to the first two subjects mentioned above--the top spinning on a flat surface, and the earth spinning through space--which are, of course, the classical problems in rotational dynamics. It need scarcely be said that Sir George Greenhill's work is abundantly cited. "In the present work my aim has been to refer, as far as possible, each gyrostatic problem directly to first principles, and to derive the solutions by steps which could be interpreted at every stage of the progress," says the author in his preface, and he has followed this aim with considerable success. It is, of course, impossible to treat many of the problems of rotational dynamics without mathematical analysis of some complexity, and a knowledge of elliptic functions and such-like weapons of the applied mathematician lies, perhaps, outside the scope of the average engineer and inventor. Professor Gray, who deplores the present ignorance of inventors in the matter of gyroscopic motion, has kept the needs of this class before him, and has taken care to arrange his matter so that those who cannot always follow the mathematical exposition given can, at least, gain a clear knowledge of the results. The first chapter, which contains no mathematical symbols, forms an excellent introduction to the subject and is quite elementary, and elsewhere in the book, when practical problems, such as the drift of a projectile, are being discussed, the nature of the investigation is stated as simply as may be. Throughout the inquiry is illustrated, as far as possible, by experiment and diagram. "Les Anglais enseignent la méchanique comme une science expérimentale; sur le continent, on l'expose toujours plus ou moins comme une science déductive et _a priori_. Ce sont les Anglais qui ont raison, cela va sans dire." In these words, the late Henri Poincaré, the greatest mathematician of his generation, praised the British tradition of teaching dynamics as an experimental subject, which is so well maintained in this book. Some specialists, no doubt, will find minor omissions in their subject, but, on the whole, with the exceptions already noted, the book is very complete. It is printed with the well-known elegance in all that pertains to mathematical symbols of the firm of Robert Maclehose, and the general production is very good. We do not understand, however, why the illustrations in the first chapter are nearly all reproduced a second time further on in the book, especially as they are mostly photographs of apparatus, which do not necessitate frequent reference. And--the question that has to be asked so often with English books--why is the index so defective? EVERYDAY EFFICIENCY: A PRACTICAL GUIDE TO EFFICIENT LIVING. By FORBES LINDSAY. W. Rider & Son. 4_s._ 6_d._ net.