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XXXI. On Evaporation from a Circular Water Surface. 

 By Nesta Thomas, B.Sc, Assistant-Lecturer .in Botany 

 in the Royal Holloway College, and Allan Ferguson. 

 M.A., D.Sc, Assistant-Lecturer in Physics in the University 

 College of North Wales, Bangor *. 



THE laws of evaporation from a circular liquid surface 

 have received a certain amount of attention, both on 

 the theoretical and experimental side, but the results of the 

 work done are by no means so well known as they might be. 

 Even now, it is not at all uncommon to find, in text-books 

 and original papers, the statement that the rate of evapo- 

 ration from such a surface is directly proportional to its 

 area, in spite of the fact that, so long ago as 1881, Stefan 

 had established from theoretical considerations that the rate 

 of evaporation was proportional, under certain specified, 

 external conditions, to the radius and not to the area of the 

 surface. And even where this fact is recognized, the con- 

 ditions under which the linear law does hold are not perhaps 

 clearly appreciated. 



Farther, in many books and papers which touch upon this 

 point there exists a certain amount of " dimensional " con- 

 fusion in the application of the theoretical equations involved ; 

 in particular, there is, in the application of the fundamental 

 formulae, some confusion between mass and volume — partly 

 traceable to the use of the equivocal term " amount " (of 

 liquid evaporated per unit time) or its corresponding 

 German equivalent " Men'ge " — which makes the checking 

 o£ calculations a matter of some difficulty. 



The existence of these sources of uncertainty is the more 

 surprising as an excellent annotated bibliography of literature 

 on evaporation has for some years been accessible -\ ; the 

 importance of an exact knowledge of the laws — involving, as 

 they do, considerations equally valuable to the physicist, 

 botanist, and meteorologist — is beyond cavil. 



We therefore propose to give in this paper a re'sume of the 

 work already done, with an account of such points in the 

 treatment of the subject as appear to us to be either obscure 

 or erroneous, and, finally, to describe some experiments 

 which we have made on the evaporation from circular water 



* Communicated by Prof. E. Taylor Jones, D.Sc. 



t Mrs, Grace J. Livingston, (i An Annotated Bibliography of 

 Evaporation." Reprinted from ' Monthly Weather Review (U.S.A.),' 

 June, Sept., and Nov. 1908, and Feb., March, April, May, and June 

 1909. 



