98 BURTON EDWARD LIVINGSTON 



with water, prevents the passage of undissolved air until an 

 air pressure of from three to five atmospheres is applied. With 

 such high pressures, and only with them, does the air force its 

 way through the porous walls, driving out more or less of the 

 imbibed water. 



As the atmometer is set up for operating, the unbalanced 

 air pressure exerted on the outside of the wet cup wall never 

 amounts to more than a meter of water column, say 10 cm. of 

 mercury at most, which is only about one-seventh of a single 

 atmosphere. Thus the films in question are never strained so 

 as even to approach the breaking point, and the pressure of the 

 air is as completely removed from the water within the cavity 

 as though the cup were of glass. On the water surface in the 

 reservoir the full atmospheric pressure is freely applied, however, 

 and this is sufficient to support a water column in the tube and 

 cup many times as long as this column ever actually is in any 

 operating instrument. 



When rain falls upon the cup, however the menisci closing 

 the pores are destroyed, the external water surface being built 

 up through addition of water from without, so that there are 

 now no menisci at the outer ends of the pore openings. The 

 result is that the water surface comes to be a free one, situated 

 some small distance outside of the exterior surface of the porous 

 wall. This free water surface receives the full atmospheric pres- 

 sure, which tends to drive water through the cup wall, with a 

 pressure as great as that of the water column extending from 

 the cup surface to the level of the water surface in the reservoir 

 below. Virtually the whole apparatus now behaves as a porous 

 clay filter with a water column below "drawing" water through 

 it. The higher the water column of the instrument the more 

 rapidly will water pass in, under these conditions. 



The rate of absorption of the standardized 8-cm. cup, 30-cm. 

 above the reservoir surface, when the cup is kept externally 

 covered by water (as in a heavy shower) may be over 4 cc. per 

 hour. 20 It is clear that such rates of absorption during rainy 



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

 16:89-93, 1913. 



