THE ability of the author of this work to give a lucid exposition of an abstruse subject is a matter of common knowledge; and hence the fact that the book contains two of his lectures delivered at the London Institution last November is in itself sufficient commendation. However, be this as it may, we have no hesitation in saying there could hardly be a clearer explanation of Prof. George Darwin's theory of tidal evolution than that contained in the work before us. The hypothesis being accepted, every feature of the past and future condition of our satellite is described in a most comprehensive manner. It is first shown how, when the earth was rotating on its axis with an enormous velocity, the tidal action set up by the sun caused a portion to become detached and form our satellite. The employment of the term “conservation of spin” facilitates considerably the demonstration of the fact that as by tidal action the spin of the earth decreases—as our day lengthens—so must the dimensions of the moon's orbit be increased, and the length of the month therefore become proportionally greater. The application of Prof. Darwin's theory to other members of our system is also inquired into; and although the author does not attempt to go back to the first stage in the evolution of celestial species, he shows that tidal evolution is an extension of the hypothesis that does so. Indeed, the book is replete with information, and by the general scientific reader will be found exceedingly interesting.
Time and Tide: A Romance of The Moon.
By Sir Robert S. Ball (London: Society for Promoting Christian Knowledge, 1889.)