REVIEWS 499 



constraint of a point (pp. 8, 19). This way of proceeding seems to take the 

 mind of the student away from the fact that the essence of analytical geometry 

 is that many co-ordinates of points are connected in pairs by one fixed equation. 

 Further, it seems rather a debatable point whether the equations of conies should 

 be postponed beyond equations of pencils of lines and systems of circles. Finally, 

 the old-fashioned way of talking of the equation " to " a curve does not sound 

 quite pleasant to modern ears. 



Philip E. B. Jourdain. 



ASTRONOMY 



Planetary Rotation Periods. By F. A. Black, F.R.S.E. [Pp. xiii + 115. J 

 (Edinburgh and London : Gall & Inglis, 1919. Price $s. 6d. net.) 



This small volume bears the sub-heading, " Two Essays on the Relations between 

 the Planets in Diurnal Rotation and in Mass." The author has discovered 

 empirically a relationship between the period of rotation of a planet and the 

 planet's mass and radius, which gives an approximately constant value for the 

 various planets of the solar system. It does not appear that the relationship 

 has any theoretical importance. Bode's empirical law is an example of another 

 somewhat similar relationship. The contents of the volume could have been 

 compressed into some half-dozen pages. Nothing essential is gained, and clarity 

 is lost, by incorporating detailed elementary arithmetical computations for testing 

 the rule ; further, the use of symbols and a simple formula would have avoided 

 pages of rules, in which unnecessary distinction is drawn between the larger and 

 the smaller planet. We do not doubt but that the majority of readers will find 

 the book too tedious to wade through it to the end. 



H. S J. 



PHYSICS 



Notes, Problems, and Laboratory Exercises in Mechanics, Sound, Light, 

 Thermo-Mechanics, and Hydraulics. By Halsey Dunwoody, Acting 

 Professor of Natural and Experimental Philosophy at the U.S. Military 

 Academy. [Pp. vi + 369, with 241 figures in the text.] (New York : John 

 Wiley & Sons ; London : Chapman & Hall, 1917. Price 13^. 6d. net.) 



This book would appear to have been compiled rather hurriedly, in order to cope 

 with an inrush of students at the United States Military Academy at the outbreak 

 of war. The compiler states that it is intended "as a reference-book for use with 

 any text on the general subject." No doubt he found it convenient for his special 

 purpose to have such a collection bound up in a single volume, but that hardly 

 justifies its export to this country. The first portion of the book consists of a set 

 of notes on Mechanics, printed by permission of Prof. Cross, of the Massachusetts 

 Institute of Technology. These notes are quite satisfactory as a summary of the 

 contents of a textbook of intermediate character, making use of the notation of 

 the calculus. Next comes a short section dealing in quite ordinary fashion with 

 Graphical Statics ; it would appear to be the only part of the book written by 

 Prof. Dunwoody himself. The third section consists of some 900 problems taken 



