196 BOTANICAL GAZETTE [September 



of c is diminished; and when the evaporating surface practically 

 fills its environment, c approaches zero. On the other hand, as the 

 area at which c is determined is made smaller, the value of c indefi- 

 nitely increases. 



As the evaporating area increases, the value of n increases, 

 until when the evaporating area practically fills its environment, 

 the value of n = 1 (that is, the air passing off will be saturated and 

 the evaporation will be in direct linear proportion to the rate of 

 wind movement). Conversely, when the size of evaporating area 

 is diminished, the value of n decreases, until it reaches the vanish- 

 ing point, when the evaporating area becomes indefinitely small. 

 When n becomes zero, the value of w n will equal 1, regardless of the 

 numerical value of w, since any number raised to the zero power 

 equals one. In other words, when the evaporating area is indefi- 

 nitely small, it cannot measurably increase the humidity of the 

 surrounding air; hence changes in wind movement would have no 

 effect upon the rate of evaporation. Instead of c we may there- 

 fore write , and for n write 7—- > * n w hich a * s an Y area expressed 

 in terms of the area used when c was determined, and k is a con- 

 stant. The formula then becomes y = ^-7- . Again, if 



c{w) k + a 

 a 

 the wind movement used when c and k were determined be made 

 unity, and other wind movements be expressed in terms of it, 

 since 1 raised to any power is 1, the formula for that wind move- 



ment becomes y = , from which the value of c can readily be 



calculated from any experimental value of y. This statement of 

 the formula can also be used in experiments in which the wind 

 movement is not varied, and may hence be considered as unity. 

 The details of such an experiment are given in table I, where 

 c = o . 984, w = 1 , a = 1 , and a t = 1 . 654. Since w = 1 throughout, the 

 determination of k was unnecessary. 



Table II gives another experiment of a similar nature, in which the 

 wind movement was 91 .43 liters of air per hour throughout, which 



