ATMOMETRY AND THE ATMOMETER 103 



Height of Cups above Reservoir. The influence exerted by the 

 he'ght of the water column above the porous cup, upon the rate 

 of absorption during showers, is very pronounced unless the 

 non-absorbing mounting is used, as has been shown, and an 

 appreciation of this influence gives rise to the question what 

 may be the relation of this height to the rate of evaporational 

 water loss? This is an important matter, for the cups are oper- 

 ated at various levels and it is not always convenient for the 

 reservoir to be immediately beneath. Furthermore, as evapo- 

 ration proceeds and the water level in the reservoir is lowered, 

 the height of the water column in question is of course increased. 

 Especially is this true when a burette or other tall and slender 

 container serves as reservoir. 



The question here raised is important but its answer is simple ; 

 there is no appreciable relation between the rate of evaporation 

 from a porous cup and the height of the water column below it. 

 On theoretical grounds, the higher the cup above the reservoir 

 the more concave should become the very small water-air menisci 

 closing the pores of the wall. It should follow then thatthe vapor 

 tension of these water surfaces must be decreased, and the rate of 

 evaporation should also fall. The fact, however, is as above stated 

 and the thinker is left, as frequently, to make hypotheses to 

 explain it. It seems probable that the reason why no influence 

 of the height of the water column is demonstrable is this : that the 

 alteration induced in the films, and hence in the evaporation 

 rate, by as great a change in column height as is ordinarily pos- 

 sible, is too small to be detected by ordinary methods. 



It was first experimentally shown that variations in column 

 height of upwards of a meter were quite without influence on the 

 rate of water loss from the cup. 25 Afterwards variations of 

 about two meters were tested, with the same result. But the 

 most conclusive proof of the proposition before us was obtained 

 by Mrs. Edith B. Shreve, working in the writer's laboratory, at 

 Baltimore. Cups of the ordinary form were mounted on small- 

 bore glass tubes about a meter in length, and the lower end of 

 each tube was allowed to dip beneath the surface of mercury in 



25 Livingston, loc. cit., 1906. Page 27. 



