February 9, 1922] 



NATURE 



167 



relate to the calculation of the constants of cojls, 

 and the theory of absolute electro-dynamometers and 

 current balances, a domain of the subject to which 

 Prof. Gray has himself so largely contributed. His 

 well-known calculation of the mutual inductance of 

 two single layer coils the axes of which intersect at 

 any angle has led to the realisation of an absolute 

 rlectro-dynamometer, constructed at the Bureau of 

 Standards, the constant of which can be calculated 

 to a high degree of accuracy. It was shown by 

 Prof. Gray that if the coils of the instrument are 

 concentric and have lengths \/3 times their radii, 

 all the terms between the first and the seventh vanish 

 in the zonal harmonic series for the mutual induct- 

 ance and the couple between them, and the remain- 

 ing terms amount to only a very small correction if 

 the dimensions of the inner coil are small in com- 

 parison with those of the outer. Consequently the 

 couple is given very accurately by the first term 

 alone of the series — that is, it can be calculated on 

 the assumption that the inner coil is suspended in 

 a perfectly uniform field equal to the field at the 

 centre of the fixed coil. 



It is shown how the values of the mutual induct- 

 ance of two coaxial single-layer coils (including the 

 important case of a helix and a coaxial circle), the 

 self-inductance of a single-layer coil, and the 

 mutual attraction of two coaxial coils, such as those 

 of a current balance, can also be deduced from the 

 general formula. Many other cases are worked out 

 in the chapter on the calculation of inductances, 

 which is much extended in the new edition, and the 

 description and illustrations of the current balances 

 of the National Physical Laboratory and the Bureau 

 of Standards, and the electro-dynamometer of the 

 latter institution, form a valuable feature of the 

 book. 



Other portions of the book which are much ex- 

 panded are those dealing with magnetometry, 

 measurements in alternating current circuits, the 

 distribution of alternating currents in cylindrical 

 conductors, the comparison of resistances, and the 

 absolute measurement of resistance. The recom- 

 mendations of the International Conference on Elec- 

 trical Units held in London in 1908, embodying the 

 definitions of the international ohm, ampere, and 

 volt, and the specification of the Weston normal cell, 

 are given in appendices. 



Much that was in the old edition has of necessity 

 l)een omitted from the new ; the omitted portions, 

 however, mainly of a theoretical nature, are not 

 directly connected with methods of measurement, 

 and have been fully treated in Prof. Gray's "Treat- 

 ise on Magnetism and Electricity." The result is a 

 more complete and a better arranged account of 

 methods of electrical measurement and the calcula- 

 tions connected therewith. 



NO. 2728, VOL. 109] 



D 



In its new form the book is in one volvime, and 

 the larger page, the absence of small type, and the 

 numbered sections will make the book more accept- 

 able to the reader. Typographical errors are re- 

 markably few for a work of this size, and in the 

 few cases where results are stated erroneously (as, 

 for instance, the expression for the capacity of a 

 condenser on p. 749) the reader will find no diffi- 

 culty in supplying the correction. 



Of the great value of the book there can be no 

 question, and it may be confidently anticipated that 

 the new edition wall be appreciated as highly as was 

 its predecessor. E. T. J. 



The Art of Prehistoric Man. 



Prehistory: A Study of Early Cultures in Europe 



and the Mediterranean Basin. By M. C. Burkitt. 



Pp. XX + 438. (Cambridge: At the University 



Press, 192 1.) 355. net. 



URING the last two decades .great progress 

 has been made in our knowledge of pre- 

 historic man, especially by discoveries in the caves 

 of France and Spain. Most of the results are 

 published in technical memoirs in serials not easily 

 accessible, and it is difficult to follow them without 

 much previous study and extensive reading. Mr. 

 Burkitt has therefore done good service by the pre- 

 paration of his volume on " Prehistory," which 

 summarises the whole subject and enables both the 

 student and the general reader to appreciate its 

 present position. He himself has taken an active 

 part in much of the research, in association with 

 the Abbes H. Breuil and H. Obermaier; he thus 

 writes from personal knowledge, and adds sufficient 

 details of some of the most interesting localities to 

 make his text-book a useful guide for those who 

 wish to visit them. 



Other English books, especially those of Lyell 

 and Boyd Dawkins, have already given a good 

 general account of the discoveries of early man in 

 the caves of this country, so that Mr. Burkitt has 

 done well in devoting attention chiefly to France 

 and Spain. His " outline of the history of the 

 subject," however, fails to give due credit to the 

 English pioneers, whose systematic work in Brix- 

 ham Cave, Kent's Hole, and Wookey Hole is not 

 even mentioned. MacEnery and Pengelly are over- 

 looked, and Mello's discovery of the drawing of a 

 horse's head on a piece of bone from the Cresswell 

 caves is wrongly described, thus throwing doubt on 

 its authenticity. On p. 76 bone is said to have been 

 " first utilised in Upper Mousterian times," and 

 when this statement, based on French and Spanish 

 experience, is contradicted on p. 89 by a casual 

 reference to the English discovery of a large bone 



