IT is necessary, in the case of a book planned in such an unfamiliar form as this, that a somewhat full explanation of the principles on which it has been written should be offered to the teacher or student into whose hands it may fall. The business of the teacher of "First-year Physics "seems to me to comprise three things. The first and chief of these, without which, indeed, the others are hardly worth while, is to inspire in the student a keenness for careful and scientific thought. Next in importance is instruction in the actual subject-matter of Physics. And the last is the teaching of matter useful for the subsequent career of the student today, especially, this cannot be entirely ignored.