ATMOMETRY AND THE ATMOMETER 147 



in darkness. The radiant energy absorbed is converted (with or 

 without passing through the phase of heating the water and cup 

 wall) into the heat of vaporization of water. The dark cup be- 

 comes little if at all warmer than the white, at least in summer 

 at Tucson, Arizona, where many of the tests of this new instru- 

 ment have been carried out. Both cups are always considerably 

 cooler than the air, which makes it evident that no absorbed 

 heat is conducted outwardly from the cup. The difference in 

 water loss from the two cups is thus an approximate measure 

 of the difference between the amounts of radiant energy absorbed. 

 This difference tells nothing definitely about the radiation 

 striking the dark cup, no doubt some of the latter is reflected, 

 but a much larger proportion is reflected from the white cup than 

 from the dark one. 



For studies in plant transpiration it was necessary to find out 

 to what degree the dark atmometer behaves like plant foliage, 

 when exposed to variations in sunlight intensity. Tests of 

 this matter were carried out by exposing plants and instru- 

 ments, standing side by side, to full sunshine and to various 

 degrees of shade, for consecutive time intervals, and deter- 

 mining the transpirational losses from the plants and the evap- 

 oration from the instruments. The results 28 showed that the 

 dark cup is affected in quite the same manner, qualitatively, 

 as are plants, by changes in sunshine intensity. The radio- 

 atmometer was thus established as a valuable instrument in 

 studies of plant transpiration. It appears from the paper just 

 cited that different plants are differently affected by variations 

 in sunlight intensity, as was indeed to be expected, the power 

 of absorbing solar radiation is thus shown to be an important 

 physiological property of plants, a property requiring study 

 before plant water relations can be logically analyzed. 



The radio-atmometer is, then, an instrument for measuring 

 the effectiveness of solar radiation as an accelerator of evapora- 

 tion. This is exactly what its name implies. It has not been 

 shown to be available for measuring solar radiation as a whole, 



28 Livingston, B. E., Light intensity and transpiration. Bot. Gaz. 52:418- 

 438, 1911. 



