144 BURTON EDWARD LIVINGSTON 



by the surface in question. Practically the only radiant energy 

 in nature, with which students of evaporation and plant trans- 

 piration are apt to be concerned, is that from the sun. Our 

 discussion will have mainly to do with this. 



To make the matter before us clear, let it be supposed that 

 an ordinary standardized porous cup is operated in the open, 

 in the vertical position. As the sun rises in the morning its rays 

 strike one-half of the evaporating surface, their direction being 

 approximately perpendicular to the long axis of the cup. Now, 

 if any of the impinging radiant energy (heat, light) is absorbed 

 by the cup, this should increase the rate of water loss on the 

 sunny side. With a given intensity of impinging radiation the 

 amount absorbed by the surface is determined by the absorbing 

 power of the latter and by the angle at which the rays strike. 

 In the case here supposed, the white porcelain cup possesses low 

 absorbing power and a large proportion of the impinging energy 

 is reflected. But the white cup does of course possess some 

 absorbing power. Of the various parts of the illuminated sur- 

 face, that portion receiving the rays most nearly perpendicularly 

 must of course absorb the most energy. This portion is rep- 

 resented by a line on the cylindrical surface, marking the middle 

 of the illuminated area. At the edges of this area the angle of 

 incidence of the radiation vanishes, and between the middle line 

 and the edge this angle ranges in magnitude from 0° to 90°. 

 But, for a given kind, intensity and direction of radiation, the 

 total absorption of energy should remain approximately the 

 same for the whole illuminated area taken together, no matter 

 what side of the cup is illuminated. If the average angular 

 exposure of the cup to the impinging radiation might be kept 

 the same throughout the day, and if all parts of the cup sur- 

 face might have just the same absorbing power, then the effect 

 of sunlight in accelerating evaporation should simply be added 

 to the effects of the atmospheric evaporating power, from minute 

 to minute or from hour to hour. 



As the sun carries out its daily march, however, the angle 

 or incidence of its rays upon a vertical cylindrical cup varies 

 markedly. At noon the area of the cup receiving rays perpendicu- 



