396 



NATURE 



[Dkcember 9, 19 1 5 



English Local Government Board, which insists 

 on a quarter-mile distance from even a small 

 group of houses. 



We have not left space for discussing the 

 chapters on child hygiene, on milk and other food 

 supplies, water supplies, housing, nuisances, 

 sanitary law, and vital statistics ; but in each 

 of these the English sanitarian will find 

 useful points for comparison with our own 

 methods. The last chapter deals with pub- 

 licity ; and here is, perhaps, the most characteristic 

 feature of public health work in the States. In 

 relation to the Press, exhibitions, lectures, motion 

 pictures, etc., useful hints are given for bringing 

 home the lessons of sanitation to the general 

 public. 



OUR BOOKSHELF. 

 North America during the Eighteenth Century: 

 A Geographical History. By T. Crockett and 

 B. C. Wallis. Pp. vi + ii6. (Cambridge: At 

 the University Press, 191 5.) Price 2^. net. 

 The authors have collaborated in an interesting 

 experiment, and have wisely chosen for their first 

 essay (for we presume it is a prelude to others) 

 a region in which the facts of history are easily 

 correlated with those of geography. In one sense 

 it is only another account of the rise of the United 

 States of America, but in a different sense it is 

 a new story, for it tells the history of a century 

 In the light of the place where it occurred. One 

 can imagine oneself in America and watch the 

 drama unfold. We are glad to see that the 

 authors invert the old term and speak of a geo- 

 graphical history, for not only should geography 

 precede history in course of study, but the term 

 historical geography has fallen on evil days so far 

 as school books are concerned. In most cases, 

 except for a preliminary chapter and a map or 

 two, it has no relation to geography. 



This book begins with the usual preliminary 

 chapter, but the succeeding ones are not disap- 

 pointing. The influence of routes and relief, and 

 the question of place relations, are kept to the fore 

 throughout, and very useful are the terse sum- 

 maries at the end of each chapter. There are 

 many useful black and white maps. In the way 

 of criticism we could wish that the first two map's 

 were a little clearer, and that the authors had cur- 

 tailed the length of some of their sentences. But 

 we welcome the volume as a most illuminating 

 book. R. N. R. B. 



First Aid in the Laboratory and Workshop. By 



A. A. Eldridge and Dr. H. V. A. Briscoe. Pp. 



32. (London : Edward Arnold, 1915.) Price 



15. net. 



The authors of this little book, who have been 

 m charge of first aid organisation in chemical and 

 physical laboratories, have found that the ordin- 

 ary text-books devote too much space to serious 

 fractures and other injuries, but give little in- 

 formation regarding ordinary accidents, such as 

 . NO. 2406, VOL. 96] 



are apt to occur in laboratories and workshops, 

 for instance, burns produced by chemicals, eye m- 

 juries, shocks produced by electric currents, and 

 poisoning. They have therefore written this pamph- 

 let to meet this need. It is prefaced by a commen- 

 datory foreword from Sir Alfred Keogh, and we 

 heartily endorse his* praise. The directions are 

 terse, clear, and correct. 



Determinative Mineralogy : With Tables for the 

 Determination of Minerals by Means of their 

 Chemical and Physical Characters. By Prof. 

 J. Volney Lewis. Second edition. Pp. vii + 

 155. (London: Chapman and Hall, Ltd., 

 1915.) Price 6s. 6d. net. 

 The present edition differs from the first — re- 

 viewed in our issue for January 15, 1914 (vol. xcii., 

 p. 550) — chiefly in the restatement with each 

 table of the classificatory characters and tests 

 leading up to it. The supplementary tables at 

 the end have been extended to include specific 

 gravity and chemical composition ; and many 

 more delicate tests have been introduced in both 

 the text and the tables. 



LETTERS TO THE EDITOR. 

 [The Editor does not 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. No notice is 

 taken of anonymous communications.] 



The Principle of Similitude. 



(i) In his article under the above heading (Nature, 

 March 18, 1915, p. 66) Lord Rayleigh deduces, by the 

 method of dimensions, an equation for the rate of 

 heat transfer between a solid body and a stream of 

 fluid in which it is immersed. Commenting on this 

 equation, M. RIabouchinsky (Nature, July 29, p. 591) 

 remarks that heat, temperature, length, and time are 

 treated in the deduction as independent units; and 

 that if we suppose only three of these units to be 

 "really independent" we obtain a different and less 

 definite result. 



In a further note (Nature, August 12, p. 644) Lord 

 Rayleigh acknowledges the interest of the question 

 suggested by M. Riabouchinsky, and indicates the 

 direction in which the solution of the apparent diffi- 

 culty is to be sought. But since he does not pursue 

 the subject further and the reader may feel as if left 

 in mid-air, It seems worth while that the point raised 

 by M. Riabouchinsky should be somewhat further 

 elucidated. 



(2) The question whether any real doubt has been 

 thrown on the validity of Lord Ravleigh's equation 

 hinges on the answer to the question whether tem- 

 perature can be derived from energy, length, and time, 

 i.e. from mass, length, and time. 



What do we mean when we say that a given kind of 

 physical magnitude can be "derived" from certain 

 other kinds which we call fundamental? We mean 

 simply that experience has shown that if we use, or 

 combine, certain particular magnitudes of the funda- 

 mental kinds in a prescribed way, we thereby deter- 

 mine a magnitude of the derived kind, the size of this 

 resulting derived magnitude being dependent only on 

 the sizes of the particular fundamental magnitudes 

 with which we started, when once the method of using 

 them has been specified. For example, we know that 

 if we construct a rectangle of altitude / on a base I 



