10 INTRODUCTION. 
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 antici- 
pated. 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, there- 
fore, 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 pre- 
cipitation 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. 8. 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 $250 was placed at my disposal by the American Association 
for the Advancement of Science. This was for the purpose of extend- 
ing the sequoia ring-record from 2,200 years in length (the result of 
preceding collection) to 3,000 years. This material was collected in 
