Evaporation from a Circular Water Surface. 309 



surfaces under " every -day " conditions — a point which 

 so far as we know, has received very little notice. 



That aspect of the question which is of botanical interest 

 we have already discussed* ; here we confine ourselves to a 

 treatment of the matter purely from the physical side. 



Apart from the tendency to assume a priori that the 

 evaporation from a liquid surface is proportional to the 

 area exposed, it is probable that the definite statement of 

 Pouillet f bad great influence in fixing ideas on the matter ; 

 and it was not till 1881 that Stefan J, on theoretical grounds, 

 advanced the true law of evaporation. A comparison of the 

 equations of diffusion with those of electrostatics shows that 

 the amount of evaporation per unit time from a circular 

 surface of radius a is given by 



V-4falog.|=&, 

 r— pi 



where k denotes the coefficient of diffusion, P the atmo- 

 spheric pressure, and p x and p the pressure of the vapour 

 at the surface and very far away from it respectively. 

 If p and p v be small with respect to P this becomes § 



P 



Stefan further shows, by an extension of the argument to 

 elliptical surfaces, that u in einem ziemlich weiten Inter- 

 vals die Capacitat einer elliptischen Platte von jener einer 

 gleich grossen kreisformigen nur wenig verschieden ist." 



He also obtains expressions for the evaporation from a 

 definite section of the liquid surface, and gives an approxi- 

 mation which shows how the evaporation is affected by the 



* « Annals of Botany/ April 1917, p. 241. 



t "Elements de physique exp^riiuentale et de meteorologie, 1837" 

 (abstracted in Mrs. Livingston's bibliography, p. 25). 

 t Wied. Ann. xyii. p. 550 (1882). 

 § This equation is misquoted as 



in Preston's ' Heat ' (p. 291, 1894 edition) and in Brown and Escombe's 

 paper " On the Static Diffusion of Gases and Liquids .... in Plants " 

 (Phil. Trans., B, 1900, p. 251). In the revised edition of Preston 

 (1904, note p. 357) the equation is corrected, but another error remains 

 to which reference will be made later. 



