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 
ig 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 sites 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 
eylinder 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 
