361] B. E. Livingston 163 
the nature of the pans themselves must be exactly alike. It 
is thus both theoretically and practically impossible to express 
the surface factor in the atmometric index by a unit that rep- 
resents merely extent of surface. 
Since the complex of internal conditions that make up the 
capacity of any surface to produce evaporation is very difficult 
of analysis, we may avoid the necessity of this analysis by 
simply using atmometer surfaces that act alike. Then the 
surface factor of our unit of measurement becomes the surface 
of our instrument (with whatever characteristics it may have), 
and we do not need to enquire what may be its area, etc. In all 
the studies so far carried out with porous clay and paper sur- 
faces for measuring the evaporating power of the air, I have 
never been led to determine the area of the surface employed ; 
it would have been useless to do so, although such a surface 
is easily measured. We are thus led to the proposition that 
the atmometric index is to be expressed in terms of (1) a 
weight unit of water, (2) a time unit, and (3) a given stan- 
dard instrument. All these desiderata are supplied in such 
a statement as this: that the evaporating power of the air in 
a given locality and for a given period is such as to produce 
the evaporation of so and so many grams of water per hour 
from a standard spherical porous-cup atmometer. No unit of 
area is considered, although all the internal characteristics of 
the instrument are implied by its name. ; 
It is clear that it makes no difference what sort of surface 
we may use as standard, but we must use the same standard 
throughout any series of comparative measurements, and when 
several instruments are needed we must be sure that their in- 
ternal characteristics are as nearly alike as possible, as far as 
these characteristics may influence the rate of evaporation. 
The only feasible way to compare a number of instruments 
in this last regard is to place them all in the same environ- 
ment (as far as environmental characteristics may influence 
the rate of evaporation) and then compare their evaporation 
rates. If these rates differ this must be because of internal 
