REVIEWS 313 



readers, it suffers from two disadvantages. Frequent reference is made 

 throughout to Bowditch {American Practical Navigator, pubhshed by the 

 U.S. Hydrographic Office) ; and, although this is doubtless an advantage to 

 American readers, it is the reverse for English readers. Further, the 

 symbols used are not those which have been generally adopted in this 

 country, and their unfamiliarity will undoubtedly be found a drawback. 



For these reasons the book can hardly be recommended to students 

 in this country. In addition, a price of 6s. net for a book containing only 

 about 70 small pages seems unduly high. 



Those readers who decide to follow the author's methods and notation 

 will find the book of blank reduction forms, which is published as a com- 

 panion to it, of use. The calculations are performed on the front of a 

 printed form, which is adapted for use with either logarithms or spherical 

 traverse table, whilst on the back is a diagram for plotting the position of 

 the Sumner line. 



H. S. J. 



Space, Time, and Gravitation : an Outline of the General Relativity Theory. 

 By A. S. Eddington, M.A., M.Sc, F.R.S., Plumian Professor of 

 Astronomy and Experimental Philosophy, Cambridge. [Pp. vii + 

 218. with frontispiece and 18 figures in text.] (Cambridge : at the 

 University Press, 1920. Price 15s. net.) 



The generaUsed relativity theory of Einstein has been objected to by its 

 opponents because of the radical revolution which it necessitates in our 

 conceptions of space and time, and in other physical ideas, and because of 

 the difficulty of comprehending the nature of the new " law of gravitation." 

 The advocates of the theory have been challenged to come out into the 

 open and, instead of taking refuge behind a maze of difierential invariants, 

 to explain, in a manner which the man in the street can understand, what 

 is involved in the theory. This may not have been the reason which in- 

 duced Prof. Eddington to write this book, but he has, at any rate, provided 

 an admirable answer to the challenge. If the opponents of generaUsed 

 relativity will but read the volume with an open mind, it is difficult to 

 believe that they will not be convinced. 



The book commences with a prologue entitled " What is Geometry ? " 

 which takes the form of a conversation between an experimental physicist, 

 a pure mathematician, and a relativist : the dependence of the bases of 

 geometry upon physical ideas is made clear, as well as the possibility of 

 conceiving a space with properties differing from those of a Euclidean space. 

 The first four chapters then deal with the foundations of the special rela- 

 tivity theory, the relative significance of the terms "space " and " time," and 

 the simplicity which results from considering time as a fourth dimension. 

 The following three chapters deal with the newer modifications and expound 

 the principles of Einstein's argument without introducing mathematics. 

 Then follow two chapters devoted to the tests of the theory ; a careful 

 discussion is given of the evidence afforded by the displacement of the sun's 

 spectral lines, and possible ways of escape, should experiment definitely dis- 

 prove the displacement, are indicated. Chapter IX deals with the questions 

 of momentum and energy, and it is shown how the principles of conservation 

 of energy and momentum follow from the theory. 



The remaining chapters deal with some of the more speculative develop- 

 ments of the theory : such as the significance of absolute rotation, the nature 

 of infinity, and the relationship between electricity and gravitation. The 

 theory of the latter is not yet in a final state, but it holds out remarkable 

 promise of including electrical fields also in the natural geometry in which 

 gravitation finds its explanation. 



