138 



EARL S. JOHNSTON AND BURTON E. LIVINGSTON 



a lateral opening in this tube above the resistance. The resist- 

 ance, at the lower end of the supply tube, should be such that 

 when the manometer registers no suction on the part of the 



cup the rate of flow of water through the re- 

 sistance is lower than the lowest rate of evapo- 

 ration to be dealt with. With higher rates of 

 evaporation suction is developed and the mer- 

 cury column of the manometer is displaced, 

 while the relatively increased water pressure 

 from without causes a more rapid flow through 

 the resistance in the tube. The greater is the 

 evaporation rate the higher does the mercury 

 rise in the proximal arm of the manometer. 

 Of course the resistance must be small enough 

 so that somewhat less than an atmosphere of 

 pressure may drive water through the tube at 

 a rate as great as will ever need be measured. 

 After proper calibration, a scale may be at- 

 tached to the manometer of such an arrange- 

 ment, to be read in terms of cubic centimeters 

 per hour. Any line of this scale represents the 

 rate of water movement through the tube un- 

 der the pressure indicated by the manometer 

 when its mercury meniscus stands at that line. 

 With changes in the rate of evaporation from 

 the porous cup above, the mercury column rises 

 or falls, and if the manometer is of small diam- 

 eter (as related to the surface exposed by the 

 cup), the lag in the manometer adjustment is 

 very slight. 



The first instruments of this kind were con- 

 structed by one of the writers, assisted by Mr. 

 H. C. Sampson, at the Desert Laboratory in 

 the summer of 1914. The porous cups were 

 arranged on bottles in the ordinary way and 

 two forms of resistance at the lower end of the 

 supply tube were employed. One of these was 



Fig. 1 . Porous 

 sphere atmometer 

 with burette at- 

 tachment. A, po- 

 rous sphere; B — B', 

 supply tube (with 

 cock) from reser- 

 voir; C — C, supply 

 tube from burette; 

 D, burette cap; E, 

 air-intake. Sphere 

 is filled by suction 

 at open end of bu- 

 rette. 



