316 Miss N. Thomas and Dr. A. Ferguson on 



water-surfaces under different external conditions. The ex- 

 perimental methods employed were quite simple. A series 

 of circular crystallizing dishes of radii varying from two to 

 ten centimetres was exposed under certain fairly definite 

 conditions, each dish in any given experiment being filled to 

 a definite depth d below its rim. The vessels after being 

 weighed were placed on a table, separated from each other 

 by distances several times greater than their own diameter, 

 and were left exposed for a definite period. They were then 

 weighed again, the resulting rate of loss (E) by evaporation 

 being expressed in grams per hour. Assuming the rate of 

 evaporation to be given by 



E = Ka», 



by plotting log E against log a, K and n can be graphically 

 determined. In all cases it was found that the curve so 

 obtained was linear to a very fair degree of accuracy, so 

 that the values of k and n could be determined without 

 appreciable ambiguity. The barometric height, hygrometric 

 state of the air, and maximum and minimum temperature 

 experienced during any given experiment were also recorded. 



The experiments were carried out under the following 

 conditions : — (A) in a dark room, having blackened walls 

 and a floor space of about 400 square feet. This room was 

 chosen for its steady temperature qualities, the temperature 

 over a period of 24 hours never varying more than one or 

 two degrees ; (B) in a large room used as a general labora- 

 tory, well lighted from above, and having a floor space of 

 about 600 square feet ; and (C) in the open air. 



The greater portion of the experiments were made in the 

 summer of 1 VJ16 in the University College of North Wales. 

 Some readings were taken during the cold spell of last winter 

 at the Hoyal Hollo way College, but these readings are not so 

 numerous as we could wish ; we hope, should circumstances 

 permit, to continue the experiments, using much larger 

 surfaces, as we believe that the results, apart from their 

 purely physical interest, may be of service to meteorologists. 



It is not necessary to give full details of all the figures 

 obtained in the various experiments. In Table I. below is 

 shown the result of an experiment made under condition (A). 

 This shows the accuracy obtainable in one given experiment 

 and will serve as a sample of the rest of our observations. 

 The first column gives the radii (a) of the dishes used, the 

 sixth column gives the observed rate of evaporation in 

 grams per hour, and the seventh column shows the rate of 



