Evaporation from a Circular Water Surface. 319 



At constant depth changes in the external circumstances 

 in the direction of greater atmospheric disturbance do not 

 make so large a change in the value of n as might be 

 expected from the distortion of the lines of flow which must 

 necessarily ensue. Thus, for d = 0*5 cm. the mean value 

 of n is about 1*52 in series A, 1*58 in B, and 1*67 in C. 

 During this last experiment, which was performed in the 

 open air, the liquid surfaces exposed were continually ruffled 

 by the wind, but the increase in ??, though quite distinct, is 

 relatively small. 



Series A contains sufficient observations to show quantita- 

 tively how n and K vary with d. Considering K first, if we 

 assume an equation of the type 



K=p 1 + q [ e-\ d J 



we find that the results are fairly well satisfied by 



K=-005 + '025<r 2 *. 



Exact correspondence is not to be expected, as the values of 

 p, q, and s, which are independent of d and of a, the radius 

 of the surface, are affected by the external variables men- 

 tioned above. In series A, however, the external conditions 

 were sufficiently constant to justify taking mean values 

 where more than one series of readings was taken for a 

 given value of d. 



The graph of the above equation was sketched on squared 

 paper, and the agreement between the observed and calculated 

 values of K is shown below. 











Table III. 











rf(.an.) ... 



00 



0-3 



! 



0-5 0-7 



1-0 



1-2 



1-5 



2-0 



25 



30 



K(calc)... 



■030 



•011) 



014-011 



0080 



•0070 



•0060 



•0054 



•0051 



0050 



K(obs.)... 



•031 



013 



015 -010 



0079 



•0063 



•0051 



•0053 



•0053 



•0049 



With the exception of the value for d = 03 cm., it will be 

 seen that the agreement is as close as could be expected. 



In the same series the value of n is fairly well represented 

 bv an equation of the type 



-sd 



n=p — qe 



if we assume for the constants values which give 

 n = 2'0-'60e- d . 



Z2 



