REVIEWS 345 



manner in which the author illustrates the importance of gyroscopic forces in 

 various cases. In the fifth chapter a brief presentation of practical applications 

 of the gyroscope is given. Obry's method of directing a torpedo, Schlick's method 

 of steadying a ship, and Brennan's mono-rail are here lucidly described. The 

 plate illustrating Schlick's apparatus gives a good idea of the dimensions of the 

 gyroscope employed. 



The remaining portion of the book is concerned with a more complete 

 mathematical treatment of the subject, and maintains the same high level of 

 clearness. Chapters are devoted to the steady motion, general motion, and 

 stability of motion of a top. Chapter viii. is devoted to the deduction of the 

 equations of motion referred to moving axes. The occasional use of the symbol 

 M for the resultant angular momentum — usually designated by h — is to be 

 regretted. The deduction of the component velocities when referred to moving 

 axes is not convincing. We should much have preferred the method in which 

 a vector is resolved along any arbitrary direction, this direction being then made 

 to coincide with each of the axes in turn. 



Throughout the work the author insists upon a careful study being made of 

 the physical dimensions of all quantities involved in any equation. An excellent 

 feature of the book is the manner in which the quantities involved in angular 

 motion are compared with the more familiar but similar quantities in linear 

 motion. The book contains an excellent collection of examples, drawn largely 

 from practical applications. Answers are given to all the unworked examples. 

 The diagrams are particularly clear, and the text is practically free from errors. 

 The book is one that will repay careful study, and forms a welcome addition 

 to the literature of this subject. 



J. S. G. Thomas. 



An Introduction to the Science of Radioactivity. By Charles W. Raffety. 

 [Pp. xii + 208.] (London : Longmans, Green & Co., 1909. 4^. 6d. net.) 



Mr. Raffety's book is, as he modestly describes it, a popular introduction to 

 the subject for the general reader. The student will do better with more 

 authoritative works. 



To come to detail ; the author would, on p. 7, leave one with the im- 

 pression that Cornish pitchblende has an abnormally low activity. It has recently 

 been shown that the value for the ratio of radium to uranium content is the 

 same for all pitchblendes. The determination of what has been called the 

 " Thomson Constant " — e — the charge on the ion — is dismissed with a few lines 

 (p. 26): the account is so brief as to be nearly useless and unintelligible 

 Page 28 might profitably be rewritten. 



Definitions of anode and cathode should have preceded the short account of 

 electrical discharges through gases. Indeed, Faraday's original " sun-rising and 

 sun-setting" definitions would have been interesting and appropriate in a book 

 of this nature. One finds (p. 144) the old and wrong explanation of the rotation 

 of a paddle-wheel subjected to bombardment by cathode rays. The effect is a 

 radiometer one, and is not a measure of the momentum of the cathode ray 

 stream. 



Mr. Raffety's views on insulators are not practical : paraffin wax is not very 

 good for electroscopes — its natural hygroscopicity, and the almost invariable 

 presence of a residual charge, render it taboo for quantitative work. We read 

 further and find ebonite condemned : we disagree entirely— it is excellent, and 



