A sad commentary on the low health-ideals which now exist is that to most people the expression "_to keep well_" means no more than _to keep out of a sick-bed_. Hitherto, the subject-matter of hygiene has been considered in its relation to disease rather than to health. In this manual, on the other hand, it is treated in its relation to (1) the preservation of health; (2) the improvement in the physical condition of the individual, and (3) the increase of his vitality. In short, the objects of the manual are positive rather than negative. It aims to include every practical procedure that, according to the present state of our knowledge, an athlete needs in order to make himself superbly "fit," or that a mental worker needs in order to keep his wits sharpened to a razor-edge. For this reason some suggestions, which might otherwise be regarded as of minor importance, have been included and emphasized. While it is true that a moderate infraction of some of the minor rules of health is not inconsistent with maintaining good health in the sense of keeping out of a sick-bed, such infraction, be it ever so moderate, is utterly inconsistent with good health in the sense of attaining the highest physical and mental efficiency and power. Future advances of knowledge will doubtless occasion additions to, or modifications of, the conclusions stated herein, and these will form the subject of subsequent publications by the Institute. In order that the Institute may have at its disposal the latest and most authoritative results of scientific investigations, its Hygiene Reference Board was created. The present book is the first general statement of the conclusions of this Board after a year of careful consideration. These conclusions are the joint product of the members of the Board, with the active co-operation of the Director of Hygiene of the Institute. They may fairly be said to constitute the most authoritative epitome thus far available in the great, but hitherto neglected, realm of individual hygiene. The Chairman of the Board has exercised the function of editor, and is responsible for the order and arrangement of the material. Friends of the Institute may help its work by spreading the ideas given in the following pages and by increasing the number of its readers. Such profits as may be received by the Institute from the sale of this book will be devoted to further philanthropic effort by the Institute. IRVING FISHER, EUGENE L. FISK. New York, Sept., 1915. CONTENTS PAGE INTRODUCTION 1 CHAPTER I AIR