244 



NA TURE 



[December 30, 190^ 



which has been fostered by his training and the habit, 

 born of tradition and the old method of education, of 

 looking a thing up in a book rather than discovering 

 it by observation. One of the consequences of the 

 movement, and one which we hope will prove per- 

 manent, has been the establishment of school gardens. 

 Anyone who knows village schools where gardens 

 exist knows the pride that teachers and scholars 

 alike take in them, and their great value from every 

 point of view. A school garden can be made to 

 furnish a vast amount of matter for school lessons, 

 and in addition it instils into the boys that love of 

 gardening so characteristic of the English life of 

 to-day. 



The teacher is bound to have text-book help in man- 

 aging his garden ; the proper arrangement of his 

 crops, the times of sowing, the pests or diseases likely 

 to be troublesome, are all matters in which he needs 

 guidance. He cannot atford to make mistakes, his 

 scholars' parents are sometimes expert gardeners, 

 always critical, and ever ready to derive amusement 

 from his little efforts. The book before us will be 

 found very useful in this respect by the teacher, and 

 the instructions for working are quite clear and have 

 been tested with satisfactory results in the school 

 gardens of Oxfordshire. The book is copiously illus- 

 trated; indeed, we find a whole page devoted to the 

 photograph of a wheelbarrow and an ordinary water- 

 ing can. The teacher who intelligently follows the 

 instruction given may quite expect his garden to be 

 successful from a horticultural point of view, and will 

 have little to. fear from the carping village critics. 



But we do not think this book represents the last 

 word on the subject. Not enough is made of the 

 garden as a means of education, in spite of a highly 

 suggestive chapter by Mr. Mcadon on "Discovery 

 Lessons," which shows a full appreciation of the 

 possibilities in this direction. We should like to 

 have seen the book dominated by the spirit of the 

 experimenter; instead, we find it dominated by the 

 spirit of the horticultural instructor, whose personality 

 comes out on every page, even to the amiable weak- 

 ness for the long Latin name that we ever associate 

 with the professed horticulturist. It must be admitted, 

 however, that there are difficulties in the way of an 

 e.xperimental school garden. A garden often becomes 

 much too personal an affair to be made the subject 

 of experiment even by the man of science, and how 

 shall the village schoolmaster treat it any more im- 

 personally? The spirit of competition is there; each 

 boy wishes his plot to be the best, and the teacher 

 wishes the garden as a whole to be at least as good 

 as the allotments ; experiments, therefore, cannot 

 come in, as he has no room for failures. We are 

 aware, of course, that some schools make trials with 

 artificial manures, but the schemes that we have seen 

 have been entirely empirical, and designed to increase 

 the crop rather than to yield information. We believe 

 that ultimately the school garden will be as successful 

 educationally as it now is horticulturally, and although 

 the present book does little towards helping on this 

 reform it will be found of real value for the school 

 garden as at present conducted. E. J. Russell. 



NO. 2096, VOL. 82] 



ELECTRIC MOTORS. 



The Alternating Current Commutator Motor and the 

 Leakage of Induction Motors. By Dr. Rudolf 

 Goldschmidt. Pp. viii + 210. (London: The Elec- 

 trician Printing and Publishing Co., Ltd., 1909.) 

 Price 6s. 6d. net. 



THOSE who are acquainted with Dr. Goldschmidt 's 

 writings will open this book with the expecta- 

 tion of finding a very intricate subject treated in clear 

 and simple style, and this expectation will be fully 

 realised. There is no padding, and consequently the 

 reader must not skip, but if he follows the author 

 conscientiously step by step in his close and methodical 

 reasoning he will find his labour amply repaid. 



A good deal has been written about the commutator 

 motor, but we have never come across a treatise 

 written so clearly and in such simple language. The 

 simplicity of treatment is not attained by malcing in- 

 admissible propositions. It is true the author takes 

 us first through the theory of the so-called perfect 

 motor, having no losses and no leakage, but after 

 establishing the main principles which must count in 

 any theory he goes on to introduce step by step those 

 disturbing influences which are inseparable from the 

 practically possible motor, and at every step he finds 

 an easy way of taking account of these influences. 

 The treatment is in the main graphical, and the 

 author's position in the old controversy between the 

 analytical and graphical school is shown by a pas- 

 sage on p. 30, which runs as follows : — 



" The preference for the mathematical or graphical 

 representation is a matter of taste, but I think that 

 many people will agree with me that a very simple 

 diagram, as the present one, will lead more quickly to 

 a clear result, and can more easily be borne in mind, 

 than a more or less complicated formula." 



The first part of the book, dealing with the com- 

 mutator motor, contains six chapters — introductory, 

 the series motor, the repulsion motor, the Latour- 

 Winter-Eichberg motor, some special types, and 

 finally examples of motors, with views and curves ot 

 performance, but not many technical data of construc- 

 tion. The only example illustrated by dimensioned 

 working drawings is a 60-h.p. motor made by the 

 Oerlikon Co. The brevity of style is certainly com- 

 mendable, but in some places it is carried too far. 

 Thus on p. 44, when dealing with tlie minimum flux 

 required for sparkless commutation, the author gives 

 w'ithout proof a formula in which the total flux, that 

 is, the flux per pole multiplied by the number of 

 pairs of poles, is shown to be proportional to the 

 square root of a fraction containing in the nominator 

 the product, horse-power, volts, and length of arma- 

 ture, and in the denominator the product revolutions 

 per minute and diameter of armature. As he says 

 that this formula " will do good service in formulat- 

 ing a general idea of the amount of flux required," 

 we may fairly expect that he should give a proof of 

 it. Another matter in w^hich a somewhat fuller treat- 

 ment might well be expected is the Deri (not Dery, as 

 the author writes) motor. One page can hardly be 

 considered sufficient to deal with a motor which pre- 

 sents so many interesting features, and is also, from 



