This series has been prepared for use in the junior-high-school grades. The authors have been guided in this work by the recommendations set forth in the report "A Program for Teaching Science," in the Thirty-first Yearbook of the National Society for the Study of Education (1932), Part I. In this report of the National Society for the Study of Education there is a list of principles and generalizations which, taken collectively, define the major contributions of science to human welfare and to human interests. This list, with some modifications, has been used for guidance in the selection of instructional material for these books. Like the two preceding volumes, this book is organized into relatively few teaching units. These are divided into conveniently arranged chapters. Each unit develops understanding of some large feature in the environment, over which man is exerting some measure of control. The subject matter of a unit has been brought together because it belongs together for the development of an understanding of the means whereby this control has been attained. At the end of each chapter there are many carefully prepared aids to learning, which take the form of direct experiences. These direct and vicarious experiences supplement one another in developing for the learner an enlargement of understanding of important principles and generalizations, and serve to develop an ability to use scientific methods of work.