AN IMPROVED NON-ABSORBING POROUS CUP 



ATMOMETER 



JOHN W. SHIVE 

 The Johns Hopkins University, Baltimore, Md. 



Since Livingston's 1 description of the rain correcting atmometer 

 appeared in 1910, a large number of these instruments have come 

 into use. Up to the present time, however, the instrument has had 

 the disadvantage of not being self-contained. The automatic 

 mercury valves which operate to prevent the water, absorbed 

 by the porous cup in times of rain, from entering the reservoir, 

 are externally situated. This renders the valves liable to break- 

 age and rather difficult of adjustment. For this reason it seemed 

 well to modify the instrument in such a way as to be self-contained 

 and at the same time to reduce the liability of breakage and the 

 difficulty of adjustment to a minimum. With this end in view 

 the present form of the instrument was devised. 



The arrangement of the different parts of the instrument is 

 shown in diagram in figure 1. From the reservoir (F) two glass 

 tubes (A and B) extend upward through a paraffined cork stop- 

 per, and then through a two-perforate rubber stopper into the 

 porous cup, one passing to the tip of the cup, the other just to 

 the upper surface of the rubber stopper. These tubes are of 

 small bore, about 0.8 mm. inside diameter. Each is bent into a 

 U and continued upward as A ' and B' . One (A ') extends through 

 the paraffined cork stopper and ends about 5 cm. above it. The 

 other (B') extends upward 6 cm. to 8 cm. and is again bent into 

 a U (inverted), thus forming a loop, and terminates near the 

 bottom of the reservoir. The tube B is expanded into a small 

 bulb C, at its lower extremity,- and the tube A' , is expanded into 

 a similar bulb 1 cm. to 2 cm. from its lower end. The tube E, 



1 Livingston, B. E., A rain-correcting atmometer for ecological instrumentation. 

 Plant World 13: 79-82, 1910. 



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