270 



of the garden. 



Until June, 1907, the meteorological records at the Garden 

 include only the dates and amounts of precipitation, and the 

 temperature of the air and that of the soil at two depths. The 

 amount of precipitation, however, is not an index of the amount 

 of water available to vegetation. Part of the meteoric water 

 drains away through the soil before it is used, while a portion of 

 it evaporates from the surface of the soil into the air. It is the 



approac 



either 



tthe 







given a 



lown fact that the rate of evaporation 

 ds upon the relative humidity of the sui 

 ing air. Relative humidity, in turn, varies with the temp 

 of the air, and with the environment. Thus, for a given 1 



vary with the area of the surface and with the depth of th< 



d the 



mpora 



substan 



fied by the nature of the substance, and with the amount 

 moisture it contains. Thus, for example, water will evapot 

 more rapidly from one square foot of water-surface than fr 



depth of, say, one quarter of an inch, than it will from 

 same area over a depth of one foot. Also the same amount 

 water will evapo, 



Shru 



e the 



ind foliage 

 itive humidity of the 



•, thus 



inge 



The experiments described in this paper form part of a more 

 extended investigation, inaugurated by Dr. Burton E. Livingston, 

 of the Desert Botanical Laboratory, of the Carnegie Institution, 

 at Tucson, Arizona. Evaporimeters of uniform pattern, and 

 standardized, have been distributed to some twenty-seven stations 

 in the United States, ranging from Orono, Maine, on the east, to 

 California, on the west, and from Bozeman, Montana, on the 



