In these days of numerous text-books the author who dares still further to increase the number owes to the public, at least, an explanation in which he shall set forth his purpose, however far short of accomplishment he may have fallen. Accordingly, in this volume, the author's hopes have been: - (i) To prepare a text-book adapted to the needs of first-year students in physics at Northwestern University. One fear in this connection is that the transfer of a course of lectures from flexible manuscript to rigid type metal may correspond to that stage of development which, in the life history of an animal, is known as rigor mortis. This difficulty, it is hoped, may be avoided by varying, from year to year, the illustrative phenomena employed in the lecture demonstrations. (ii) To keep the treatment elementary, and yet include all the fundamental principles of physics; and at the same time to bind them together with "connective tissue" in such a way as to make clear to the student the essential unity of the subject. (iii) Not merely, or even mainly, to impart information, but to set before the student a large and compact body of truth obtained by a method which shall remain for him, throughout life, a pattern and norm of clear and correct thinking. To succeed completely in an undertaking so ambitious as that suggested by the threefold purpose just stated is more than the author dares to hope. To my friend, Mr. John Mackenzie of Minneapolis, I am indebted for those paragraphs in Chapter III which describe Professor Osborne Reynold's remarkable theory of gravitation. The imperfections of the present text would have been many more except for the clever revision of the proof by my colleague, Professor R. R. Tatnall, and by my friend, Professor A. A. Knowlton of the Armour Institute.