THE RAIN'-CORRECTING ATMOMETER 



91 



operation of the valves) depends upon the height of the cup above 

 the water level in the reservoir. In the ordinary field use of 

 the cup the usual height is perhaps about 25 to 30 cm. The mer- 

 cury column necessary- to support this water column is 1.9 to 2.2 

 cm., so, taking into account the volume of the tubing per centi- 

 meter, the error for each operation of the valves is only about 0.01 



Diagram showing construction of the rain-correcting atmometer. (Reprinted 

 from Livingston, The Plant World 13: 79. 1910.) 



cc. The back flow allowed in the operation of the valves is thus 

 seen to be quite insignificant in the field vise of the cup. 



An idea of the amount of water absorbed during rains by the 

 standard cup when unprotected by a mercury valve may be had 

 from the following experiment. Standard cups, not supplied 

 with the non-absorbing device, were subjected to the action of 



