168 Altmometric Units [366 
With open pans it is easier to measure depth than volume, for 
rough approximations. It is true that volume or weight read- 
ings from other types of instrument than those employing an 
open pan, may also be multiplied by a constant throughout the 
series, and this constant might be the area of the surface em- 
ployed, or any other number that may be chosen. But it can- 
not be too strongly emphasized that such treatment is to be 
applied only to series of readings that are already comparable, 
and that no constants can be found by which readings from 
different types of instruments may be rendered comparable. 
The use of depth units in comparing water losses from open 
pans has introduced and supported a fallacy that is extremely 
hard to combat in the minds of those who have not given the 
subject of atmometry serious attention. This fallacy is based 
upon the mistaken idea that the area of the evaporating sur- 
face is the only surface characteristic that can influence the 
rate of evaporation. If different sizes of pans are employed 
the readings are incomparable, and they remain incompar- 
able even after each one has been divided by the area of its 
own water surface. Readings must be stated as from a cer- 
tain instrument, in any event, and the application of an areal 
coefficient only complicates matters. To avoid the continua- 
tion of this fallacy, as much as may be, it is highly desirable 
that all atmometric readings be stated in terms of weight or 
volume, even though they were originally obtained by measure- 
ments of depth. 
The worst feature of the use of depth units in pan atmome- 
try is that it has led to another fallacy, by which these depth 
units are taken to be equivalent to the other depth units that 
are employed in the measurement of rainfall. The two classes 
of units look alike but they are widely different in their mean- 
ings. An example may illustrate this very important point. 
Suppose that the rainfall for a certain place is found to be 
75cm. (of depth) for a certain year, and suppose that the 
observer states that the evaporation from a Weather Bureau 
pan for the same period was 90 cm. (also of depth). In such 
a case students of climatology have been led to say that evapo- 
