A Study of Evaporation' and Plaxt Distribution'. 207 



The only series of observations on the intensity of evapor- 

 ation in the United States,* which has been heretofore avail 

 able, and that used by Transeau. is that made by T. Russell f for 

 the period July, 1887, tn June, 1888, inclusive. These are not 

 direct observations, but deductions from readings of the wet 

 and dry bulb thermometers and the barometer, and aie of 

 uncertain accuracy on account of the incom] leteness of the 

 fornnila by which they were computed. It was theiefore 

 deemed worth while to attempt a series of direct evaporation 

 observations over the United States throughout the summer 

 months and to compare the results with the general vegetation- 

 al characters of the various parts of the countrv. The measure 

 U'ed was the rate of water loss from the porous cup atmo- 

 meter, which is affected by external conditions in much the same 

 manner as aie ordinary plants, and which automatically 

 sums the records, so that readings may be taken at any 

 suitable intervals of time. This work was carried out---' under 

 the auspices of the Department of Botanical Research of the 

 Carnegie Institution. 



The plan was first put into operation in the spring of 1907. 

 Porous cups were sent to twenty-four stations, fairly well dis- 

 tributed over the United States, and were operated, with 

 weekly readings, for a period extending from May to September. 

 The cups were placed in the open, with free access of sunshine 

 and of wind. The center of the cup was fifteen centimeters 

 above the soil surface, so that the records may be considered 

 as approximately proportional to the evaporating power of 

 the air as this would affect the transpiration of a plant of about 

 the above named height, giowing in the open. ft The details 



♦Perhaps the best series of obsen-ations on the intensity of evaporation which has so 

 far been recorded is that of T Okada, Evaporation in Japan. Bui. Cent. Met.Obs.. Japan. 

 1904. No 131. A general review of the literature of evaporation of throughout the world 

 has been published by Grace J. Livingston, An Annotated Bibliography of Evaporation. 

 Monthly Weather Review for June. Sept.. and N'ov , 190S. and Feb.. March. April. May and 

 June. 1909. The student of this subject will find many suggestions and much of general 

 interest in the recent publication by Briggs.L.J .and Belz. J. O., Dry farming in relation to 

 rainfall and evaporation. Bulletin 188. Bur Plant Ind . U. S. Dept. Agric. pp. 71. 1911 



tRussell, T., Depth of evaporation in the United States. Mo. Weather Rev . 16: 

 235-9. 1888. 



* *The instrument was first devised in its essentials by Babinet (Compt. rend 27: 

 529-30. 1848). It was later independently devised by A Mitscher.i.-h il.andw Ver- 

 suchsstat. 60 63. and 61: 32'.1 1904) and by the writer (Publ 50. Cirnegie Institution. 

 1906). The literature of this form of atmometer is presented in Plant World 13; 111. 

 1910. On a modification of the instrument, see Transeau. E. X.. A Simple Vaporimeter. 

 Bot. Gaz. 49: 459-60. 1910. 



t tAs has been later pointed out (Bot. Gaz. 191 1). evaporation from the white clay cup is 

 not influenced so much by sunshine as is transpiration from green leaves. Thus the results 

 here reported do not adetjuatcly include the influence exerted by sunshine. But. since 

 the regions of highest evaporation intensity are usually those of greatest solar illumination, 

 our records may probably be considered as showing at least the correct order of magni- 

 tudes, even if the influence of sunshine be included. 



