

 heat into work by their means. One of the least worked at de- 

 partments of both magnetism and thermodynamics is the connexion 

 between them. There is here a large field for the exercise of 

 experimental and analytical skill which has apparently escaped even 

 the searching attention of German seekers after original investiga- 

 tions suitable for doctorate theses. 



This subject of thermodynamics and that of the rate at which 

 magnetization takes place are the two departments in which we 

 are at present most obviously ill-informed. They are both seriously 

 complicated by irreversible actions, but this should render their 



