272 THE PLANT WORLD. 



a cork stopper nearly to the bottom of a glass jar of the 

 "Mason" pattern. The cork stopper fits the jar somewhat 

 imperfectly, or has a slight groove cut in its margin to allow 

 access of air. Above the jar the tube passes through a 

 conical cap of cloth coated with shellac, which serves to shed 

 rain water and prevent its direct entrance to the apparatus. 

 A file mark is made on the wall of the jar near the shoulder, 

 and the tube is pushed into the cup far enough to allow its 

 free end to reach the bottom of the jar and still leave a space 

 of ten centimeters between the file mark and the base of the 

 cup. A pint, quart or half-gallon jar is used, according to 

 the evaporation rate and the time period during which the 

 instrument is to operate between fillings. The crudeness of 

 the instrument is counterbalanced by its simplicity and by the 

 ease with which all parts excepting the cup may be obtained 

 in almost any locality. The arrangement of the parts is 

 shown diagramatically in fugure. The file mark on the jar 

 is indicated at O in the figure. 



In setting up this atmometer, the jar is partially filled 

 with distilled water, the cup (after being soaked in distilled 

 water for a short time to remove air) is filled and its stopper 

 pushed home, the tube is filled, and the free end of the latter 

 is quickly thrust to the bottom of the jar, or until the cork 

 stopper rests on the edge of the latter, care being taken that 

 no air bubbles enter the tube. The jar is next filled to the 

 file mark, the cork stopper placed in position, and the instru- 

 ment is ready for operation. 



When the porous clay of the cup is saturated with water, 

 as it is when the cup is filled, the surface tension of the water 

 films closing the pores at the outer surface is so great that air 

 cannot penetrate, and the cup remains filled, although it is 

 above the level of the water in the jar. Evaporation pro- 

 ceeds from the surface of the wet clay, and water is drawn 

 into the pores to replace what has been lost. The water thus 

 removed from the cavity of the cup Is in turn replaced by 



