160 Atmometric Units [358 
ATMOMETRIC UNITS 
By Burton HE. LivincsTon 
The increasing interest in atmometry’* and the fact that 
this subject is becoming recognized as of general and funda- 
mental importance in many branches of scientific and practical 
endeavor, make it desirable that there be some uniformity in 
our conceptions as to the units employed in atmometric meas- 
urements. To approach the subject it is first necessary that 
the purpose of atmometric observations be clearly in mind; 
much vagueness still prevails in this connection. The rate 
of evaporation of water from any surface is dependent on 
two sets of conditions. One set (internal ones) are effective 
in or behind the surface and the other (external ones) are 
effective in front of the surface, that is, in the gas phase of 
the system. The internal conditions are the characteristics 
of the evaporating surface and include such features as the 
concentration of solutes in the liquid water, the influence ex- 
erted by the presence of a solid in which the water is imbibed, 
the shape and extent of the surface, its direction of exposure, 
its ability to absorb or emit radiant energy, the heat-conduct- 
ing capacity of the material back of the surface, etc. The 
external conditions include primarily four characteristics of 
the space in front of the evaporating surface: the temperature 
of the gas phase, its moisture condition, the influence of move- 
ment or circulation of the gas over the surface, and the effec- 
tive intensity of impinging radiation. I have used the term 
*I employ the word as synonymous with and shorter than atmid- 
ometry, just as I have adopted atmometer in place of its rival, atmid- 
ometer. Both are etymologically correct, but the one formed from 
the root atmo, besides being shorter, has received the sanction of an 
international meteorological congress. Atmometer seems to have 
been coined by Sir John Leslie, 1813. (See Livingston, B. E., 
“ Atmometry and the porous cup atmometer.” Plant World 18: 21-30, 
51-74, 95-111, 143-149. 1915. Other papers are there cited, ) 
