December 13, 1895.] 



SCIENCE. 



811 



forces cannot be shown, nor can induction by 

 motion of the circuits without alteration of the 

 currents. This can be made possible by a simple 

 alteration. If the middle weight, instead of 

 rolling on a fixed rail, roll on the bar connect- 

 ing the two outer carriages, the coefficients of 

 induction will vary with the position of the 

 middle mass, and moving it along its bar while 

 one of the outer masses is moving will cause the 

 other to move, etc. The centrifugal force tend- 

 ing to make the middle mass roll along its bar 

 will represent the magnetic force between the 

 currents. 



A number of interesting cases of induction 

 are worked out, including a simple case of 

 ' throttling of an alternating current ' and vari- 

 ous practical problems connected with trans- 

 formers. The explanation of Elihu Thomson's 

 interesting repulsion experiments is also simply 

 given. Various electromagnetic measurements 

 are worked out, including several methods of 

 'determining the ohm.' Finally the effects of 

 dielectric currents are treated, and the motion 

 of the ' Faraday tubes ' and of the energy 

 through the field. The case of propagation of 

 plane electromagnetic waves is taken up, and the 

 experiments of Hertz described. All this with 

 the assumption of no more mathematical knowl- 

 edge than ' an acquaintance with the Elemen- 

 tary principles of the Differential Calculus.' 

 The reviewer was so struck with the absence of 

 integral signs in the book that he counted them, 

 and was surprised to find that there were actu- 

 ally fifty. Of differential equations there were, 

 however, eighty-six. It will be granted, how- 

 ever, that in a book of over five hundred pages 

 this is not too many. To return to the question 

 of how this is done : ■ It is, after all, by stating 

 facts in language which, while avoiding the no- 

 tation of the calculus, employs its essential con- 

 cepts. We question somewhat whether this is 

 not putting on an appearance of simplicity that 

 is but apparent. For instance, it seems doubt- 

 ful whether the expedient of dividing an area up 

 into meshes, multiplying the force by the area of 

 each and adding, is to be preferred to using the 

 term surface integral in the first place. The 

 method of the book may be characterized as that 

 of dealing with phenomena in infinitely small 

 pieces. It only remains to use the language of 



limits and to integrate in order to have a com- 

 plete treatment. If the author may be accused 

 of calculus-dodging, however, he has done it so 

 well that he may be forgiven, and the student is 

 bound to be pleased. We can only congratu- 

 late those students who have the good fortune 

 to study this subject under the personal guidance 

 of Prof. Thomson, and we predict a large sale 

 for the book. 



Of Prof. Nipher's book, a number of the state- 

 ments already made of Thomson's may be made. 

 In spite, however, of the word ' advanced ' in 

 the sub-title of the former, and of the word 

 ' elementary ' in that of the latter, it must be 

 admitted that Thomson's contains a good deal 

 more meat. Prof. Nipher states also that his 

 book ' ' is designed for the use of students who 

 have but recently begun to use the processes of 

 the calculus, and it has been an incidental aim 

 of the author to assist the pupil in acquiring 

 possession of the machinery of mathematics. 

 There has been no attempt to avoid any legiti- 

 mate analytical method because it is not popu- 

 larly known, but on the other hand there has 

 been an attempt to avoid wasting the time of 

 the reader over puzzles and obscurities which 

 are made difficult and called easy. ' ' This attempt 

 has certainly been carried out. The student 

 will not waste his time if he reads this book. 

 It has evidently been written with a view to 

 the needs of the engineering student, who has 

 been almost ignored by Prof. Thomson. It 

 will do this engineering student good to read 

 Prof. Nipher's chapter on electrostatics, and 

 on energy, even if they do not assist him to 

 design dynamos. Here again we have the 

 parallel action of dielectrics and magnetizable 

 bodies clearly brought out, a matter which can 

 hardly be too strongly emphasized. Prof. 

 Nipher recently announced the existence of 

 ' Ohm's Law ' for dielectrics as if it were some- 

 thing new, whereas the matter must have been 

 evident to anyone familiar with the geometrical 

 meaning of Laplace's equation, and was, if we 

 mistake not, known to Faraday. Prof. Nipher 

 has also taken the pains to invent the terms 

 perviance, diviance and perviability, to denote 

 the electrostatic analogues to conductance, re- 

 sistance and conductivity. We trust that this 

 will not go on to all the cases in which similar 



