by a large number of examples drawn from common experiences. It is hoped that the student will in this way learn to connect the principles of physics with those of daily life. There are several well-known logical methods of introducing the fundamental concepts of mechanics. In an elementary text it seems desirable to make a choice largely on pedagogical grounds. The weight of bodies, with which we are all familiar, has been chosen as a measure of force. This choice has made it possible to begin with statics and its application to liquids and gases, and to postpone the more difficult subject of dynamics. The author has not hesitated to use such terms as momentum, voltage, and the like before accurate definitions are given. For example, the term mass, as something different from weight, is introduced before any attempt is made to explain the difference. The student is encouraged to think about that difference and hence to be better prepared when the explanation is finally reached. While this may sometimes appear to give a lack of rigor, yet the advantages are obvious to an experienced teacher. Certainly the student cannot understand definitions or generalizations when he is not familiar with specific facts. A chapter on meteorology has been introduced, for two reasons : (1) a knowledge of some of the facts and principles is of practical importance; and (2) there are in this subject some first-class examples of the applications of the principles of physics to phenomena which are within the understanding and observation of all. In the early portions of the text the more familiar units of the British Engineering system are used. Gradually the units of the international scientific system, the centimeter-gram-second system, are introduced; later they are used almost exclusively. The British Absolute system of units, which is rarely found outside of textbooks, is not used. An attempt has been made to state laws and equations in immediately useful forms. Formulas giving numerical relations between different units are repeatedly introduced in order that the solutions of problems may not be largely exercises in arithmetic. The ideas and facts of modern physics are freely included; for example, modern notions of the atom and the electron are used wherever helpful. However, some of the more perplexing ideas (such as those of the quantum theory), which properly belong to more advanced texts, are omitted. The division of the text into a large number of chapters and sections makes its use easier for the instructor who, on account of limited time, is forced to omit portions of the subject matter. The rather large number of divisions has made it possible to segregate some of the more difficult parts of the subject. For example, part of the subject of electrostatics has been reserved for a chapter which is reached only after the student has become more familiar with the concept of potential. Summaries are given at the end of those chapters where they will be of the greatest service. It is suggested that the student prepare for his own use similar summaries in those cases where they have been omitted. Answers to only the more difficult problems are given (in an appendix). It is hoped that this plan will tend to prevent the abuse of answers and yet meet the needs of those students who use them properly. It is expected that this text will be supplemented by suitable demonstration and laboratory experiments. Nothing can take the place of classroom drill as conducted by a good teacher. It is impossible for the author to give proper credit to all those to whom he is indebted for many of the ideas and methods that have been used. Much help has been received during the actual preparation of the text. The author is under special obligations to Professors H. B. Lemon, of the University of Chicago, and R. A. Millikan, of the California Institute of Technology, who read the manuscript; to Professors S. R. Williams, of Oberlin College, F. R. Watson, of the University of Illinois, and G. W. Stewart, of the University of Iowa, who read the proofs; and to his colleagues Professor H. M. Reese, who read much of the manuscript, and Messrs. H. E. Hammond and R. T. Dufford, who read all the manuscript. O. M. STEWART |