PERMIT me to thank Prof. Tait for his kind and amusing criticism of my little book. I am struck with comic horror at the thought that anything in the preface can be construed into a comparison between works like Thomson and Tait, Clerk-Maxwell and W. K. Clifford, with such elementary picture-books as Deschanel and Ganot. I do not indeed share Prof. Tait's contempt for these “foreign” books; a student will find in them details, about (say) barometers or air-pumps, for which he may search the other works mentioned in vain. I did not urge students to read Thomson and Tait, because to those who can the advice is superfluous; to those who cannot it is disheartening. I did, and do, recommend such junior students as we get at provincial colleges to read easy works on Physics—not always because they contain a profound and satisfactory statement of principles, for how few of them do, but because they explain a multitude of details and experimental developments with which it was unwise to encumber a little book dealing mainly with vital principles, and aiming at being, in its humble way, an introduction to the classics of the science.