10 JOHN W. SHIVE 



cup above the water level in the reservoir. The mercury in B 

 B' ', is drawn into the bulb C, where water rising from the reser- 

 voir is freely allowed to pass, supplying, in the usual way, the 

 water lost from the surface of the cup. The mercury columns 

 in the tube A A', and B B', remain in equilibrium in the position 

 indicated in figure 1, so long as the water loss from the surface 

 of the cup by evaporation equals or exceeds the absorption 

 from without by any part of its surface. In times of rain, when 

 the water loss from the surface of the cup by evaporation is less 

 than the absorption from without, the automatic mercury valves 

 become reversed. The mercury column falls in A, and rises in 

 A', at the same time that the mercury in bulb C, drops into the 

 tube below and rises in the tube B' (the height to which the mer- 

 cury rises in this tube depending upon the height of the cup above 

 the water level in the reservoir), thus effectually preventing 

 water from entering the reservoir from this direction. The 

 readings obtained give the actual evaporation minus the error 

 introduced by the volume change required for the operation 

 of the mercury valves, the value of the error thus introduced 

 depending upon the number of complete reversals of the valves. 



Harvey 2 has estimated the error permitted by the Livingston 

 mercury valves to be no greater than 0.01 cc, assuming the 

 glass tubing used to have an inside diameter of approximately 

 0.8 mm. and assuming the cup to be not more than 30 cm. above 

 the water level in the reservoir. In the present modified form of 

 the instrument the volume change required for the operation 

 of the automatic mercury valves is no greater than that esti- 

 mated by Harvey. It is evident, therefore, that the error 

 involved in one or several complete reversals of the valves is 

 quite negligible excepting in cases where extreme accuracy is 

 essential. 



2 Harvey, E. M., The action of the rain-correcting atmometer. Plant World 

 16: 89-93, 1913. 



