December i, 1910] 



NATURE 



135 



particularly the by no means easy genus Pentaceros — 

 are practically revised, within set geographical limits, 

 adds enormously to the value of this conscientious 

 monograph. 



Though the work is for the most part descriptive — 

 almost commendably so in an age of easy speculation 

 — the author takes pains to set all his species in their 

 due relations. Of Falmipes sarasini he observes that 

 its differences from its congeners are almost of generic 

 value, and of Valvaster that its peculiarities are almost 

 sufficiently exclusiv-e to give it rank as an independent 

 family. He also discusses the position of the irre- 

 concilable genus Metrodira, which he has no hesitation 

 in establishing among the Linckiidae. Nor has he 

 forgotten to notice the small parasitic mollusca found 

 on species of Stellaster and Palmipes. 



This is Prof. Koehler's sixth memoir of the fine col- 

 lection of Echinoderms of the Indian Museum, and, 

 as India is still meta incognita so far as the Echinoidea 

 are concerned, we trust that it is not the last. 



One criticism, however, may be offered of this 

 memoir, as of its precursors of the series, namely, 

 that it is too exclusively addressed to the specialist. 

 Species are examined and described with acumen, but 

 there are none of those synoptical tables, of educa- 

 tional value, which the student has almost a right to 

 expect in a museum publication that treats in its 

 entirety one large component of a fauna. If the 

 author would crown his labours in this field by pub- 

 lishing synopses of the families, genera, and species 

 of Indian Echinoderma, he would " thereby highly 

 oblige " many to whom, though they are not experts, 

 whatsoever passeth through the paths of the seas is of 

 interest. 



EXPERIMENTAL ELECTRICITY AND 

 MAGNETISM. 

 (i) Practical Electrical Engineering for Elementary 

 Students: An Elementary Laboratory Course for 

 Students of Electrical Engineering in Trade and 

 Technical Schools. By W. S. Ibbetson. Pp. xii + 

 155. (London: E. and F. N. Spon, Ltd.; New 

 York : Spon and Chamberlain, 1910.) Price 3s. 6d. 

 net. 



(2) Practical Electricity and Magnetism: A First 

 Year's Course. By R. Elliott Steel. Pp. viii+175. 

 (London: G. Bell and Sons, Ltd., 1910.) Price 2s. 



(3) Elementary Experimental Electricity and Mag- 

 netism. By W. T. Clough. Pp. viii + 255. (London: 

 Methuen and Co., Ltd., 19 10.) Price 2^. 6d. 



'T'HE first two books are intended to cover first-year 

 -L laboratory courses, in the one case for technical 

 students, in the other for beginners in science at a 

 school or college. The usual differences are to be 

 noted between them, the second being far more 

 theoretical than the first, and also containing sections 

 on magnetism and electrostatics, which the other does 

 not touch. 



(i) The system employed by Mr. Ibbetson is excel- 

 lent, and could with advantage be adopted in any 

 technical schools in which a similar scheme is not 

 already in operation. It is perhaps to be regretted 

 NO. 2144, VOL. 85] 



that no experiments with magnets are included, un- 

 less the student is intended to have taken a pre- 

 liminary course in electricity and magnetism under 

 the heading of physics. 



Slight alterations in the experiments will have to 

 be introduced in different laboratories to suit diverse 

 conditions, as the book has evidently been written to 

 fit the apparatus employed in one particular school. 

 This defect shows itself most prominently in the too 

 narrow specification of the instruments to be used 

 in the experiments, instead of general advice to 

 enable students to select instruments and resistances 

 suitable in their range and capacity. 



The connection diagrams are, on the whole, very 

 good, the boldness of their drawing being a valuable 

 feature. They could be improved, however, especially 

 in the cases of more complicated experiments, by 

 simplification of the drawings of instruments and 

 switches and the avoidance of cross-overs wherever 

 possible. 



In experiment xxxv. the use of a standard resist- 

 ance in connection with the calibration of a voltmeter 

 seems quite unnecessary-, and confuses the experiment 

 with the calibration of an amperemeter. 



" Shunt dynamo, separately excited," is a ven,- con- 

 tradictory term, which appears on p. 144; moreover, 

 the experiment can be performed just as well with a 

 generator with a low-resistance field winding. 



The complete lack of any reference to the error 

 due to the voltmeter current when measuring resist- 

 ance by the ammeter-voltmeter method is a serious 

 defect. When measuring efficiency in the photometry 

 experiment, the connections are made so as to avoid 

 this error, but no reason is given. On the whole, 

 however, the experiments are detailed with the care 

 and exactness so essential when dealing with elemen- 

 tary classes. 



(2) Mr. Steel's book contains instructions for carrv- 

 ing out a great many experiments, but the language 

 is hardly concise enough for scientific work. The use 

 of supply mains and accumulators is avoided, which 

 renders the book suitable for some few laboratories 

 but unsuitable for many others. The comparative 

 absence of diagrams of connections is a great draw- 

 back, and the few which appear are not good 

 examples for students to copy from. 



(3) Mr. Clouch's book is intended to act as a 

 theoretical text-book, as well as a practical guide in 

 the laboratory, for students preparing for the elemen- 

 tary examinations in the subject. 



Magnetism and statical electricitv together occupy 

 the first one hundred and fifty pages of the book, and 

 voltaic electricity- the remaining one hundred. Voltaic • 

 work is explained from a statical point of view in a 

 method somewhat unusual at the present day. 



The diagrams and illustrations are plentiful, and 

 the type is varied so as to call into prominence the 

 most important passages. Numerous exercises are 

 given, chiefly drawn from recent examination papers, 

 which should be very helpful for intending candidates. 



A few omissions are noticeable, e.g. no reference 

 is made to the moving-coil type of galvanometer, 

 which is more frequently used than the suspended- 

 magnet tvpe. 



