HYDROLOGY OF NEW YORK 



137 



in the report of the Chief of the Weather Bureau the average yearly 

 velocity of wind is found to vary from about 3 miles to 16 or 18 

 miles. With other conditions the same, evaporation will be much 

 larger with a higher wind velocity. 



The preceding summary of evaporation relations further shows 

 that evaporation will vary in some degree in proportion to 

 pressure, temperature, moisture — which may be taken to include 

 dewpoint, relative humidity, vapor pressure, precipitation, and 

 cloudiness — and, finally, in proportion to average velocity of the 

 wind. It may also be expected to vary in some degree in propor- 

 tion to electrical phenomena — thunderstorms, auroras, etc. — but 

 as yet we know so little about these that they can be no more 

 than mentioned. The writer, however, believes that studies in the 

 direction here indicated would be very prolific of results. For 

 this purpose two or three stations, observing all the elements 

 herein enumerated, should be established in each catchment area. 



In the present study an attempt has been made to correlate 

 these elements with the runoff, but, aside from the rainfall, the 

 data are too indefinite for satisfactory results. It is for these 

 reasons, with others, that the writer is able to give only tentative 

 conclusions in regard to the relation of rainfall to the runoff of 

 streams. 



Persistency of evaporation. The persistency of the amount of 

 evaporation for any given stream at about the same figure through 

 long periods of time was first pointed out by Messrs. Lawes, Gil- 

 bert, and Warrington in their classical paper On the Amount and 

 Composition of Rain and Drainage Waters Collected at Rothamp- 

 sted, published in the Journal of the Royal Agricultural Society 

 of England for 1881. As to why evaporation exhibits such per- 

 sistency these distinguished authors consider it largely due to the 

 fact that the two principal conditions which determine large 

 evaporation — namely, excessive heat and abundant rain — very 

 rarely occur together. The result is, specially in the English 

 climate, a balance of conditions unfavorable to large evaporation. 

 In a wet season, when' the soil is kept well supplied with water, 



