536 



NATURE 



[January i6, 191; 



irentarv character, and those who have seen 

 Greenhill experimenting with bicycle wheels at 

 former mathematical congresses will not be sur- 

 prised to find these experiments detailed here. The 

 chapter on the screw propeller contains a summary 

 of the principal theories and formulse that have 

 been proposed, but, as the author points out, all 

 these are based on certain assumptions. The 

 discontinuous motion past the blades of a pro- 

 peller is far too complicated to admit of analytical 

 investigation. The reference on p. 105 to the 

 method of fixing two screws on the same shaft 

 so as to neutralise the rotational angular momen- 

 tum of the wake^ seems deserving of careful atten- 

 tion. This system may not have been of much 

 advantage when applied to the early screw 

 steamers, but with the increasing size and speed 

 of modern flying machines, an arrangement of the 

 kind may not improbably prove indispensable, 

 especially in a rare medium like air. Without 

 ';ome such plan increased speed can only be 

 obtained by increasing the revolutions of the 

 screw or decreasing the pitch, and in the latter case 

 the proportion of energy wasted in the rotation 

 ol the wake increases correspondingly, as does 

 also the couple, tending to turn the aeroplane over 

 sideways unless it is provided with twin-screws. 

 The last chapter deals with the pneumatical prin- 

 ciples of an airship, that is to say, mainly ele- 

 mentary hydrostatics. 



For the student of exact science, this book will 

 afford a good account of the principles and 

 formulae which determine the forces acting on a 

 flying machine when it moves through the air. 

 The study of the motions which an aeroplane 

 undergoes under the action of these forces is 

 another matter altogether. On the other hand, 

 those who require to apply formulae to actual 

 machines will have a guarantee that the formulae 

 in this book are at least sound deductions from 

 the assumptions on \\hich they are based. 



G. H. Bryan. 



MUNICIPAL TRADING AND CURRENCY. 

 (i) Principles and Methods of Municipal Trading. 

 By Douglas Knoop. Pp. xvii 4- 409. (London : 

 Macmillan & Co., 1912.) Price los. 6d. net. 

 (2) The .Statidard of Value. By Sir David Barbour, 

 K.C.S.I., K.C.M.G. Pp. xvi + 242. (London: 

 Macmillan & Co., 1912.) Price 65. net. 

 (i) ' I "*HE subject of municipal trading presents 

 J_ an initial diilficulty of definition. The 

 economic or "reproductive " undertakings of local 

 authorities may be divided into trading enterprises 

 carried on for profit, and those of which the 

 charges do not fully cover the cost and which are 

 therefore subsidised. Mr. Knoop confines himself 

 NO. 2255, VOL. 90] 



to the former, but includes those businesses which 

 are sometimes carried on for profit and sometimes 

 are not. .A.s a result, his work has gained in 

 completeness and thoroughness. 



The arrangement of the book is admirable, and 

 each chief division of the subject, such as Extent, 

 Management, and Selling or Labour Policies, is 

 separately dealt with. Moreover, both the general 

 treatment and the criticism are, with a few ex- 

 ceptions, ample and well-informed. One may 

 mention especially the case of reserve funds and 

 renewals, in which the author's strictures appear 

 most just, and the judicious summing-up regarding 

 the " writing-down " of the commercial to the 

 housing value of cleared sites. The author's con- 

 clusions limit the municipal management of com- 

 petitive enterprises to those which tend to become 

 monopolies ; otherwise, " with one or two small ex- 

 ceptions, it is strongly to be deprecated." This 

 appears unduly pessimistic, but few will deny the 

 very great value of the book as a history, descrip- 

 tion and criticism. 



(2) Sir David Barbour, a former Financial 

 Member of the Council of the Governor-General of 

 India and one of the Royal Commission on Gold 

 and Silver, aims at placing on record the results of 

 his wide administrative and financial experience. 

 His book deals primarily with the causes of appre- 

 ciation or depreciation, whether in a gold standard 

 or under a system of bimetallism. Regarding the 

 latter, he holds that a definite verdict either way 

 is not possible, but is doubtful whether the adop- 

 tion of a gold standard would have taken place 

 if all its consequences could have been foreseen. 



There is a slight preliminary treatment of some 

 general questions of economic theory which bear 

 upon the subject, and a fuller one of the Quantity 

 Theory and of the relations of money, credit and 

 prices. Here the author is inclined to under- 

 estimate the modifications of the theory that are 

 brought about by the influence of credit, at any 

 rate in the case of wholesale transactions. Again, 

 it is not the total output of fresh gold, but the 

 I part of it that is used for coinage, including re- 

 serves of bullion, that influences the level of prices. 

 The second half of the book is both the more 

 valuable and the more intei'esting part of it. The 

 points are well made that to lower general prices, 

 decreased cost of production must be accompanied 

 by increased output ; and that " the fall in the gold 

 price of silver was due to the fall in the gold price 

 of commodities produced in the gold standard 

 countries and exported to the silver standard 

 countries." The whole book is clear and read- 

 able, and, embodying as it does the theoretical 

 conclusions of a man of wide practical experi- 

 ence, contains much that is of interest and value. 



N. B. De.'vrle. 



