148 BURTON EDWARD LIVINGSTON 



though its readings no doubt approximate such measurements 

 to a greater or smaller degree. It is further to be emphasized 

 that this instrument gives no information as to the quality of 

 the radiations received; it is only as energy (familiarly, as heat) 

 that varying or differing intensities are to be studied by this 

 means. Thus, as is shown in the paper last cited, actinometric 

 measurements of solar intensity may be expected frequently to 

 be in disagreement with those obtained by means of the radio- 

 atmometer. 



Not the least important consideration in regard to the radio- 

 atmometer is the fact that, like the atmometer itself, it is an 

 automatically integrating instrument; if read hourly it gives the 

 total effectiveness of absorbed radiation for each hour, if read 

 weekly or monthly it gives weekly or monthly summations, etc. 



Standardization of Dark Cups. The dark cups for the radio- 

 atmometer are standardized in the same manner as are the white 

 ones, only the standardization must never be carried out in 

 sunlight. Ordinary diffuse daylight seems to be of too weak 

 intensity sensibly to affect the water loss from the dark cup. 

 From the nature of the case it will be seen that the corrected 

 readings of a dark and of a white cup should be practically iden- 

 tical for the hours of darkness. At sunrise these two corrected 

 values (of hourly readings, for example) begin to diverge, that 

 from the dark cup becoming greater than the other, and they re- 

 main divergent throughout the day unless it is very dull. At 

 sunset the two values again come together. The amount of 

 divergence, for any hour, etc., is a measure of the intensity of 

 solar radiations for that period, in so far as such radiation may 

 accelerate evaporation from an imbibed surface. 



With some preliminary work on the relation of rate divergence 

 (of the two cups) to visible brightness of sunshine, the radio- 

 atmometer should become available as a simple and exceedingly 

 sensitive sunshine recorder. 



The Spherical Atmometer Cup. It has been said that the cups 

 for radio-atmometry must be placed at a certain angle with the 

 vertical and that this angle must be altered every few days", 

 with the advance of the season. It was at once obvious that 



