320 Evaporation from a Circular Water Surface. 



Table IV. shows the agreement between the observed and 

 calculated values of n. 



Table IV. 



d (cm.) 



n (calc.) 

 n (obs.) 



o-o 



1-40 

 1-43 



03 



1-56 



1-60 



0-5 



1-64 



1-52 



0-7 1 1-0 

 1-70 j T78 

 1-69 1-78 



1-2 



1-82 



1-86 



1-5 i 20 2 5 



1-87 

 1-85 



1-92 

 1-99 



1-95 

 1-98 



30 



197 



1-97 



The value for ^ = 0*5 cm. excepted, the agreement is fairly 

 good, when it is remembered that the temperature, pressure, 

 and humidity, all of which affect the constants of the above 

 equations, must necessarily vary to a greater or less degree 

 during the progress of any given experiment. 



Wo may say then that the evaporation from a circular 

 surface of radius a, at a depth d below the rim of the con- 

 taining vessel, is given in a steady atmosphere by an equation 

 of the form 



where pi, qi, s 1? p, q, and s are independent of a and d, 

 but vary with temperature, pressure, humidity, and wind- 

 velocity. The dependence of these quantities on the above 

 variables is a matter for future investigation, but it may 

 safely be assumed that, in all ordinary conditions, for sur- 

 faces from 2 to 10 cm. in radius, when the value of d is 

 greater than about 3 cm., the value of the exponent of a in 

 the above equation is constant and equal to 2. 



One further point may be noticed. It is customary to 

 give evaporation results in linear measure — inches or centi- 

 metres, as the case may be. The above results show that in 

 such cases the amount of evaporation recorded will be a 

 function of the radius of the vessel used. For, if d be the 

 depth evaporated per unit time, the mass evaporated will be 



E = Tra^pd, 

 and if the law of evaporation be 



E = Ka», 

 then 



d— —a 11 * 

 Trp 



and is only independent of a when n = 2. If we assume 



