162 Atmometric Units [360 
the index of the evaporating power of the air. From this last 
expression comes the new term, the atmometric index of the 
locality and time period considered. 
The atmometric index is the relative measure of the evapo- 
rating power of the air, and it is to be expressed as a possible 
time rate of doing work; it is an index of a power. The unit 
of measurement must therefore be a unit of work, but it may 
just as well be a unit of process rate, if the same process be 
always employed. Thus it may be stated as the amount of 
water evaporated per unit of time. If the liquid water were 
always at the same temperature this would actually be a 
measure of work. That the water of evaporating surfaces 
varies in temperature has been thus far neglected in this 
whole line of enquiry, the errors thus arising being relatively 
small in the present early stages of our studies. 
To determine the numerical value of the atmometric index, 
we must also consider a factor representing some standard unit 
of surface. It has been seen above that the power of any sur- 
face to give off water vapor is determined by the character- 
istics of that surface, and that the extent of the surface is not 
by any means the only characteristic that needs to be con- 
sidered. The shape and the direction of exposure of the sur- 
face must be taken into account, and also the influence of the 
non-aqueous materials that are in or behind the surface, ete. 
It is therefore impossible to employ a surface unit: defined by 
area alone. As soon as this is realized all attempts to ex- 
press the atmometric index as a time rate of evaporation from 
a square centimeter (etc.) of free water surface are seen to be 
quite useless. A free water surface ig more or less nearly 
plane and more or less nearly horizontal (depending upon the 
wind velocity, among other things), but it may be of any 
shape or size, and all these characteristics are important. 
With a given set of aerial conditions two different atmometer 
pans, for instance, can give off the same amount of water per 
hour, per square centimeter of surface, only when they are of 
the same size and shape. Also, the depth of the water and 
