on the pines of northern Arizona and the results all tabulated, when it occurred to the writer to compare the annual growth of Flagstaff trees directly with the 8 or 10 years of rainfall records taken at the United States Weather Bureau station recently established there. It was immediately seen that the accuracy with which tree-growth as shown in the rings may represent annual rainfall was far greater than anticipated. In a considerable number of cases, but especially in the dry climate groups, this has been found to be in the neighborhood of 70 per cent, which is raised substantially by applying a formula to allow for some degree of moisture conservation. At the present time, therefore, it is possible to lay a foundation for this study directly in the fact that the rings of trees form an approximate measure of the rainfall.
When the studies were carried to northern Europe an equal exactness in following the rainfall was not found, but a direct correlation was discovered between tree-growth and solar activity. Subsequent groups have been obtained from moist regions of the United States, and one is led to believe that this altered reaction is a question of precipitation and that it must be kept well in mind in any application of the methods hereafter described.
Since the beginning of this investigation, in 1901, assistance has been received from several sources which it is a pleasure to acknowledge at this time. Mr. T. A. Riordian, of Flagstaff, had 24 sections of the early Flagstaff group cut from the ends of logs and shipped to me. Mr. Willard P. Steel assisted in the measuring of the first 25 sections and a number of friends helped in the tabulation. Mr. C. H. Hinderer, of the United States Forest Service, at Prescott, Arizona, assisted in procuring the Prescott groups. Mr. H. S. Graves, Chief of the United States Forest Service, gave me several letters of introduction to foresters in Europe, by which I was greatly assisted in procuring the 9 European groups. I am glad to express my obligation to Dr. H. H. Jelstrup of Christiania, Professor Gunnar Schotte of Stockholm, Professor Dr. A. Schwappach of Eberswalde, and Professor A. Cieslar of Vienna, for especial aid in this connection. Assistance in completing the Vermont group was given by Mr. M. H. Douglass and others, and for aid in procuring the Oregon group I am glad to mention the excellent work of Mr. Robert H. Weitknecht, who was for a time connected with the United States Forest Service at Portland, Oregon. I am indebted to Mr. George A. Hume, of the Sanger Lumber Company, for important help in connection with the sequoia groups. In 1914 a grant of $200 was received from the Elizabeth Thompson Science Fund for study upon the correlation between tree-growth and solar variation. In 1918 a fund of $260 was placed at my disposal by the American Association for the Advancement of Science. This was for the purpose of extending the sequoia ring-record from 2,200 years in length (the result of preceding collection) to 3,000 years. This material was collected in