338 Mr. G, H. Knibbs : Mathematical Analysis of 



Some of the deductions of Houdaille are very important ; 

 for example, that 



(a) Increase in rate of evaporation with air movement 



is very rapid for low velocities, but becomes 

 sensibly proportional to the wind velocity when 

 it attains 4 metres per second ; 



(b) The increase is independent of the difference 



between the vapour-tension at the water surface 

 and that of the air ; 



(c) Increase of evaporation due to solar radiation is 



sensibly proportional thereto ; 



(d) The utilization of solar heat varies greatly according 



to temperature, humidity, and intensity of in- 

 solation ; 



(e) Electric tension causes rapid increase of rate of 



evaporation *. 



Schierbeck in 1896, in his article u Sur la vitesse de 

 Fevaporation au point de vue special des relations physio- 

 logiques,'' f confirms Stefan's formula. He found that the 

 coefficient of evaporation was directly proportional to the 

 absolute temperature (see, however, reference to Marvin's 

 work hereinafter), and he gave an expression for the volume 

 of vapour passing through a cross-section of unit area in unit 

 time at a temperature of 0° C. and pressure 760 mm. 



More recently, after making a series of elaborate experi- 

 ments at Paris, Prof. F. H. Bigelow submitted a somewhat 

 elaborate formula in which there was a coefficient containing 

 thermodynamic constants; a term denoting saturation vapour- 

 pressure at the temperature of the water surface ; a term 

 denoting the vapour-pressure at the dew-point of the sur- 

 rounding air ; and a term depending on wind velocity : and 

 he says : — " We have shown that the following terms enter 

 vitally into an evaporation formula, viz., surface-water 

 temperature, dry-bulb and wet-bulb temperatures, and 

 velocity, and besides these there are apparently some un- 

 known terms concealed in the thermodynamic coefficient " J. 



In an article on a proposed new formula for evaporation § , 

 0. F. Marvin points out that " absolute evaporation/' viz., 



* Annates ecole not. agric. Montpellier, vi. pp. 197-247 (1892). 



t Overs. 7c. Banske Forhandl. i. p. 30 (1896). 



% Monthly Weather Review, 1908, p. 445. 



§ Monthly Weather Review, February 1909, pp. 57-61. 



