The appendix contains a copy of the working plan followed by the U.S. Forest Service in the extensive investigations covering the mechanical properties of the woods grown in the United States. It contains many valuable suggestions for the independent investigator. In addition four tables of strength values for structural timbers, both green and air-seasoned, are included. The relation of the stresses developed in different structural forms to those developed in the small clear specimens is given. In the bibliography attempt was made to list all of the important publications and articles on the mechanical properties of wood, and timber testing. While admittedly incomplete, it should prove of assistance to the student who desires a fuller knowledge of the subject than is presented here. The writer is indebted to the U.S. Forest Service for nearly all of his tables and photographs as well as many of the data upon which the book is based, since only the Government is able to conduct the extensive investigations essential to a thorough understanding of the subject. More than eighty thousand tests have been made at the Madison laboratory alone, and the work is far from completion. The writer also acknowledges his indebtedness to Mr. Emanuel Fritz, M.E., M.F., for many helpful suggestions in the preparation of Part I; and especially to Mr. Harry Donald Tiemann, M.E., M.F., engineer in charge of Timber Physics at the Government Forest Products Laboratory, Madison, Wisconsin, for careful revision of the entire manuscript. SAMUEL J. RECORD. YALE FOREST SCHOOL, _July 1, 1914_. CONTENTS PREFACE PART I THE MECHANICAL PROPERTIES OF WOOD Introduction Fundamental considerations and definitions Tensile strength Compressive or crushing strength Shearing strength Transverse or bending strength: Beams Toughness: Torsion Hardness Cleavability PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD Introduction Rate of growth Heartwood and sapwood Weight, density, and specific gravity Color Cross grain Knots Frost splits Shakes, galls, pitch pockets Insect injuries Marine wood-borer injuries Fungous injuries Parasitic plant injuries Locality of growth Season of cutting Water content Temperature Preservatives PART III TIMBER TESTING