ESSAY-REVIEW 



THE PHILOSOPHY OP PHYSICS. By Prof. A. W. Porter. 

 D.Sc, F.R.S., being a review of Physics : The Elements. By Norman 

 Robert Campbell, Sc.D., F.Inst. P. [Pp. ix -f 565.] (Cambridge, at the 

 University Press, 1920. Price 405. net.) 



" It is not the facts but the explanation of them that matters." Such 

 is the motto which precedes the preface to this book. The subject-matter 

 has been accumulating since 1904 : first of all in the form of separate essays 

 (some of which have already appeared in print). In 1919 an attempt was 

 made at co-ordination, but this met with diflSculties. " I found that I had 

 forgotten about the matter so completely that, at the outset, the manuscript 

 might have been that of an unkno\vn author. It would have been wiser to 

 read it, bum it, and start afresh." This was not done, however. " The 

 co-ordination of the separate essays is still incomplete, and there are several 

 instances where the point of \dew is rather different in difierent parts of the 

 book." " The other fault is a complete lack of reference to the works of 

 others." 



The book is primarily intended to meet the needs of professional phy- 

 sicists. It assumes throughout entire famiharity with all the facts and 

 theories of ph^.'sics, ancient and modem. Its object is not to add to scientific 

 facts. " Briefly, it may be said that what is aimed at is not investigation 

 or exposition, but criticism." " Criticism of this nature has secured a large 

 part of the attention of pure mathematicians for the last thirty years, and 

 has become almost a new branch of their study. In this treatise on 

 phj-sics I hope to extend in some measure such criticism to a portion of 

 experimental science." 



Thus far, we have allowed the author to speak for himself. He is aware 

 that his is not the first attempt. Reference is made to Poincare, whose 

 essay's abound in criticism of the fimdamental conceptions of ph^'^ics. But 

 others have also ventured on the same ground — mention may be made of 

 Kirchhofi, W. K. Clifford, Mach, Karl Pearson, Hertz ; while we leave im- 

 mentioned the large number who, though adequately trained in the laws 

 of thought, have not had the equipment necessary on the ph^rsical side. 



The range of the book can to some extent be gathered from the tities 

 of the chapters. From Part I we select : The nature of la^vs, The explanation 

 of la\\-s. Theories, Chance and probabiUty, The meaning of science ; and from 

 Part II, Fimdamental measurement. Physical number. Numerical laws and 

 derived magnitudes. Units and dimensions. Errors of measurement. Mathe- 

 matical ph>-sic3. It is impossible in a short re\iew to do more than select, 

 almost at random, one or tn'O cases for special consideration. There is 

 more suitabiHty in such a random selection, because the author's own illus- 

 trative cases often seem to have the same character. One of the chief faults 

 we have to find is the lack of orderly development of the subject. 



To illustrate the author's methods take first the consideration (p. 40) 

 of Hooke's law, " stated in the form that the extension of a body is propor- 

 tional to the force acting upon it." This is the first example of a physical 



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