April 13, 1905] 



NATURE 



557 



examination results, as a comparison of tlie sets of 

 examination papers included at the end of the volume 

 with the text of the book amply demonstrates, and 

 it must be added that if an observant student carries 

 out the simple experiments so clearly described at 

 various places in the volume, he will have acquired a 

 very desirable knowledge of the more important 

 features of physiology. But so much cannot be said 

 of the remainder of the text, which aims at far too 

 much statement of detail for the space available, a 

 matter in which the syllabus may be much more to 

 blame than the author. 



For example, the student who has learnt no 

 chemistry previously will not be able to digest much 

 from the description of the chemical elements given 

 in a single page, and the same is true of the descrip- 

 tion of the chief inorganic compounds and the organic 

 compounds of the body, each dismissed in less than a 

 page. 



The valuable habit of coordinating knowledge in 

 the form of tables is visible at places in the book, but 

 summaries have a wav of becoming either too sweep- 

 ing or too inexact, and we fear that the pupil, 

 especially after such a concise training in chemistry 

 as we have just indicated, may be in danger of con- 

 cluding from a perusal of the table on p. 13 that the 

 body contains " mineral salts " formed from a very 

 strange combination of elements, or, from the table 

 on p. 162, that these same " mineral matters " share 

 only " in forming bene ,Tnd assist in digestion," and 

 not that they are found in every cell and tissue in the 

 body, and form as essential a constituent there as 

 the all-important proteids, which are in the same table 

 represented as the only tissue formers. 



B. Moore. 



TERRESTRIAL MAGNETISM. 

 Terrestrial Magnetism and its Causes. By F. A. 

 Black. Pp. xii-l-226. (London and Edinburgh: 

 Gall and Inglis, 1905.) Price 6s. net. 



WITH regard to the earth's magnetism, the 

 general conclusions from observations made 

 on its surface are that it is partly permanent, partly 

 induced, and subject to the effects of electric currents 

 in the earth's crust and the surrounding atmosphere. 

 Moreover, that the direct action of the sun plays a 

 comparatively subordinate part in producing the 

 observed phenomena. 



In this boolv, however, various reasons are sub- 

 mitted for the belief that the general magnetism of 

 the earth, and the constant changes thereof as shown 

 by the hourly variations of the needle, are due to 

 causes external to the earth. In short, that the earth 

 is to be considered as an electromagnet excited by 

 electric currents proceeding from the sun and im- 

 pelled towards the earth with inconceivable rapidity, 

 the orbital and axial movements of the earth through 

 these currents producing magnetic effects in a manner 

 similar to the winding of an electromagnet through 

 which a current passes. 



In order that we may believe this to be the case, we 

 must agree that the sun gives out electric waves 

 continuously in every direction equal to the work of 

 NO. 1850, VOL. 71] 



maintaining the earth as an electromagnet. For 

 example, that during the forty-five years of the last 

 century, when, according to computation from 

 observed facts, the earth's magnetic moment hardly 

 changed, these emanations were continuous. .'\t pre- 

 sent there does not appear to be any ground for such 

 a belief. 



In an endeavour to explain the hourly angular 

 variations of the needle, it is submitted that the 

 earth's magnetic poles probably occupy a consider- 

 able area round the centre of which certain centres 

 of primary attraction in them make a daily circuit, 

 due to the action of the sun as the earth rotates on its 

 axis. In addition to the " primary " magnetic pole 

 in North America, it is suggested that a " secondary " 

 pole of a similar nature must exist in northern 

 Siberia. The daily variations of the needle, both in 

 declination and dip, in the northern hemisphere are 

 then attributed to a battle for the mastery between 

 the revolving centres of attraction in the two poles 

 mentioned, modified as the magnetic equator is 

 approached by the attraction of the south magnetic 

 poles. 



As one reads through several of the first chapters 

 the fully expressed acceptance of the idea that the 

 attraction of the needle by the magnetic poles is the 

 immediate cause of its variations seems unaccount- 

 able, until a fundamental error is reached. This is 

 when the author takes it as generally agreed that, in 

 the same way as steel is attracted by the poles of an 

 ordinary artificial magnet, the magnetic needle is 

 attracted by the poles of that great natural magnet, 

 the earth. Such a statement vitiates whole pages of 

 tire arguments adduced. 



On the question of the position of the magnetic 

 equator with regard to the terrestrial equator, the 

 results of observation have also been too much 

 ignored. There have not been four crossings of the 

 two equators during the last sixty years, neither are 

 the two known points of crossing regulated by the 

 position of the magnetic poles as suggested. In the 

 Atlantic region, the point of crossing seems to be 

 chiefly regulated by local causes below the earth's 

 surface. 



It may be finally remarked that the chapter on 

 magnetic storms is the most acceptable in the book. 



OUR BOOK SHELF. 

 Mechanical Appliances, Mechanical Movements and 



Novelties of Construction. By Gardner D. Hiscox. 



Pp. 396. (London: Constable and Co., Ltd., 1905.) 



Price 12s. 6d. net. 

 This book is luxuriously printed, with clear figures, but 

 it is difficult to say more in its praise. It consists of 

 a series of short paragraphs, each with its illustration, 

 describing some mechanical or constructional device. 

 It is similar in plan to those " Centuries of Invention " 

 of which the Marquis of Worcester's was the earliest 

 (1746). The devices described are of the most hetero- 

 geneous character, old and new, important and un- 

 important, useful and useless. They are arranged_ in 

 the roughest way in sections which have no relation 

 to any natural order of classification. It is difficult 

 to see to whom such a work appeals, but in fairness 

 to the author it should be stated that a previous work 



