Bcsidts of Some Experiments upon tJie Bate of Evaporation. 53 



Hence the rate of evaporation increases while the temperature 

 rises, while the humidity ratio decreases, and while the wind increases 

 in velocity. The temperature numbers make no very symmetrical 

 curve, but the humidity numbers, within the observed limits, fall 

 pretty evenly upon the parabola 



(100 — H) -' :- 20,000 (E - e) 



where E is the total amount of evaporation from the tank when the 

 •observed mean humidity of the day is H ; '''■ e being the average 

 ^evaporation when the mean humidity of the day is between 80 and 

 100 per cent. That is, since e is not very great, the depth of w^ater 

 •evaporated w^ill vary approximately as the square of the number 

 representing the dryness of the air. Also the rate of evaporation and 

 the velocity of the' wind increase together, and in pretty much the 

 same ratio. This last, however, is quite an accidental result, and 

 ■owes its existence perhaps more to the fact that during the most humid 

 months of the year — i.e., April, May, and June — the wind is lightest, 

 whereas the air is dryest — in October, November, and December — 

 when the strongest winds occur. 



It w^ould appear that the station is too sheltered to allow the 

 wind to exert its maximum influence upon gauges on or near 

 the ground. Indeed, if we consider all the observed daily 

 amounts of evaporation occurring in a moderately dry air, say 

 for mean daily humidity ratios lying between 30 and 40 per cent., 

 during the year 1900, in all 75 occasions, we get 36 days with 

 sh wind- velocity below 140 miles per day, ! and a concomitant average 

 ■evaporation of '234 inch per day ; and 39 days wdth a wind-velocity 

 greater than 140 miles per day, and a concomitant average evapora- 

 tion of '239 inch per day. There w^ere, moreover, eleven of these 

 occasions when the wind-velocity w^as less than 100 miles per 

 day, and the average evaporation about one-fifth of an inch ; and 

 ■eleven other occasions when the wind-velocity was greater than 200 

 miles per day, and the average evaporation about one-quarter of an 

 inch per day. Thus the Kenilworth experiments upon evaporation 

 are not definite enough to reveal a wind factor anything like so 

 influential as might have been anticipated, or at any rate to isolate 

 it from amidst the variety of other contributing agencies. 



Returning again to the question of the dryness of the air, we get 

 the following average quantities of evaporation for assigned moisture 

 conditions, from the pan, the tub, and the Piche gauge respectively : — 



* Computed by the aid of the Greenwich factors. 



t As recorded by a Robinson anemometer whose cups are 45 feet above the 

 ground. 



