58 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Gila, and the Asuncion, whose two branches, the Altar and Magdalena, head 

 near the Santa Cruz and flow southwestward through northwestern Mexico 

 to the Gulf of Mexico. The subjects to which attention was chiefly devoted 

 were, first, the relation of the climate of the country (with its twofold rainy 

 season in winter and in summer) to the climatic zones of the earth as a 

 whole ; second, the length of time that arid conditions have endured and the 

 effect of prolonged aridity upon topography ; third, the effect of changes of 

 climate upon topography ; and fourth, the climatic history of the country 

 from the Glacial period to the present time. The last subject proved most 

 fruitful. It involved, among other matters, a study of such diverse subjects 

 as the nature of the climate of Arizona during the Glacial period ; the action 

 of floods and the origin of the alluvial terraces found along all the rivers ; 

 the traces of a highly civilized and unexpectedly dense pre-Indian population, 

 not only in the river valleys now inhabited, but in scores of places now unin- 

 habitable; and the economic condition of the Indian before the coming of 

 the white man. 



At the end of the field season a general view of southern Arizona was 

 obtained by a ride of 370 miles from Tucson westward to Yuma by way of 

 Phoenix. This was followed by a few days devoted to a study of the pecu- 

 liar features of the Salton Sea, especially its abandoned strands, both old and 

 new. Throughout the entire season it was found that, although the deserts 

 of southern Arizona differ markedly from those of most parts of Asia in 

 certain botanical features, such as the occurrence of cacti and the surprisingly 

 large number of trees and bushes in proportion to the rainfall, but there is 

 remarkable agreement as to the topographic features and as to evidences of 

 changes of climate. 



Evaporation and Soil Moisture (by Dr. B. E. Livingston) : 



The study of evaporation and that of soil moisture has been continued. 

 Several improvements have been made in the manipulation of the porous 

 cup-atmometer, and an entirely new arrangement has been perfected whereby 

 the error due to entrance of rain into the instrument is avoided. In the rela- 

 tion of plant activity to soil-moisture, several principles have been developed. 



Dr. W. H. Brown has been employed as assistant in this work for the 

 summer of 1910. 



The problem of the physics of plant transpiration has been somewhat 

 further simplified by experimental analysis, and considerable has been accom- 

 plished on the relation between this function and the intensity of sunlight. 

 In this connection, a new integrating heliometer has been devised, which 

 promises to be as valuable in studies on the relation of transpiration to solar 

 radiation as has been the atmometer in those on the relation of transpiration 

 to the evaporating power of the air. 



The studies of Professor Livingston and Dr. Shreve, on the relations be- 

 tween the climatological factors and plant distribution in the United States, 

 which have been in progress for several years, are now nearing completion 

 and will soon be ready for publication. 



