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ATMOMETRY AND THE POROUS CUP ATMOMETER 



BURTON EDWARD LIVINGSTON 



The Johns Hopkins University, Baltimore, Md. 



II 



THE MAIN FORMS OF ATMOMETER 



Choice as to Form of Instrument 



From what has been said in the last section the question 

 arises : if the readings of one form of atmometer cannot be reduced, 

 by mathematical treatment, to readings that might have been had 

 from some other form of instrument (operating in the same place 

 and at the same time), then by what criteria is the investigator 

 to decide what sort of instrument to employ in a series of compara- 

 tive measurements? Obviously, from the very nature of evapora- 

 tion and from the medley of conditions by which it is influenced, 

 the kind of evaporation to be studied must form the basis for this 

 decision. 



Where it is desired to approximate the rate of water loss from 

 reservoirs and other large bodies of water, the floating pan is 

 doubtless the most suitable instrument; it exposes a free water 

 surface in much the same manner as does the reservoir itself. 

 Where the study in hand involves the measurement of the evap- 

 orating power of the air as this affects plant transpiration, some 

 form of saturated paper or porous clay surface is to be chosen; 

 such surfaces may be exposed in a manner fairly comparable to 

 the exposure of transpiring plant parts. To study evaporation 

 from soils, a box or pan of moist soil seems more logical as an 

 instrument than does a pan of water, though paper and porous 

 clay surfaces may be adapted to this need. In short, the surface 

 by means of which the evaporating power of the air is to be meas- 

 ured should possess as nearly the same form as possible and 



51 



THE PLANT WORLD, VOL. 18, NO. 3, 1915 



