546 



NATURE 



[April 29, 1922 



torque and efficiency at all loads can be found very 

 simply by constructing a certain circle and drawing 

 various lines. 



Theory shows that the torque developed when 

 switching one of these motors into circuit is greatly 

 increased by increasing the resistance of the rotating 

 circuits. Many inventions have been devised, so 

 that the resistance of the rotor circuits automatically 

 diminishes as the speed increases, thus securing high 

 initial torque with economic working. It is interesting 

 to learn that in the rotors of the two-phase motors 

 used in the U.S. battleship New Mexico there are 

 two windings. The outer is made of a high-resistance 

 alloy and the inner has low resistance. The outer 

 winding produces the initial torque, but the inner 

 produces the greater torque at normal speed. 



The author defines the leakage factor of a motor as 

 L1L.2/M2-1, where Lj, Lg are the inductances of a 

 stator and rotor winding respectively and M is the 

 mutual inductance between them. We much prefer 

 Behn-Eschenburg's definition, namely, i-M^/LjLg. 

 The latter is always a fraction lying in value between 

 o and I. The former varies between o and infinity. 

 We also think it better to talk about motors being 

 " in cascade " rather than " in concatenation." We 

 regard this book as an important contribution to the 

 practical theory of alternating current machinery. 



A. Russell. 



Our Bookshelf. 



Memoirs of the Geological Survey. Special Reports on 

 the Mineral Resources of Great Britain. Vol. xxiii. : 

 Lead and Zinc Ores in the Pre-Carboniferous Rocks 

 of West Shropshire and North Wales. Part i, West 

 Shropshire. By B. Smith. Part 2, North Wales. 

 By H. Dewey and B. Smith. Pp. iv + 95. (South- 

 ampton : Ordnance Survey Office ; London : E. 

 Stanford, Ltd., 1922.) 35. net. 



Reports on the lead and zinc ores of Scotland, of 

 Cornwall, Devon and Somerset, of the Lake District, 

 and of the carboniferous rocks of North Wales have 

 already appeared, and the three remaining volumes 

 of the series, dealing with British lead and zinc ores in 

 the remainder of the country, are promised shortly. It 

 is of the utmost importance in the interests of economic 

 geology that this work should be done now, before it 

 is too late ; but unfortunately it is becoming only too 

 clear that the interest is a purely academic one, and 

 that the industry of lead and zinc mining in Britain 

 is in a moribund condition. It is obviously impossible 

 that our relatively small deposits, some of which have 

 probably been worked for 2000 years, can compete 

 in the world's markets against the vast masses of 

 mineral, the development of which is of quite recent 

 date, which are to be found in the United States, 

 Australasia, Burma, etc., and it must be regretfully 

 admitted that it is impossible to bolster up an industry 



NO. 2739, VOL. 109] 



that has to contend with such crushing disadvantages, 

 both natural and artificial. For reasons that are well 

 known to all students of mineral deposits, our veins of 

 lead ore were richer and more easily worked at the out- 

 crops than they are to-day ; we are far indeed from the 

 days of Pliny, according to whom lead was found in 

 Britain near the surface of the ground in such abund- 

 ance that it was found necessary to limit strictly the 

 output. 



The volume before us describes the occurrences of 

 lead and zinc in two districts, which have probably 

 been thus grouped together on account of their marked 

 geological similarity, the ores in both occupying fault 

 fissures in the older rocks of Cambrian, Ordovician 

 and Silurian age. The individual mines are described 

 accurately and minutely, and the description is in 

 many cases supplemented by sections taken from the 

 actual mine plans. It is only to be regretted that more 

 attention has not been paid to the introductory 

 chapters dealing with the districts as a whole, parti- 

 cularly as regards the statistical portion. No summary 

 of district statistics is given for North Wales, and 

 that for Shropshire is indicated only by means of a 

 small graph, which shows the general features of its 

 rise and fall, but from which it is impossible to obtain 

 exact figures. H. L. 



Elementary Chemical Microscopy. By Prof. E. M. 

 Chamot. Second edition, partly rewritten and en- 

 larged. Pp. xvi + 479. (New York : J. Wiley and 

 Sons, Inc. ; London : Chapman and Hall, Ltd., 

 1921.) 255. net. 



The first edition of this work was reviewed at some 

 length in Nature in 1915 (vol. 96, p. 84) shortly after 

 its appearance. The subject of chemical microscopy, 

 however, received a great impetus during the war, many 

 new applications revealing themselves in the special 

 war industries, which resulted in a more extensive use 

 of the microscope in applied chemistry than at any 

 time during the last quarter of a century. Hence, a 

 new edition of this book was found necessary in 

 America, and it is somewhat disappointing to find that 

 practically no new methods or processes, and but little 

 new apparatus, are described. The lack of photo- 

 micrographs of typical microscope fields of character- 

 istic crystals produced in the tests described is still 

 very obvious, but the author on the one hand promises 

 a second book to make good this deficiency, and on the 

 other states that this present book is primarily in- 

 tended as a text-book (especially for the students of 

 Cornell University), and not as a book of reference, and 

 that the method of instruction in the Cornell course is 

 intentionally one which leads to the best results when 

 the student is encouraged to discover for himself (under 

 guidance) the characteristic morphology of the materials 

 studied. 



The same more or less antiquated crystallography is 

 retained, in which such terms as " optical elasticity," 

 " hemihedral," and " tetartohedral " constantly occur, 

 and the confusion between trigonal and hexagonal 

 crystals is so complete that the former term is not even 

 mentioned. 



This second edition is, however, an improvement, for 

 several obscure portions of the first edition have been 



