I 659 ] 



LXI1I. Notices respecting New Books. 



Production et emploie des Courants altematifs. Par L. Barbillion. 

 Paris : G-. Carre et C. Naud, 1901. ( 4i Scientia " Series, No. ] 1.) 

 Pp. 103. 



WE have already had occasion to review briefly some of the 

 volumes belonging to the " Scientia" series, which is being 

 brought out by the well-known French publishing firm, Messrs. 

 Gr. Carre et C. Xaud. The present volume of 103 pages contains a 

 digest of alternating-current theory. The title of the book is mis- 

 leading, and it seems to us that " the theory of alternating currents '* 

 would have been a much truer indication of the contents of the 

 book than the title selected by the author. The brief discussion 

 of continuous-current machines given on pp. S-14 seems entirely 

 out of place, and is, of course, very incomplete. On p. 8 we have 

 a diagram (fig. 2) of the magnetic field in a two-pole dynamo 

 which would be excusable in a pre-historic treatise on the subject, 

 but which it is surprising to find in an up-to-date book ; no 

 attempt is made to show the true course of the lines and their 

 refraction as they pass from iron to air or vice versa. Taken as a 

 whole, however, the book is a useful and concise exposition of the 

 principles underlying the applications of alternating currents, and 

 contains a good account of several interesting developments of 

 recent date. 



LXIV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 580.] 



June 19th, 1901.— J. J. H. Teall, Esq., M.A., V.P.R.S., 

 President, in the Chair. 



r PHE following communications were read : — 



■*■ 1. • On the Use of a Geological Datum.' By Beebv Thompson, 



Esq., F.G.S., F.C.S. 



A proper interpretation of geological phenomena frequently 

 requires that allowance shall be made for differential earth- 

 movements that have taken place since the period under con- 

 sideration. Present differences of level in rocks of the same age 

 may be due to actual differences in depth of the sea-floor on which 

 they were deposited ; but they may also be the result of subsequent 

 differential earth-movements. The rock selected as a datura should 

 combine as far as possible the following characteristics : —It should 

 be thin, of considerable horizontal extension, having similarity in 

 physical characters and palasontological contents over a large area, 

 and situated as near as possible, in vertical sequence, to the reference- 



