164 Atmometric Units [362 
differences in the instruments. That two porous cups or 
pans of water are of the same size, shape, color, etc., does not 
necessarily indicate that they may be expected to give like 
readings if placed in the same environment, for other, less 
easily recognized characteristics of the instruments may not 
be without influence, and the surfaces may differ with respect 
to some of these. This consideration leads to standardization 
and the use of a rotating table. 
By this procedure an index is obtained that represents rela- 
tive capacity of each of the instruments tested, to give off 
water vapor, and the index of each one is expressed as a 
coefficient of correction, a number by which the readings of 
that instrument are to be multiplied in order to give the read- 
ing that would have been obtained from the master standard 
instrument if it had been operated for the same time and at 
the same place. If an instrument is effectively just like the 
master standard its coefficient is unity. It is not possible, 
however, to obtain useful coefficients for instruments that 
differ appreciably from the standard in form, size, ete. 
Since it is necessary that several instruments be practically 
alike if their readings are to be comparable, it is highly desir- 
able that different workers use as few different forms of instru- 
ment as possible. For studies on the details of the evaporation 
process itself various kinds of surfaces are desirable, but for 
general climatic atmometry the number of kinds should be 
kept as small as may be. This seems to be not at all well 
understood, and workers who have not taken the trouble to 
appreciate just what is the purpose of atmometric measure- 
ments continue to construct new types of instruments and to 
employ them. For example, the idea is abroad that if the 
right sort of instrument might be devised its readings would 
indicate relative rates of plant transpiration. Obviously such 
an instrument would have to alter its internal conditions from 
minute to minute, just as would occur in the standard plant 
individual, and all other plants would usually differ from it. 
The idea is bootless. We do not wish to measure plant tran- 
