PREFACE. THIS TREATISE is little more than the substance of a set of Lectures given by me in the summer of 1872, at the request of the Head Master, to the 6th Form Boys of the Surrey County School. My aim was to lay before them-lads of from sixteen to eighteen, soon about to quit school for active life—not a popular account of recent discoveries or theories, such as abound in periodicals and books for the young, but such an exhibition of the principles and methods of modern science, and of some results obtained therefrom, as might show them the use of the mathematical groundwork in which they had been carefully instructed, and perhaps induce some of them to pursue their own scientific education after leaving. Finding afterwards that the subject was to be made. part of the annual examination in 1873, I made up my notes, that the Examiner might see what the course had been. He thought they might be usefully worked up into a fuller treatise: I took advice from other competent judges; and on the whole thought the experiment worth trying. Since that time various works have appeared in English, going over much of the same ground. But I |