6o6 



NA TURE 



[October i8, 1906 



we find the familiar F which stood for lepidopterid 

 flowers applied to those fitted to the visits of Diptera 

 we have ground for complaint. The same is true of 

 the introduction of new symbols for the well-known 

 A, AB, and B. Surely English standards are suffici- 

 ently different from those in use on the Continent 

 without our needlessly multiplying instances. 



The discussion which follows on the different 

 classes of flowers forms one of the most interesting 

 parts of the volume. Thus we get Knuth's curious 

 observations on the proportion of anemophilous plants 

 on the wind-beaten Halligen Islands in the North 

 Sea, where they form 47 per cent, of the flora, whereas 

 on the mainland the percentage is 215 — a case which 

 may remind us of the wingless insects of Madeira. 

 Then, again, we have details of flowers fertilised by 

 bats, birds, slugs, and snails which we think are 

 here put together for the first time in English. There 

 is also a discussion of some interest on flowers which 

 to our eyes are inconspicuous, but which neverthe- 

 less attract many visitors. Further on is a good 

 account of the well-known methods of fertilisation in 

 the yucca and the fig. 



L'nder pollen flowers, i.e. those visited for the sake 

 of their pollen, the author makes what seems to us 

 an unnecessary blot in his system of classification. 

 Thus Sarothamnus scoparius, Genista tinctoria, &c., 

 though devoid of nectar and visited solely for pollen, 

 " are not regarded as pollen-flowers but as well 

 marked bee-flowers." Even here he is not consistent, 

 since Cassia chaniaecrista and Solanum rostratum 

 are described as pollen flowers, though they too are 

 adapted for bees. 



H. Miiller's important work on the specialisation 

 of insects in relation to flowers is fully given, and 

 this is a subject often neglected for the converse 

 instances of floral adaptations. Here too is an 

 interesting account of differences in habits according 

 as the visitors are of the social or solitary bees. The 

 social class, having to work hard for a living, is 

 forced to visit flowers which the luxurious solitary 

 bee neglects. Near the end of the book is a good 

 account of the statistical method of treating the visits 

 of insects, as illustrated chiefly from MacLeod's re- 

 searches. The volume concludes with a valuable 

 bibliography comprising 3748 entries, and occupying 

 160 pages. 



The translator has done his work w^ell on the 

 whole. We must, however, direct attention to a few 

 instances of faulty rendering. Thus " Blumenblatter " 

 is translated by "floral leaves," " Saft " (nectar) by 

 "sap." But the few slips in translation that occur 

 are not serious; we have no objection to H. Miiller 

 being described as a "genial" author (p. 25), or 

 to the incorrect statement that Darwin inherited his 

 house at Down (p. 8n), except that they are due to 

 the translator, not to the author. 



But these are trifles in comparison to the fact that 

 his English is thoroughly readable, and this is a 

 standard by no means easy of attainment in trans- 

 lating from German. 



F. D. 

 NO. IQ29, VOL. 74] 



SINGLE-PHASE COMMUTATOR MOTORS. 

 Single-phase Commutator Motors. Bv F. Punga. 



Translated from the German by R. F. Looser. 



Pp. -xvi 4-187. (London: Whittaker and Co., 1906.) 



Price 4-?. 6rf. net. 

 "O ECENT advances in the application of single- 

 -*-^ phase alternating currents to electric traction 

 have given rise to a large volume of literature dealing 

 chiefly with the motors employed. The possibility of 

 working direct-current motors with alternating cur- 

 rents is by no means new, but it is only within the 

 last few years that the principles of good design have 

 become sufficiently well known to enable such work- 

 ing to be made a commercial success. 



It was perhaps inevitable that a large part of the 

 literature devoted to this subject should be somewhat 

 academic ; in any new departure of this kind the 

 experimental work which forms the basis of progress 

 is in the hands of manufacturers, to whose interest it 

 is that the information so obtained should not be made 

 public. It is, therefore, all the more interesting to 

 examine a book which is evidently written for the 

 practical man. In such a book circle diagrams should 

 occupy a subordinate position, and attention should 

 be directed to the question of proportions that may be 

 assumed in practice. 



The course adopted in this book is to set out as 

 clearly as possible what may be called the practical 

 theory of the motors, and to follow this up by applica- 

 tions of the theory to the design of actual examples. 

 This is no doubt the right course, for however valu- 

 able a knowledge of the fundamental theory may be, 

 thei-e are many points of equal importance which can 

 only be brought out in the calculation of an actual 

 motor. 



The setting out of the theory of single-phase com- 

 mutator motors has been made very clear, and 

 although circle diagrams are referred to, the author 

 states very truly that they are of little practical value, 

 and that it is better to calculate the current for a few 

 points from first principles. Particular attention has 

 been paid to the question of sparking, and its depend- 

 ence on the " transformer voltage," the " reactance 

 voltage," and the "rotation voltage" in the coils 

 short-circuited by the brushes. The effect the trans- 

 former voltage has on the general design is also clearly 

 explained, but hardly sufficient reference is made to 

 the magnetising action of the circulating current pro- 

 duced. 



Turning now to the calculation of typical motors, a 

 series motor of 60 h.p. is worked out, and also a re- 

 pulsion motor of 48 h.p. It is unfortunate that prac- 

 tically no indication is given as to how these motors 

 are rated. At present, single-phase commutator 

 motors are inevitably associated with traction work, in 

 which it is customary to speak of the one-hour rating. 

 Supposing this to be the intention of the author, it 

 must be confessed that the size of the motors is rather 

 large for their output, chiefly on account of the low 

 speed chosen. .Another objection, which is perhaps 

 more serious, is that the windings have been made 



