EVAPORATION RATES FOR SHORT INTERVALS 137 



evaporating power of the air.^ In both cases the condition con- 

 sidered is of course conceived as dependent upon rates (of molecu- 

 lar motion, etc.), but the present consideration need not enter 

 into these deeper questions. 



It must have occurred to most users of atmometers that a 

 form of this instrument that might indicate the magnitude of 

 atmospheric evaporating power from time to time, as readings 

 might be made upon a thermometer, would be exceedingly con- 

 venient for many purposes. For example, if it were required 

 to maintain the evaporating power of the air in a room at a 

 constant value, as by manipulating temperature and humidity 

 controls, it would be helpful to be able to read the evaporating 

 power at a glance, just as temperature can be read from an 

 ordinary thermometer. As it is, however, the evaporating power 

 of the air can be determined only in terms of the evaporation 

 rate from a standard surface, requiring two readings with a 

 known time period intervening. With precise methods of meas- 

 urement the required time period may be comparatively short, 

 but even the porous cup form of atmometer, as usually arranged, 

 is seldom read in practice at intervals of less than ten or fifteen 

 minutes, and the employment of open pans of water generally 

 requires much longer periods. An atmometer that might allow 

 instantaneous determination of the evaporating power of the air 

 would be a valuable instrument for greenhouses and, if it were 

 durable and easily read, it might serve a useful purpose in 

 assembly rooms, living rooms, etc. 



It is quite possible to measure a rate without two readings if 

 the rate may be converted into a pressure and thus read upon a 

 manometer scale, and this seems feasible in the case of atmos- 

 pheric rates. It is only necessaiy to introduce suitable resistance 

 to water movement in the tube that connects the porous cup 

 with the water reservoir below, and to connect a manometer to 



2 On the use of this term and objections raised thereto, see: Livingston, B. 

 E., Atmospheric influence on evaporation and its direct measurement. Monthly 

 Weather Rev. 43: 126-131. 1915. Idem, A modification of the Bellani porous 

 plate atmometer. Science, n. s. 41: 872-874. 1915. Idem. A single index to 

 represent both moisture and temperature conditions as related to plants. 

 Phvsiol. Res. 1: 423-442. 1916. 



