October 22, 1914] 



NATURE 



»97 



The Government Committee on the Chemical 

 Trades is composed of men who will command the 

 confidence of everyone, and their recommenda- 

 tions will have great weight. It must not be for- 

 gotten, however, that nearly all trades would be 

 benefited by scientific help. The attitude of mind 

 induced by scientific education is just the one re- 

 quired for the successful development of a 

 business. If the manufacturers as a whole would 

 realise this, they would see to it that their sons 

 or those who are to carry on their business, have 

 a thoroughly good scientific education, such as 

 may now be got at many of the Universities and 

 technical colleges in the country. It is not the 

 classical or the modern side of our schools which is 

 going to supply the successful manufacturer of 

 to-morrow, it is the side where a man is taught 

 to bring all his useful knowledge to bear on the 

 achievement of success. Mr. Illing^vorth's book 

 is the most successful attempt which has been 

 made to explain the situation, and we believe that 

 if the Government would send a copy of the book 

 to each manufacturer in the country, the cost 

 would be a mere nothing compared with the effect 

 which would be attained. 



OUR BOOKSHELF. 

 A Little Book on Map Projection." By Mary 



Adams. Pp. viii+io8. (London: George 



Philip and Son, Ltd., 1914.) Price 2s. net. 

 It is very satisfactory to find that at the present 

 time care is being taken that the principles of 

 map projection are being studied as soon as the use 

 of maps is seriously undertaken. At one time this 

 part of their subject was much neglected by geo- 

 graphers, and left to those whose mathematical apti- 

 tude was especially developed. In this book Miss 

 Adams aims at meeting the needs of the secondary 

 and the higher elementary schools ; and in clear 

 and simple language sets forth the difficulties of 

 an adequate cartographical representation of the 

 earth's surface. Simple explanations are given of 

 the. principal types of projection, and then the 

 distortion of the original spheroidal surface when 

 it is represented on a plane surface is explained, 

 and the compromises which have to be adopted 

 are set forth. After this preliminary exposition, 

 into which no mathematics enter, there follows a 

 more detailed discussion of the principal zenithal, 

 conical, and cylindrical projections, as well as 

 certain special projections. These are illustrated 

 by diagrams, and the explanation which is given 

 of each should enable anyone to obtain a clear 

 idea of the essential character of each kind. 



A short app>endix gives a more mathematical 

 account of Mercator's, the Zenithal, and Moll- 

 weide's projection, but with this exception no 

 demand of any but elementary mathematical 

 knowledge is made upon the reader. 



A short bibliography, which might be usefully 

 xtended by the inclusion of some well-known 

 XO. 2347, VOL. 94] 



German and Italian works, shows the student 

 where he may find a more advanced treatment 

 of the subject. The book is carefully written and 

 well adapted to those for whom it is intended, 

 and, while it cannot give them that complete 

 knowledge of the subject which mathematical 

 treatment alone can supply, it will pave the way 

 to an intelligent appreciation which is of the 

 utmost value to all who use maps. 



Chemical Engineering : Notes on Grinding, Sift- 

 ing, Separating and Transporting Solids. By 

 J. W. Hinchley. Pp. viii+103. (London: 

 J. and A. Churchill, 1914.) Price 2s. 6d. net. 

 In this series of articles, reprinted with some addi- 

 tions from the Chemical World, Mr. Hinchley 

 provides, for the use of students intending to take 

 ; up chemical engineering, a concise and practical 

 j outline of a subject of wide scope. The articles 

 are illustrated with seventy sketches and dia- 

 ■ grams, and their thoroughly practical character 

 '. will be much appreciated by students of this 

 branch of technology. 



LETTERS TO THE EDITOR. 



[The Editor does tiot hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. Xo notice is 

 taken of anonymous communications. J 



Fizeau's Experiment and the Prmciple of Relativity. 



It is well known that the principle of relativity 

 gives for the velocity of light in a moving medium 

 the expression 



f + >h., where >^=(.-:,-^*^) 



ft \ fi.- Ji o\/ 



where v is the velocity of the medium, and cjfi is 

 the velocity when at rest. The repetition of Fizeau's 

 experiment by Michelson and Morley gave for water 

 the value 0-434 + 0-02 fp"" t^^ coefficient k, whereas the 

 value of the theoretical coefficient, taking the known 

 values of ft and A for sodium light, is 0-451, which is 

 well within the limits of the possible error." 



The experiments of Gutton (Journ. de Phys., ii., 

 p. 196, 1912) show clearly that in the case of a 

 very dispei-sive medium like carbon disulphide the 

 velocity of propagation of nearly homogeneous light 

 is the group \-elocity, and not that of an absolutely 

 monochromatic train of waves. If this is universally 

 true, as Lord Rajleigh and Gouy have predicted, then 

 in the expression for the convection-coefficient k, the 

 symbol u must stand for the ratio of the velocit},- of 

 light in free space to the group velocit}-. If we call 

 the ordinar}- coefficient of refraction /*,, we have 



M'-}M- 



fo 



\ A 



nesrlecting the square of — -~ {i-e. of 0014) and 



Mo o^ 

 terms of the same order, we have 



/^o 



If we take the dispersion-formula of Ketteler and 

 calculate the values of the two last terms in this 



