

r 



1920] FREEMAN— EVAPORATION AND TRANSPIRATION • 191 



a surface is evaporating in the open, the environment is unbounded 



or indefinite. 



environment, not in typ 



but in degree, since where there is any wind movement, after a 



greater or less length of time, the evaporating surface will come 

 into equilibrium with it, and thereafter give off water at a uniform 

 rate. The larger the environment and the slower the wind move- 

 ment, the longer will be the time required to reach this equilibrium. 

 With an unbounded environment and no wind movement, theo- 

 retically it would never be reached, but for all practical purposes 

 a small evaporating surface exposed in the open in still air soon 

 reaches a uniform rate of evaporation, -which will remain constant 

 so long as the temperature and dewpoint of the air remain the 

 same. The influence of the size of the environment, therefore, is 

 capable of mathematical expression, and may enter as a factor 

 in a rational evaporation formula. When the size of the environ- 

 ment always remains the same, as in all the experiments con- 



formulae 



may 





other constants found. When the size or nature of the environ- 

 ment varies in any way, it must be calculated as a separate 

 constant, or else the other constants (which had included it) must 

 be recalculated for each new environmental condition. It is this 

 consideration which has made it impossible to write, for any given 

 type of surface, an evaporation formula which would apply to all 

 situations, even though all be in the open. The reason for this is 

 that every environment is more or less restricted by buildings, 

 trees, banks, hills, or mountains, so that no two are exactly alike. 

 It would be impossible to write a formula which would account 

 for all of these variations. So long, however, as we confine our- 

 selves to a given environment and a given type of evaporating 

 surface, we can easily account for variations in amount of surface, 

 temperature, wind movement, and dewpoint of the air. 



In the physical experiments undertaken, the evaporating sur- 

 face used was a porous cup atmometer. A constant stream of air 

 was drawn through the cylinder (over the atmometer) by a rotary 

 air pump driven by a small motor. The air was measured by a 

 water gasometer which was accurate to 1/1000 of a cubic foot. 



