given, if desired, in the second year of the high school, and the problems, the real heart of any course, have been especially chosen and especially tested for such use. For schools that have more time and can do the work more thoroughly there is provided the course represented by both the large print and the smaller print, with an abundance of additional problems in the Appendix. In addition to these two systematic courses, and as an aid to both of them, the pedagogical expedient, already successfully tried in a much less complete way, has been adopted of presenting an incidental picture course on the history of physics both as to outstanding industrial appliances and as to the "heroes of physics" whose work has made these possible and whose lives lend to the whole subject intense human interest. This consists of ninety-nine full-page illustrations with rather elaborate explanatory legends. No attempt has been made to make these illustrations an organic part of the systematic course presented in the remaining pages. They are introduced rather for the sake of arresting the student's attention, stimulating his interest, and getting before him incidentally a large number of fascinating facts and developments, which he will often not understand fully, but which will nevertheless stimulate him to further studies, and give him problems of his own to work upon. The pupil who is capable of doing more and understanding better than the average run of a class should profit greatly by this feature. A second advantage that offers arises from the fact that almost every teacher has some practical industrial development in which he is especially interested, and which, often for local reasons, he wishes to incorporate into his course, sometimes even when it is beyond the proper scope of an elementary course organized for the average pupil. Such a teacher can incorporate as many of the full-page inserts into his own systematic course as he wishes. Another new pedagogical expedient for us is the introduction of brief, telling summaries at frequent intervals for the sake of giving the pupil condensed reviews as to just what the discussion has been about and what are the important results. These have been done with very great care and should be an important aid in the simplification of the subject, and in rendering it coherent and intelligible. A further step in the same direction has been taken in the elimination of some problems and some subjects which experience has shown to be rather beyond the abilities of the average high-school pupil of today. Acknowledgments are due literally to thousands of teachers who have assisted us by trying out in the school of experience not merely our methods but our actual mode of presentation. We have had especial assistance of this sort from Edward E. Ford, West High School, Rochester, New York; Allan Peterson, East High School, Des Moines, Iowa; Willis C. Campbell, Gorton High School, Yonkers, New York; Walter L. Barnum, Evanston High School, Evanston, Illinois; E. Waite Elder, East High School, Denver, Colorado, who has in addition read all the proof; Charles F. Dutton, West High School of Commerce, Cleveland, Ohio; Dr. Hugh Dunn, Bell Telephone Laboratories, who has given us invaluable assistance on the radio sections; and E. A. Merrill of the California Institute of Technology, who has assisted in developing a new and very simple presentation of the principles of the airplane. R. A. M. H. G. G. W. R. P. Definition and Measurement of Force · Composition and Resolution Molecular Forces in Solids; Elasticity Molecular Forces in Liquids; Definition and Measurement of Work Work and the Pulley Work Fusion Evaporation and the Properties of Vapors Hygrometry · Detection of Electric Currents Chemical Effects Magnetic Ef- . Cells Secondary Cells Heating Effects Speed and Nature Reflection, Reënforcement, and Interference Transmission of Light Illumination and Photometry The Images formed by Lenses Images in Mirrors Optical Instru- Radiation from a Hot Body Electrical Radiations Cathode and |