414 REPORT OF THE COMMISSIONERS OF 



air; W, the velocity of the wind in miles per hour, and E, the evaporation in inches 

 of depth per hour. It can be shown that there is going on nearly always a condensa- 

 tion of moisture from the air, upon any water surface. At the same time there is 

 going on a loss of moisture from the water to the air. The intensity of both these 

 operations depends upon the difference in temperature between air and water. When 

 the temperatures of air and water in contact are the same, both processes stop. 

 Evaporation is, therefore, the measure of the difference of these two exchanges. The 

 experiments of Mr. Fitz Gerald and others have shown that evaporation from water sur- 

 faces is subject to a definite law, expressed by the formula just given, but evaporation from 

 the ground has never been reduced to any such simple expression. Various difficulties 

 arise which apparently render it impossible to make a single expression covering all 

 the phenomena involved. If the surface of the ground be kept constantly wet, 

 evaporation therefrom goes on substantially as from water surfaces. The main 

 difficulty, therefore, in reducing evaporation from the ground to a simple formula, is 

 largely due to uncertainty of the water supply. The demands of evaporation from 

 the surface of the ground are continuous, the same as from other surfaces, but 

 constant interruptions by either complete or partial exhaustion of the available supply, 

 complicate the action so much as to render expression by formula apparently 

 impossible. Varying demands of vegetation at different seasons also further 

 complicate the problem. 



EVAPORATION DATA. 



Evaporation data applying to water surfaces at Boston may be found in Mr. Fitz 

 Gerald's paper already referred to. For evaporation from the land surfaces of several 

 catchment areas in the United States, refer to the author's paper On the Water 

 Resources of the State of New York, No. 24 of the Water Supply and Irrigation 

 papers of the United States Geological Survey; and for evaporation data at Rochester, 

 refer to the Annual Reports of the Executive Board of the City of Rochester, 1891-98, 

 inclusive; also to the author's Report on the Genesee River Storage Surveys, and to 

 the paper on the Water Resources of the State of New York. 



For evaporation data abroad, at a number of foreign points, refer to Beardmore's 

 Manual of Hydrology and to Durand-Claye's Hydraulique Agricole et Genie Rural, 

 page 257, where evaporation data applying to Paris, Turin and Lake Fucino, may be 

 found. Tables showing the hourly variation of evaporation may be found in Mr. 

 Fitz Gerald's paper, and in Durand-Claye's. A number of tables of evaporation at 

 foreign points are given in the author's Report on the Upper Hudson Storage 

 Surveys for 1896. 



