164 Atmometric Units [362 



differences in the instruments. That two porous cups or 

 pans of water are of t'he 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, etc. 



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- 



