CLIMATIC FACTORS 



139 



When water is evaporated, the level in the reservoir falls and the 

 amount of water evaporated is read from the changes in the surface 

 level and expressed in cubic centimeters per hour or per day, from 

 the standard atmometer. The atmometers most widely used are 

 those devised and improved by Livingston (1915). As the atmometer 

 cups vary somewhat individually, each one is compared with a stand- 

 ard cup and is given a coefficient of correction. Thus if the coefficient 

 of correction for a given cup is 0.70 and the loss from the cup for the 

 given time is 40 cc, then 40 X 0.70 = 28 cc, 

 which is the rate of evaporation from the stand- 

 ard atmometer for the given place and time. 



The improved Livingston-Thone atmometer 

 mounting (Fig. 76 A) consists of a broad bottle 

 with rubber stopper through which pass two 

 glass tubes, the one (a) for filling the bottle 

 with distilled water, the other (6) for connect- 

 ing the porous cup with the water within the 

 bottle. When the instrument is set up, the 

 bottle is filled to a mark on the tube a, and 

 by measuring the water required to refill to the 

 same mark the loss for the period of exposure 

 may be readily and accurately ascertained. 



Since all types of porous porcelain atmom- 

 eters will absorb water from rain or mist, re- 

 turning it to the reservoir and thereby falsifying 

 the reading, it is necessary to equip the mount- 

 ings intended for field work with some type of 

 rain-proofing valve. The valve most widely 

 in use is one devised by Livingston and Thone 

 (1920) (Fig. 76 A, c). This consists of a column 

 of mercury, about 5 mm. high, resting on a plug of wool, absorbent 

 cotton, or other porous material e. The mercury acts as a highly 

 flexible yet perfectly sealed check valve, permitting upward move- 

 ment of water but preventing a reversal. A second plug of porous 

 material / higher in the tube prevents loss of mercury when the 

 apparatus is being handled. Other successful rain-proofing valves for 

 atmometers, devised by various workers, are described by Thone 

 (1924). 



A convenient modification of the above mounting consists in 

 making the valve in a bit of glass tubing about 5 cm. long (Fig. 76 B) 

 and attaching this to the lower end of the tube within the bottle by 

 means of a short piece of rubber tubing. 



Fig. 76.— ^.TheLiv- 

 ingstone-Thone atmome 

 ter mounting; B, a detail 

 of the rainproofing valve. 



