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its earliest adumbrations in Newton's system of Dynamics to its precise formu- 

 lation by Einstein. The modifications which its acceptance introduces into our 

 ideas concerning the measurement of space and time are very clearly outlined in 

 a succeeding chapter, and a further chapter is devoted to its application to the 

 equations of the electro-magnetic field, and to the demonstration that the con- 

 ception of the " contractile electron " to which Lorentz's researches gradually 

 impelled him, is itself a direct application of the principle of Relativity. 



The sixth chapter of the book will doubtless prove a " tough nut to crack " for 

 the reader whose mathematical training is slender, yet without it the book would 

 be incomplete ; for he who has not grasped that Relativity is not so much a 

 physical law as a general mode of thought which demands that all our physical 

 laws must satisfy a certain test, has missed the entire significance of the principle. 

 The mathematical form into which this test can be cast is most readily expressed 

 in terms of the so-called four-dimensional space of Minkowski ; and in the sixth 

 chapter there is an explanation of Minkowski's four-dimensional vectors which 

 is certainly as clear and simple as the nature of the matter allows. Two chapters 

 on the relation of the principle to mechanics and to the conception of an ethereal 

 medium conclude the monograph. As far as the writer of this notice is aware, 

 this little book is, while being a fairly complete account, the simplest exposition 

 in English of the principle of Relativity ; and its simplicity is not of that spurious 

 kind which, by avoiding the real difficulties, leaves the reader absolutely un- 

 enlightened. 



J. R. 



The Universe and the Atom. The Ether Constitution, Creation, and Structure 

 of Atoms, Gravitation, and Electricity, Kinetically Explained. By Marion 

 Erwin, C.E. [Pp. vi + 314, with 58 illustrations.] (London : Constable 

 & Co., 191 5. Price 8^. 6d. net.) 



This is not a popular exposition of modern physical research ; but an account of 

 an original vortex theory of ether structure devised by the author, who has, at 

 the same time, and perhaps unconsciously, also invented a new system of 

 mechanics. Two or three quotations bearing on this point will suffice to show 

 the importance to be attached to the book. First, among the definitions on 

 p. 150 are these : " Pressure is force, weight is force, momentum is force " ; again, 

 on p. 152, "In a general sense pressure is momentum." If some doubt is cast 

 on this statement by the fact that, on p. 178, pressure is equated to energy, a 

 final remark (p. 74) that " force represents energy " puts matters right by 

 suggesting that all five terms are synonymous ! 



The theory is entitled "The Pan Cycle Hypothesis. Invisible Composition 

 Light-waves, the Warp and Woof of the Ether-structure and of all Things 

 Material." It appears to involve a longitudinal as well as a transverse com- 

 ponent in the vibrations set up by a light wave ; certainly the final conclusion 

 violates the doctrine of the availability of energy. 



The wisest sentence in the book is the first, "To acquire a knowledge of the 

 tools to be used, and the materials to be operated upon, is the first step in the 

 successful building of any structure." It is self-evident that if the author had 

 only acted up to his own precept he would have gone no further. As it is there 

 remains a strong suspicion that he has been overwrought by too much study of 

 Ganot's Physics and the Popular Science Monthly. It should be added in con- 

 clusion that the preface is dated from the Garden City, L.I., New York. 



D. Orson Wood. 



