146 BURTON EDWARD LIVINGSTON 



atmometer cups, one shaded from the sun's rays and one exposed 

 to them, and to derive from the differences between their rates 

 of evaporation a measure of the power of the sunshine to accelerate 

 this process. But to do this it is requisite to give the two in- 

 struments the same exposure in all other respects, and the prob- 

 lem of establishing shade conditions without also modifying wind 

 conditions, for example, is a very difficult one. 



An alternative method was suggested. Let one cup remain 

 white and let the other be blackened, so as to absorb as much 

 radiation as possible, and let the two be exposed side by side with- 

 out any shade. The difference between the readings of the two 

 instruments should be a measure of the effectiveness of the sun's 

 rays (during the particular time period in question) in promoting 

 evaporation. To avoid the difficulty pointed out above, let both 

 cups have an oblique position, such that the sun's rays will fall 

 perpendicularly to the long axis throughout the day Such an 

 arrangement was first tested at the Desert Laboratory, during 

 the summer of 1907, with such promising results that consider- 

 able attention has since been given to its perfecting, as the 

 radio-atmomeier. 27 



Porous cups were finally obtained, of the usual form but dark 

 brown in color and exhibiting a high power of light-heat absorp- 

 tion. Also white cups were blackened by the use of washed 

 lampblack and also (a suggestion of Prof. F. E. Lloyd of McGill 

 University) by the use of the so-called water-proof liquid draw- 

 ing ink obtainable from dealers in artists' supplies. The black- 

 ened cups possess a slightly higher power of light-heat absorption 

 than do the brown ones, but the latter have the advantage of 

 retaining their color while the black coatings so far employed 

 are rather easily removed, as by rain, etc. Until quite recently 

 it has been impossible to obtain a black porous porcelain, though 

 an attempt in this direction has been in progress for a number 

 of years. 



During hours of sunlight the dark cup gives off more water 

 than the white one, assuming that both evaporate at the same rate 



27 Livingston, B. E., A • radio-atmometer for comparing light intensities. 

 Plant World 14: 96-99, 1911. 



