We wish to express our sincere appreciation to all who have helped in the making of this book; to Oscar C. Gallagher, Head Master of the West Roxbury High School, Boston, and George W. Hunter, Head of Biology Department, DeWitt Clinton High School, New York, who encouraged us to undertake the work; to Dr. Thomas H. Briggs, of Columbia University, who as editor has carefully read the proof and offered valuable suggestions; to Leonard 0. Packard, Head of Geography Department:, Boston Normal School, Clarence 11. Jones, Submaster, Martin (Model) School, Boston, Captain George T. Palmer, U.S.S.C., and Bertha B. Bryant, Principal, Wytnan School, Woburn, Mass., who have read and criticized portions of the manuscript; to Miriam S. Draper, Librarian of the Children's Museum, New York, who rendered able assistance especially in connection with selecting reference books; to Edna Van Buskirk, who has rendered invaluable assistance throughout; and to Hanson Hart Webster, who by his energy and consTable of Contents Part I Tue Chief Necessities of Life Introduction; UNIT I THE AIR AND HOW WE USE IT; PROJECT I THK AIR A REAL SUBSTANCE; Introduction3; Air is real; Problems 4; Topics:; Three forms of matter, 8 The atmosphere an ocean of air, 9 Air pressure, 10 Making a mercurial barometer, 10 The barometer used to indicate height, 10 The aneroid barometer, It The barometer used to forecast weather, 12 Air pressure and the suction pump, 12 The force pump, 12 Air pressure and the exhaust pump, 12 The bicycle pump, 13 Air pressure and the action of the siphon, 13 Pressure on the human body, 13 Structure of the human ear, 14 Air transmits sound, 15- The nature of sound, 16 The rate at which sound travels, 16 Compressed air, 17 Liquid air, 17 Balloons, iS Airplanes, 19; Individual Projects20; PROJECT II AIR AND FIRE; Introduction 21; Fire in its relation to our lives; Problems 22; Topics:; A fire needs air, 26 Composition of the a