Evaporation from a Circular Water Surface, 311 



was a function of their position, being greater for those 

 nearer the periphery than for those in the neighbourhood of 

 the centre of the dish. This is in accordance with Stefan's 

 theory, but the quantitative agreement is by no means exact, 

 a result which is rather to be explained by non-fulfilment 

 of the theoretical conditions than by any deficiency in the 

 theory. 



Later 3 v. Pallich * instituted experiments with a similar 

 object and obtained similar results, finding, for example, 

 that the curves of equal vapour-pressure (which, according 

 to Stefan, are ellipses) are indeed approximately elliptical, 

 but have an eccentricity of about twice the value given by 

 the theory. 



In a lengthy monograph on the physics of transpiration 

 phenomena, Renner t has given the results of some experi- 

 ments on the evaporation from free water surfaces. He 

 assumes that the rate of loss of vapour in grams per second 

 from such a surface is given by 



M = 4:kpa = Ka (where K = 4fy>), 



where k is the coefficient of diffusion of water vapour into 

 air, a the radius of the surface, and p a quantity to which he 

 gives the name of " potential difference/' and defines as the 

 difference between the weight of a c.c. of vapour saturated 

 at the given temperature and the weight of a c.c. of vapour 

 in the surrounding atmosphere. After finding experi- 

 mentally the rates of loss from surfaces of different radii, 

 he proceeds to compare the observed values with those given 

 by the equations 



M = Ka, M = K.7ra, and M = K.7ra 2 , 



finding that the formulae applicable are functions of the 

 radius of the surface. Why the equation M = K . ira should 

 have been employed is not very clear, as it merely involves 

 a substitution of the semi-circumference for the radius ; 

 possibly it is intended as a tribute to the occult qualities 

 which are sometimes associated with 7r, but in any case 

 such comparisons have no important physical significance — 

 results deduced therefrom are merely expressions of the not 

 very recondite mathematical fact that two quite unrelated 

 curves may approximately coincide over a portion of their 

 lengths. 



* Berl. Ahad. Sitzber. 106. p. 384 (1897), and Sci. Abs. i. p. 203 

 (1898). 

 t Flora, 100. p. 474 (1910). 



