June 3, 1920] 



NATURE 



419 



A clear description of the Thury system of direct- 

 current hig^h-tension transmission is given. There 

 seems to be little chance, however, of this system 

 being adopted on a large scale in this country. 

 Chap. V. describes the uses of a storage battery 

 in connection with a dynamo. Brief descriptions 

 of the various types of booster used in this con- 

 nection are given and will be helpful to the 

 student. 



In chap. vii. a brief reference is made to 

 Fourier's theorem. The coefficients are obtained 

 by drawing the graphs of curves the equations of 

 which are of the form 



y=j{x) cos (27rn.\7X), 

 and then getting their area by the planimeter. 

 Although theoretically correct, we think that the 

 method would prove laborious in practice. We 

 think also that the error for high harmonics would 

 probably be large, as the planimeter reading gives 

 the difference between many areas. 



The author suggests that large choking coils 

 should be constructed in the form of a toroid, 

 the diameter of the circular cross-section of which 

 equals 0-31 iD, where D is the mean diameter of 

 the toroid. This is Maxwell's solution for the 

 most economical coil; and the author has found 

 practically that the expression g^sn^D gives Its 

 inductance, where n is the number of turns. This 

 is in good agreement with Maxwell's formula, 

 ^Trn-D—i.e. 9-43n2D. It has to be remembered, 

 however, that Maxwell's formula is only a rough 

 approximation. If we use Rayleigh's formula we 

 get g-Ggn^D for the inductance. The 4 per cent, 

 difference between theory and experiment is prob- 

 ably due to the assumption that the current is 

 uniform all over the cross-section of the toroid. 

 If we use Rayleigh's formula it will be found that 

 we get very appreciably different dimensions for 

 the most economical choking coil. 



The discussions of parallel running, trans- 

 formers, converters, and induction and commuta- 

 tor motors are all instructive and suggestive^. The 

 last chapter, on phase advancers, is a strong and 

 convincing plea for their more general adoption 

 in practice. Considerable economies can be effected 

 by their use. A. Russell. 



British Iron Ores. 



The Iron Ores of Scotland. By M. Macgregor, 

 Dr. G. W. Lee, and G. V. Wilson. With con- 

 tributions by T. Robertson and J. S. Flett. 

 (Memoirs of the Geological Survey, Scotland : 

 Special Reports on the Mineral Resources of 

 Great Britain : Vol. xi. Iron Ores {continued).) 

 NO. 2640, VOL. 105] 



Pp. vii 4- 240. (Edinburgh: H.M.S.O. ; South- 

 ampton : Ordnance Survey Office.) Price 10s. 

 net. 



THE present volume forms a continuation of the 

 important series of memoirs on the iron ores 

 of Great Britain which the Geological Survey has 

 been issuing for some time past. Unlike some of 

 the previous ones, the subject here discussed 

 affords little scope for original geological investi- 

 gations, the principal deposits of iron ore being 

 very well known and having often been described. 

 As is, however, very truly remarked by Sir Aubrey 

 Strahan, the information concerning them is 

 scattered throughout a large number of publica- 

 tions, and it is a great advantage to the student 

 of the subject to have it all brought together in 

 one volume. The authors have done their work 

 carefully and painstakingly, and the result of their 

 labours has been to render available a very com- 

 plete and minutely accurate record of the known 

 Scottish iron-ore deposits. 



The book is divided into seven chapters. The 

 first gives an introductory and historical account 

 of the subject, containing much interesting matter ; 

 attention should, howeverj be directed to a mis- 

 take as to the nature of the old Catalan furnace. 

 The author writes : " These furnaces were of the 

 Catalan type, . . . and castings were apparently 

 made direct from the furnace itself." These tw3 

 statements are incompatible, for the essential' 

 feature of the Catalan process was that it produced 

 malleable iron, and not cast iron, direct from the 

 ore. 



The next four chapters are devoted to thi 

 bedded iron ores of Carboniferous age, these being 

 subdivided mainly according to their geographical 

 distribution. In a sixth chapter the bedded 

 Raasay ironstone of Jurassic age is described, 

 and in the last chapter a number of minor occur- 

 rences, which are grouped together under the 

 heading " Haematite in Veins and Beds " — a some- 

 what infelicitous title, seeing that true haematite 

 is conspicuous by its absence. 



As in the previous volumes of the series, the 

 least satisfactory portion of the present one is that 

 relating to the estimated tonnage of ore reserves. 

 It ought to be made thoroughly clear that the 

 tonnage of ore as estimated by a geologist repre- 

 sents a quantity many times greater than that 

 which the miner can hope to recover in actual 

 practice. The iron industry of Scotland requires 

 some 2^ million tons of ore annually, about one- 

 fifth of which is obtained from native Scottish de- 

 posits, and it is poor consolation to the Scottish 

 ironmaster, who for a good many years past has 



