386 



NA TURE 



[February 24, 1898 



science owes him a heavy debt. The magnetic circuit, and 

 the mode of testing the performance of dynamo machines 

 by means of characteristic curves, which themselves 

 give valuable information regarding the magnetisation 

 of the circuit, really did for the dynamo machine very 

 much what practical thermodynamics and the indicator 

 did for the steam-engine. 



After an account of the General Properties of Mag- 

 netic Circuits, Dr. du Bois proceeds in his Chapter vii. 

 to a discussion of the Analogy of the Magnetic Circuit 

 with other Circuits. Here the author, as we think very 

 properly, condemns the use of the term "Ohm's Law" 

 to express the fact that magneto-motive force in the 

 circuit divided by the magnetic reluctance is equal to 

 the flux of induction in the circuit. This, it has always 

 appeared to us, is using the term " Ohm's Law " to 

 describe what is a mere result of definition. The 

 equality of the magneto- motive force multiplied by the 

 permeance (permeance = i/reluctance) of the circuit to 

 the flux of induction is a result of definitions of induc- 

 tion and field intensity, and to thus use it is to introduce 

 into magnetics the confusion which many have fallen 

 into with regard to the law of Ohm in voltaic circuits. 

 In the latter case, the law, properly speaking, expresses 

 the proportionality of the current in a conductor to the 

 difference of potential between two points in the con- 

 ductor near its ends when the conductor is not the seat 

 of any internal electromotive force. This result has no 

 counterpart in magnetic circuits, and no " Ohm's Law " 

 holds there. 



In Chapter viii. the Magnetic Circuits of Dynamos or 

 Electromotors is dealt with, the work of the brothers 

 Hopkinson, the empirical formulae for the magnetisation 

 curves of such machines given by Frolich and others are 

 all described, and the application of all this thoroughly 

 practical theoretical discussion is focussed on the con- 

 struction and arrangement of the field-magnets and 

 armature of the machine. 



We are immediately carried on in a natural sequence 

 to magnetic cycles and hysteresis, and the immensely 

 important researches on the magnetic properties of iron 

 carried oiit by Ewing and others. The results in this 

 field are, however, before all our readers specially in- 

 terested in the subject, and the subject is so large and 

 so full of detail, that we cannot with any advantage 

 continue our sketch of the contents of the book. 



The magnetic circuits of electromagnets, containing 

 an account of the devices used by Ewing and the author 

 to obtain very intense fields, and of methods of measuring 

 such fields, is given in Chapters ix. and x. Up to about 

 fifteen years ago determinations of field intensities had 

 been confined to measurements of the earth's field- 

 intensity or intensities of like magnitude, and a paper 

 {Philosophical Magazine, 1882) by the present writer, 

 describing methods which w,ere in use in the Glasgow 

 University Physical Laboratory, contained perhaps the 

 first published statement of how the field between the 

 poles of a powerful electromagnet could be quickly and 

 accurately measured. 



In this connection also we have one of the author's 

 notable contributions to magnetics, in his account of the 

 researches which he carried out at Berlin on the magneto- 

 optical method of measuring intense fields. 

 NO. 1478, VOL. 57] 



Here we must conclude our review of a most fasci- 

 nating book. It cannot be praised too highly as a piece 

 of work sound from every point of view, and tending 

 to the advancement of knowledge. Dr. Atkinson and 

 his colleagues have performed their work of rendering 

 the book into English very carefully indeed, and on the 

 whole the version reads like a book originally written, 

 and well written, in English. Only in one or two places 

 have we found, in a fairly extensive comparison of the 

 German and English, that the sense has not been exactly 

 caught. For example, at p. 340 a method of investiga- 

 tion there referred to is rather hardly treated by being 

 described as "circuitous." The German "umstandlich " 

 ought here to be rendered by " circumstantial." 



A. Gray, 



AUDUBON. 



Audubon and his Journals. By Maria R. Audubon. 

 With zoological and other notes by Elliott Coues. 

 Svo. Two vols. Vol. I., pp. xiv 4- 532 ; Vol. II., 

 pp. viii -I- 554. Portraits, copies of diplomas, photo- 

 gravures, &c. (New York : Charles Scribner's Sons, 

 1897.) 



NATURALISTS and many who are not naturalists 

 will find this an entertaining book. It gives in 

 great detail the incidents of certain years of Audubon's 

 life, years in which he was carrying on his work in the 

 field, or else meeting every day men who are still note- 

 worthy. The book is founded upon unpublished journals 

 and letters, of which parts are given at length. All has 

 been corrected by recollections handed down in the 

 family, and the zoological information has been revised 

 by Dr. Elliott Coues. A summary of the naturalist's life 

 is prefixed. We have now a full and lively account of 

 what is most memorable in the life of Audubon. 



Some readers who are not naturalists will turn with 

 curiosity to the passages in which mention is made of 

 places which are now populous American cities, but which, 

 when Audubon dwelt there, or passed through, were 

 backwoods settlements. Others will be glad to note 

 small particulars concerning the naturalists, merchants, 

 and men of letters whom Audubon saw during his visit 

 to Europe in 1826-29 — Cuvier, Bewick, Jameson, Selby, 

 Jardine, Rathbone and Roscoe. Others again will study 

 the character and methods of a man, who partly by real 

 merit, and partly by the good-luck of becoming known to 

 men who could write, has been widely accepted as a 

 typical naturalist. Some of us can hardly remember the 

 time when we had not heard of Audubon as the man 

 who faced all dangers and fatigues, caring nothing for 

 comfort or profit, if only he could learn more and more 

 of the wild creatures which fly from the face of man. 

 There is matter here for readers of very different kinds, 

 and all should thank those who have rescued so many 

 old papers, and arranged them so carefully. 



Audubon was a real naturalist, if ever there was one. 

 He had the passion for observing and drawing birds 

 almost as a gift of nature. As a boy and young man he 

 was fit for nothing else, but the responsibility which a 

 family brings made him in middle life a sensible man of 

 business. He was never a man of science. He never 

 received from others, nor gave himself any kind of training 

 in scientific method ; he never studied the anatomy of 



