Outside the Yale Faculty we have received equally cordial assistance. Professor T. C. Chamberlin of the University of Chicago, to whom, with his permission, we take great pleasure in dedicating this volume, has read the entire proof and has made many helpful suggestions. We cannot speak too warmly of our appreciation not only of this, but of the way his work has served for years as an inspiration in the preliminary work of gathering data for this volume. Professor Harlow Shapley of Harvard University has contributed materially to the chapter on the sun and its journey through space; Professor Andrew E. Douglass of the University of Arizona has put at our disposal some of his unpublished results; Professors S. B. Woodworth and Reginald A. Daly, and Mr. Robert W. Sayles of Harvard, and Professor Henry F. Reid of Johns Hopkins have suggested new facts and sources of information; Professor E. R. Cumings of Indiana University has critically read the entire proof; conversations with Professor John P. Buwalda of the University of California while he was teaching at Yale make him another real contributor; and Mr. Wayland Williams has contributed the interesting quotation from Bacon on page x of this book. Miss Edith S. Russell has taken great pains in preparing the manuscript and in suggesting many changes that make for clearness. Many others have also helped, but it is impossible to make due acknowledgment because such contributions have become so thoroughly a part of the mental background of the book that their source is no longer distinct in the minds of the authors. The division of labor between the two authors has not followed any set rules. Both have had a hand in all parts of the book. The main draft of Chapters VII, VIII, IX, XI, and XIII was written by the junior author; his contributions are also especially numerous in Chapters X and XV; the rest of the book was written originally by the senior author. CONTENTS PAGE I. The Uniformity of Climate 1 II. The Variability of Climate 16 III. Hypotheses of Climatic Change 33 IV. The Solar Cyclonic Hypothesis 51 V. The Climate of History 64 VI. The Climatic Stress of the Fourteenth Century 98 VII. Glaciation According to the Solar Cyclonic Hypothesis 110 VIII. Some Problems of Glacial Periods 130 IX. The Origin of Loess 155 X. Causes of Mild Geological Climates 166 XI. Terrestrial Causes of Climatic Changes 188 XII. Post-Glacial Crustal Movements and Climatic Changes 215 XIII. The Changing Composition of Oceans and Atmosphere 223