266 LIVINGSTON— CLIMATIC AREAS [April i8, 



of atmometer be generally adopted, and many weather services are 

 at present furnishing data upon evaporation as well as upon the 

 other climatic factors more commonly recorded. On account of 

 various difficulties arising from the use of a free water surface for 

 measuring evaporation, the most valuable instruments now available 

 determine the evaporation rates from the surface of an imbibed 

 solid, such as bibulous paper or porous porcelain. For plant ecology 

 the porous cup atmometer^ appears to be the most satisfactory of 

 these instruments, and it seems to be rapidly rising in the esteem 

 of agriculturists and others who are interested in this line of study. 

 This instrument has the advantage, for our purpose, that its evapo- 

 rating surface is so exposed as to be fairly comparable to the evapo- 

 rating surfaces of plants. 



The only systematic information which the United States Weather 

 Bureau has furnished upon the geographical distribution of evapo- 

 ration intensities is comprised in the report of Russell's" studies. 

 This author employed Piche atmometers at nineteen stations and 

 derived a formula from the results thus obtained, by which the 

 monthly evaporation rates for many other stations were derived. 

 His operations extended over a single year, from July, 1887, to 

 June, 1888, and a very valuable chart of evaporation in the United 

 States resulted therefrom. 



During the summers of 1907 and 1908 I carried out a compara- 

 tive study of evaporation intensities throughout the United States, 

 under the auspices of the Department of Botanical Research of the 

 Carnegie Institution, using the standardized porous cup atmometer. 



* On the porous clay atmometer, see : 



Babinet, T., " Note sur un atmidoscope," Coiiipf. Rend., 27 : 529-30, 1848. 

 Marie-Davy, H., " Atmidometre a vase poreaux de Babinet," Nouf. Met., 

 2: 253-4, 1869; Mitscherlich, Alfred, " Ein Verdunstungsmesser," Landw. 

 Vcrsiichsstat.. 60: 63-72, 1904; also 61: 320, IQ04 ; Livingston, B. E., "The 

 Relation of Desert Plants to Soil Moisture and to Evaporation," Carnegie 

 Inst. Wash. Publ. 50, Washington, 1906; "A Simple Atmometer," Science, 

 N. S., 28: 319-20, 1908; "A Rotating Table for Standardizing Porous Cup 

 Atmometers," Plant World, 15: 157-62, 1912; also other literature there 

 referred to. 



' Russell, Thomas, " Depth of Evaporation in the United States," .l/o. 

 Weather Rev., 16: 235-9, 1888. 



