166 Atmometric Units [364 



be that of a small, spherical droplet of water suspended freely 

 in the air. This is not practicable, but the Livingston stan- 

 dard spherical porous cup seems to approach this desideratum 

 as nearly as is possible when general ease of manipulation is 

 considered. Since we have been able to obtain these porcelain 

 spheres I have regarded the quest for a practically perfect 

 surface as at an end. Some form of imbibed porous surface 

 is undoubtedly best for general purposes, and students of 

 ecology, ventilation engineers, agriculturists, etc., should 

 avoid the free water surface if possible, unless it is desired that 

 the results obtained be roughly comparable with those obtained 

 by the Weather Bureau. 



A third requirement is not as important now as it will be 

 later, after more atmometric data have been collected. This 

 is, that the instrument should be like some form previously 

 used, so as to give data that may be comparable with at least 

 some of those already on record. 



Of the different forms of imbibed porous surface there are 

 four that should be more or less generally useful : the Piche 

 paper disk, the Bellani porous clay disk, the Babinet cylinder 

 and the Leslie sphere, the last three having been recently im- 

 proved in our own work. Probably more measurements have 

 been made with the Piche paper disk and with the Livingston 

 standard cylinder than with any other types of instrument, 

 but the Piche instrument has serious practical shortcomings 

 and the sphere is more nearly perfect than the cylinder. The 

 cylinder will remain an important instrument for a long time 

 but the sphere will almost surely .replace it eventually. A 

 fourth feature of the porous sphere may be emphasized as 

 desirable, namely, the ready applicability of this type of in- 

 strument to the measurement of effective radiation intensity, 

 which is the other aerial condition of evaporation besides the 

 evaporating power of the air. Since we have been able to 

 obtain black porous spheres of the same size as the white ones, 

 the spherical form has become almost essential in all work in 

 atmometry involving radiation. The two spheres, one white 



