104 BURTON EDWARD LIVINGSTON 



a suitable vessel. As evaporation went on mercury rose in the 

 tube, following the receding water column. It is easily possible 

 thus to obtain a mercury column over 60 cm. in height. It 

 was conclusively shown that the rate of evaporation is not 

 appreciably less with such a mercury column than it is with only 

 the usual water column Such being the case the question of the 

 influence of height of cup above reservoir, as far as the operation 

 of the atmjometer is concerned, may be dismissed. 



The Influence of Air in the Cup. If air bubbles enter the cup 

 they must either remain there or pass out by dissolving and 

 diffusing through the water of the cup wall. To remove such air, 

 the cup is removed and refilled, in the case of the absorbing 

 form; or strong and intermittent suction is applied to the lateral 

 tube, with the non-absorbing form. 



The most obvious effect produced by air in the cup is the 

 variable error caused by the thermometer and barometer action 

 of this air. It is to be remembered that such a body of gas is 

 subjected to less than atmospheric pressure and that barometric 

 and temperature changes in -the surroundings will produce corre- 

 sponding changes in the gas volume, much more pronounced 

 changes than these similarly produced in the liquid. These 

 changes, transmitted to the reservoir, are the sources of error 

 here in view. 



Another effect of air arises from the fact that such a mass 

 of gas, lying at the top of the atmometer cup, prevents free 

 contact of the porcelain wall with the internal water at that point. 

 In so far as this occurs it is clear that the renewal of water in that 

 part of the porous wall thus cut off from direct supply, must 

 occur by lateral or longitudinal movement through the wall. 

 Thus the portion of the' porous porcelain which is on both sides 

 in contact with gas must operate quite as does the disk or cylinder 

 of the paper atmometer already discussed. This being the case, 

 the same sort of difficulty is conceivable here as occurs with the 

 paper form when the paper is large and the evaporation intense; 

 that is, water may be removed from the outer surface of the air- 

 protected portions of the porous wall at a rate faster than it can 

 be supplied by capillary movement from adjacent, directly 



