THE RAIN-CORRECTING ATMOMETER 



93 



There remains the consideration that small areas of the cup 

 may, during a driving shower, be exposed to evaporation and that 

 some water loss may thus occur. As is shown by one of the tests 

 above described, water might evaporate from one side of the 

 cup and be absorbed on the other side to an equivalent amount, 

 the level in the burette remaining without change. The absorp- 

 tion of slight amounts of dew might also introduce a correspond- 

 ingly slight error in the readings. No arrangement has been 

 devised which will care for these contingencies, but the error thus 

 involved is assuredly not one which will play any tangible part 

 in evaporation measurement for a long time to come. 



It may be concluded that the error of rain absorption by the 

 ordinary porous cup atmometer is decidedly not to be neglected 

 in humid regions, and that the errors of this sort which are still 

 permitted by the Livingston mercury valve are so extremely 

 small as to be entirely negligible in all operations so conducted as 

 to expose the instruments to rain or dew. 



As a possible improvement on the apparatus as described by 

 Livingston, it may be suggested (though this is probably included 

 in the remark about, possible modifications made in the original 

 publication on this subject) that a good stopcock might replace 

 the second mercury vah^e, the function of this valve being merely 

 to allow ready filling of the cup (when first placed or when a 

 change is made for restandardization) and to close the opening 

 thus used after the cup is filled. 



The tests here considered were carried out at the Desert Labora- 

 tory of the Carnegie Institution, during the summer of 1912, 

 and the writer wishes here to express his obligation to Prof. 

 B. E. Livingston for numerous suggestions bearing upon the study. 



