EVAPORATION AND PLANT DEVELOPMENT. 27-? 



more from the jar below, this being raised by atmospheric 

 pressure upon the free water surface. 



After the lapse of a time period the cork stopper is 

 loosened, slightly raised and slipped sidewise as far as the 

 tube will permit (its lower surface resting on the edge of 

 the jar) and distilled water is poured from a graduated ves- 

 sel into the jar till the water level is brought again to the 

 file mark- The amount of water required for refilling is the 

 amount which has been evaporated during the preceding 

 period. 



Only pure water can be used in this atmometer, on ac- 

 count of a rapid clogging of the pores of the cup due to the 

 formation of crystals when impure water is useci. Water 

 from an ordinary still is satisfactory. To prevent bacterial 

 and algal growth in and on the cup, which might result in 

 the clogging of the pores, the addition of about three per cent 

 of formaldehyde to the water is recommended. This has no 

 perceptible effect on the operation of the instrument. 



The cups as received from the factory are of course not 

 uniform in their porosity, and it is necessary to standardize 

 them by comparing there evaporation rates under the same 

 conditions. A coefficient of correction is thus obtained, which 

 is applied to the ciirect readings after the instruments have 

 been placed in the localities to be studied. In standardizing, 

 a certain cup (or several cups with equal rates) are set 

 aside as standard, and not used excepting for these tests. 

 Suppose cup A and Standard cup S to evaporate under the 

 same conditions and in the same time the quantities a and s 

 respectively. Then /v=- -^' , where A' is the coefficient of 

 correctic>n for the cup A. When A is in operation it is only 

 necessary to multiply the reaciings obtained therefrom by this 

 coefficient, to obtain the reading which cup S would have 

 given for the same time and place ; from the above equation, 

 s == /v a= While the cups do not appreciably alter their por- 

 osity during several months of use with pure water, as has 



