NOTICES OF BOOKS. xvii 



how the apparatus is to be put together. There are, however, two con- 

 spicuous exceptions in figs. 137 and 147, in which the bends in the 

 glass tubes will certainly incite the beginner to make even a worse 

 pucker than usual. A full-page picture of the tower of Pisa is also 

 hardly in keeping with the other figures. 



On the whole the book will be found very suitable for those who 

 require a book which combines the characteristics of a decidedly ele- 

 mentary text-book with those of a good laboratory-manual. 



Theory of Electricity and Magnetism. By Charles Emerson Curry, 

 Ph.D. With a preface by Prof. Boltzmann. Pp. 442. London : 

 Macmillan & Co., 1897. 



This work embodies the substance of the course of lectures on 

 Electricity and Magnetism given by Prof. Boltzmann at the University of 

 Munich, Dr. Curry having, however, considerably amplified the lectures 

 and added examples and deductions for illustrating the subject matter. 



The characteristic of the author's method of treating the subject 

 lies in that starting with the differential equations of the electromagnetic 

 field he then proceeds to deduce from them the explanation of all electric 

 and magnetic phenomena, by means of mechanical analogies, and in 

 this he follows a course quite different from that adopted in the class 

 of text-book in favour in this country. This method of procedure of 

 course necessitates, on the part of the student, a considerable acquaint- 

 ance not only with the experimental part of the subject but also with 

 the theories of the subject. The work forms in fact an illustration of 

 the standpoint from which the theory of electricity and magnetism is 

 studied on the continent, and will be found of much use to those whose 

 physical and mathematical equipment is sufficiently good. 



Starting with the assumption that in every volume-element of the 

 ether a motion exists, and that it is to this motion that electric pheno- 

 mena are due, and that further the displacement produced by this motion 

 can be represented by a vector which he calls the " tonic vector," the 

 author by making certain assumptions as to the connection between the 

 kinetic and potential energy, etc., and this vector deduces by means of 

 Hamilton's principle the equations"of the motion of the ether. By a 

 slight modification these equations are then reduced to Maxwell's form. 



In the interpretation of the equations the conception of two 

 incompressible fluids, the positive and negative real electricities, is 

 introduced, together with the idea of free electricity and of electrical 

 polarisation. As an analogy simply meant to illustrate the subject 

 this may be all right, but as a physical theory it is of little use. 



After considering Maxwell's form of the equations the author goes 

 on to show by what assumptions and in what manner they may be 

 converted into Helmholtz's form. He also shows how Helmholtz's 

 equations may be deduced from the empirical laws. The subject of 

 Hertzian waves, of the propagation of longitudinal ether waves and 

 of electric and magnetic striction are also dealt with. 



The reasoning is on the whole clear and explicit, and the book is 

 remarkably free from errors. 



