some Experiments in Climatological Physiology. 339 



the number of molecules passing out of the water, varies 

 as T/B, where T is the absolute temperature and B is the 

 barometric pressure. But the effective or apparent evapora- 

 tion is the " absolute evaporation " diminished by the number 

 of molecules returning to the water. The higher the tem- 

 perature of the air, the greater its capacity to receive and 

 disseminate the moisture, and this is greatly facilitated by 

 wind. Bigelow remarks that the rate of evaporation 

 dE/dt may be discussed under two forms, viz., 



(i.) dE/dt varies as (T,.T a /T rf )g, 



and (ii.) dE/dt varies as (T, + T a -Trf)^, 



in which T 5 is the temperature of the water-surface, T a that 

 of the air, T^ that of the dew-point, measuring therefore 

 the actual quantity of aqueous vapour ; V is a measure o£ 

 wind velocity and B of the barometric pressure. After 

 showing how the Bigelow and Dalton equations are obtained 

 from (i.) by neglecting certain elements among the conditions, 

 Marvin submits the formula 



in which Be/Bs is merely the tabular difference in the 

 ordinary table of saturated vapour-pressures for successive 

 temperatures, representing thus the rate of change with 

 temperature corresponding to the temperature of the water- 

 surface ; e, is the saturation vapour-pressure corresponding 

 to T„ and e& is the saturation vapour-pressure corresponding 

 to Trf. G/ed therefore varies with wind-velocity and dryness. 

 Marvin remarks that " evaporation by pure diffusion takes 

 place only in perfectly quiescent air, and we know the 

 process is a very slow one." Hence he concludes that in all 

 ordinary cases the wind velocity must be taken into account, 

 in all open air conditions, and indeed generally *. 



7. Nature of Formula for Wind Effect. 



In order to deduce evaporation results when the air is in 

 movement, it will probably be found necessary to take it into 

 account in two different ways. Thus, for example, wind, or 



* An extensive bibliography of treatises on Evaporation by Mrs. Grace 

 J. Livingston will be found in the Monthly Weather Review, 1908, 

 pp. 184-186, 301-306, 375-381 ; and 1909, pp. 68-72, 103-109, 157-160, 



