ATMOMETRY AND THE ATMOMETER 145 



larly is usually (depending upon latitude and season) but a 

 small portion of the hemispherical tip. At sunset again, this 

 area is approximately represented by the middle line of the 

 illuminated area, just as at sunrise, but on the opposite side 

 of the cup. If it were not for the fact that the absorbing power 

 for radiation, of the white porous porcelain, is very low, these 

 conditions would vitiate the atmometric readings as indicating 

 the evaporating power of the air. Actually, this atmospheric 

 evaporating power can be measured only in the absence of appre- 

 ciable radiation; practically, the absorbing power of the white 

 porcelain is so low that the entire influence of sunshine may be 

 neglected, for most purposes at least. The radiation is received 

 but is almost entirely reflected and not absorbed. 



The mean angular exposure of the cup to the sun's rays, may 

 be held constant by placing the instrument obliquely instead 

 of vertically, the tip pointing northward (in the northern hemi- 

 sphere) and the whole cup so tilted that the sun's rays will con- 

 tinue all day to impinge perpendicularly to the long axis of the 

 cup. Such a position is not the same throughout the year, how- 

 ever, and frequent adjustments are necessary; the cup should be 

 the most nearly vertical at the winter solstice and most nearly 

 horizontal at that of summer.. Such changes in the position of 

 the cup might result in differences in its effective exposure to air 

 movement, and for most lines of study it is probably best — and 

 it is certainly satisfactory — to give the cup the vertical position 

 and to neglect the possible variation in exposure due to angular 

 variations with reference to solar radiation. This refers to or- 

 dinary measurements of the evaporating power of the air, where 

 the influence of sunshine is not particularly considered. 



The possible influence of sunshine upon evaporation, having 

 been once appreciated, it became desirable to devise some means 

 by which this influence might be approximately measured. 

 Especially is this needed for analytical studies of the conditional 

 control of plant transpiration, where the absorption of radiant 

 energy by foliar surfaces is frequently more important than the 

 atmospheric evaporating power itself. 



At first thought it seems feasible to expose two quite similar 



THE PLANT WORLD, VOL. 18, NO. 5, 1915 



