December 22, 1923] 



NA TURE 



897 



mended that " continuous current " and " virtual 

 value " should be used instead of " direct current " and 

 " effective value." The younger generation has simply 

 reversed these decisions. Our sympathies are with 

 the authors who strive to model their nomenclature on 

 the very latest recommendations, and find later that 

 changes have been made. The constant strivings 

 of electrical engineers after standardisation in specifi- 

 cations have done much to stabilise the industry'. 



The authors in many places where there is doubt give 

 the variants, as, for example, effective virtual and root 

 mean square (R.M.S.), ground and earth, and several 

 other synonyms. They measure both magnetic in- 

 duction B and magnetic force H in the same unit, 

 namely, the gauss, which is defined to be one line of 

 magnetic flux per square centimetre. 



From the teacher's point of view, however, this leads 

 to hopeless difficulties. We can recommend this book 

 to those engineers who have a sound knowledge of 

 theory and want to know the latest practical problems 

 which the engineer has to solve. 



Geologic Structures. By Bailey Willis. Pp. xi + 295. 



(New York and London : McGraw-Hill Book Co. 



Inc., 1923.) 175. dd. 

 This book is essentially different from James Geikie's 

 " Structural and Field Geology," which makes its 

 appeal through its fine presentation of rocks as they 

 actually appear on bare surfaces of the crust. The two 

 works may well stand side by side. Prof. Bailey Willis 

 concerns himself here with the mechanics of rock- 

 displacement and rock-folding, and illustrates these by 

 photographs of his series of models made to illustrate 

 the structure of the Appalachians. He uses mixtures 

 of wax, plaster, and turpentine, producing strata that 

 yield very variously to mechanical stress. The defor- 

 mation of an incompetent series under load provides 

 material that returns, as it were, into the core of a 

 rising arch formed by competent strata that can lift a 

 load when laterally compressed, or into the core of a 

 sy-ncline when the competent series lies below them 

 and is bent downwards, displacing matter in the 

 depths (p. 148), Hence we have highly crumpled series 

 between strata of more simple curvature. The shear- 

 ing of materials in sediments as well as in schists, so 

 that new parting-planes are set up, accompanied by 

 thinning and elongation of the mass, is frequently 

 brought before us in this stimulating volume. More- 

 over, we never lose sight of the tridimensional character 

 of the structures described. There is a valuable chapter 

 on field-methods, in which the author remarks (p. 28) 

 that " the explorer should have the pluck of an 

 American and the self-respect of a Chinese." The book 

 provides geologists with very pleasant reading. 



G. A. J. C 



Differential Equations. By Prof. H. B. Phillips. 



Pp. vi + 78. (New York : J. Wiley and Sons, Inc. ; 



London : Chapman and Hall, Ltd., 1922.) 65. 6d. 



net. 

 Dr. Phillips's little book is not a treatise on differential 

 equations in the ordinary sense. He does not deal 

 with any but the most elementary equations, and 

 his aim is purely utilitarian, namely, to provide 

 " thorough drill in the solution of problems in which 

 the student sets up and integrates his own differential 



NO. 2825, VOL. I 12] 



equation." There are a very large number of problems, 

 with some worked out in the text. The problems are 

 from all branches of applied mathematics,, physics, 

 physical chemistry, etc. We can certainly advise 

 students of these subjects to become acquainted with 

 the easier types of differential equations through 

 the agency of Dr. Phillips's attractive and readable 

 book. 



A few criticisms of detail may perhaps be allowed. 

 In the example on p. 6 the minus sign should be used 

 at once in the form dRldt= —kK, instead of leaving 

 the negative in the form of an incidental result of the 

 calculation. On p. 25 something should be said about 

 the geometrical properties of homogeneous equations 

 of the first order. The definition of phase angle on 

 p. 66 is incorrect. There are also a number of mistakes 

 and misprints. S. B. 



An Introduction to the Study of the Compounds of Carbon, 

 or Organic Chemistry. By Ira Remsen. Revised 

 and enlarged with the collaboration of the author 

 by Prof. W. R. Omdorff. (Macmillan's Manuals 

 for Students.) Pp. xii + 567. (London: Macmillan 

 and Co., Ltd., 1923.) 10s. net. 

 Remsen's text-book has for many years been regarded 

 as perhaps the best introduction to organic chemistry. 

 It is extremely well written and not obscured by tedious 

 details, and is well within the student's capacity. 

 Theory is kept within bounds, and one feels that to 

 the author, at any rate, organic substances are not 

 chalk marks on blackboards. In the new edition the 

 essential character of the book is preserved, but by 

 omitting illustrations and directions for experiments, 

 it has been found possible to bring the text thoroughly 

 up-to-date and to include some rather more advanced 

 material. Very little calling for criticism can be 

 found, but it is suggested that the theory of esterifica- 

 tion on p. 67 is unsound, and that ethylene is not 

 most conveniently prepared from the dibromide (p. 

 276) : Newth's method is not even mentioned. Again, 

 on p. 282, some account should have been taken of 

 Chattaway's work. Apart from such trifles, the book 

 is clear, up-to-date, and accurate, as well as readable. 



Tracks of British Birds. Edited by H. Mortimer 

 Batten. Life size. Printed on cloth chart, 20 in. 

 by 30 in. (Edinburgh and London : W. and A. K. 

 Johnston, Ltd., 1923.) /[s. net. 

 This forms a companion chart to " Tracks of British 

 Animals," already noticed in these columns, and follows 

 the same general lines. Four categories of birds are 

 represented, namely, swamp birds, ground birds, perch- 

 ing birds, and birds of the seashore, each with about 

 ten examples. The tracks are reproduced life-size, and 

 a few brief explanatory notes on the general subject 

 are given at the foot of the chart. Organisations such 

 as Boy Scouts and Girl Guides, in which instruction in 

 the craft of the country side occupies a good deal of 

 attention, will find this chart invaluable, and it will be 

 welcomed by teachers of Nature Study in schools as a 

 most useful aid to the teaching and cultivation of 

 powers of observation. The use of the word mavis as 

 the common name of the song-thrush is, we believe, 

 only general north of the Tweed, and we suggest the 

 addition of the latter name for the benefit of those 

 wlio are not familiar with the Scotch term. 



