4 s J 



NATURE 



[October 12, ign 



they were able to run 40,000 km. Information of 

 this kind is simply valueless. 



A good deal of space is devoted to air as an insulat- 

 ing material, and reference is made to the investiga- 

 tions of Ryan, Steinmetz, Scott, and Mershon (the 

 latter's name being persistently spelled Merthon). 

 There is, however, nothing said about Watson's re- 

 search on the increase of the dielectric strength of 

 air under high pressure, and the practical application 

 he mad.' of it, and brought before the Institution of 

 Electrical Engineers a few years ago. The best 

 chapter in the book is that on porcelain as an insu- 

 lator. Here we get some definite data, both as to the 

 various types used and the methods of testing. 



(2) Formulae for armature windings are a fascinating 

 subject to the mathematician. It is, of course, pos- 

 sible to represent any winding, no matter how com- 

 plicated, by some mathematical expression, or even to 

 devise a general winding formula of formidable aspect 

 which, by a suitable choice, of coefficients and con- 

 stants, may be reduced to represent any winding, even 

 the simplest. To an accomplished mathematician 

 there must be a great temptation to treat the subject 

 in this comprehensive way; and we may feel grateful 

 to the author for having withstood the temptation 

 and given us a treatise which restricts the subject 

 to those windings that are of importance to the 

 manufacturer. But these he treats thoroughly, not 

 only as mathematical problems, but also from the 

 manufacturer's point of view, that is, including de- 

 tailed descriptions of the various special tools required. 

 The use of winding tables instead of drawings of 

 connections is, of course, well known, but the author's 

 way of arranging the winding table is extremely use- 

 ful. It contains not only the sequence of the con- 

 ductors (or in a coil winding of the coil sides) proper, 

 but also the sequence of slots and commutator sec- 

 tions, so that the division of current between the 

 armature circuits and the potential difference between 

 any two conductors or any two commutator segments 

 can be seen at a glance. 



The use of open slots and binding hoops is recom- 

 mended for peripheral speeds under 6000 feet a 

 minute; beyond that wooden wedges should be used, 

 or, better still, slots with an overlapping lip. As 

 regards equipotential connections the author seems to 

 think them of equal importance for wave and lap 

 winding, and he fails to point out that with four-pole 

 machines, even if lap wound, they are not nearly so 

 necessary as in a six-pole machine. This is, however, 

 a minor matter, and, moreover, does not affect the 

 system of winding, which is, strictly speaking, a 

 geometrical and not a dynamical problem. 



In the second part of the book we come to windings 

 for alternating-current machinery. These are gener- 

 ally more simple than direct-current windings, but the 

 author treats them with equal thoroughness. Even 

 such questions as the shape of the e.m.f. curve as 

 influenced by the number of slots per phase per pole, 

 the comparative merits of six-phase and three-phase 

 windings for converters, those of two-phase versus 

 three-phase generators, and other more theoretical 

 matters receive due attention. The special tools re- 

 NO. 2189, VOL. 8/] 



quired are fully discussed and illustrated, and ques- 

 tions of insulation, such as the thickness of cotton 

 covering, thickness of micanite tubes, covering and 

 protection of coil ends, are not overlooked. The work 

 is profusely illustrated both with line drawings and 

 with perspective views of windings in different stages. 

 These are very clear, and, as examples, well chosen. 

 Polyphase winding diagrams are shown in different 

 colours, so that it is easy to follow out the circuits 

 for each phase separately. Giseert Kapp. 



AGRICULTURAL BACTERIOLOGY. 



(1) Handbuch der landwirtschaftlichen Bakteriol 



By Dr. F. Lohnis. Pp. xii + 907. (Berlin : Gebriider 

 Borntraeger, 1910.) Price 36 marks. 



(2) Landwirtschaftlich - bakteriologisches Praktikum. 

 Anleitung zur Ausfiihrung von landwirtschaftlich- 

 bakteriologischen Untersuchungen und Demonstra- 

 tions-Experimenten. By Dr. F. Lohnis. Pp. vii + 

 156. (Berlin : Gebriider Borntraeger, 191 1.) Price 

 3.40 marks. 



AS the fertility of virgin soils becomes exhausted 

 and the world's population increases, man will 

 become more and more dependent on artificial methods 

 for increasing the fertility of the soil and on intensive 

 cultivation. Research during the last twenty years 

 has shown how important lowly forms of life are in 

 these connections. The microbiologist has already 

 indicated that, for instance, by increasing the nitrogen- 

 fixing micro-organisms in the soil by inoculation, or 

 by partial sterilisation whereby forms inimical to them 

 may be destroyed, much may be done to increase the 

 yield of crops without the actual addition of plant-food 

 elements by means of manures. 



(1) There has of late been an enormous output of 

 work from various laboratories and experimental 

 stations, and the first work under review gives an 

 admirable summary of the more important researches 

 on agricultural microbiology. It is a large volume of 

 900 pages, and has been compiled with true German 

 industry from the original papers, the references to 

 which are given. The latter occupy from a quarter 

 to one-half, and in many cases three-quarters, of every 

 page, often with a line or two of comment or explana- 

 tion appended, so that the book forms a very complete 

 bibliography of the subject, and it is pleasing to note 

 that British, Colonial, and American work appears 

 to have received full recognition, which is not always 

 the case in German literature. The volume is divided 

 into five main sections, which respectively deal with 

 the occurrence and activity of micro-organisms in 

 (1) fodder and agricultural foodstuffs, (2) the 

 "retting" of flax and of hemp, and tobacco fermenta- 

 tion, (3) milk and milk products, (4) manure, and 

 (5) the soil, and the treatment of each is very com- 

 plete. Thus, in the first named, after a general dis- 

 cussion of the number and nature of micro-organisms 

 in fodder and foodstuffs (hay, straw, grain and sei ds, 

 meal and roots), the subjects of the influence of micro- 

 organisms in the preparation, heating, and firing of 

 hay, in ensilage, in the decomposition of the starchy, 

 saccharine, protein, and fatty constituents of fodder, 

 &c, on digestion in the animal which consumes them, 



