ATMOMETRY AND THE ATMOMETER 67 



periods of frost; porous cups burst and leave blanks in the record, 

 open pans freeze and introduce misleading readings into the 

 record, readings which are not directly comparable with those 

 from the same instrument with liquid water. 



THE POROUS CUP ATMOMETER 

 The Porous Cups 



The atmometer cups now generally used in ecology, agricul- 

 ture and forestry are practically of the same form and size as 

 those described by the writer in 1906. The only improvement has 

 been to make the closed end more nearly hemispherical, less 

 pointed, than was the case at first. This change was made in 

 1913. These cups are cylindrical, about 13 cm. long and 2.5 cm. 

 in diameter, closed at one end and strengthened at the other by 

 a thickened rim. The wall is from 3 to 4 mm. thick, with the 

 exception of the rim, which is about twice as thick. They are 

 white, with a smooth, porous and absorbent exterior surface. 

 The cup is closed, in use, by a rubber stopper which bears the 

 tubing connection to the reservoir. 



Since it is absolutely essential that the evaporating surface 

 change as little as possible, it is unsafe, in handling, to allow 

 contact of the hands with the porous surface. To avoid the 

 necessity of this, the basal portion (at the open end) of each cup 

 is made water proof; details of manufacture have so far made 

 use of a glaze for this purpose impossible and ordinary orange 

 shellac in alcohol has been mostly employed as coating. Melted 

 sulphur, into which the previously heated cup is dipped, gives a 

 more serviceable coating, in some respects, but its application is 

 somewhat troublesome. Before the coating is put on each cup is 

 numbered on its rim, so as to be easily identified in use. The 

 dipping in shellac is repeated till a satisfactory covering is 

 obtained. 



The original standard cup of 1907 was free of coating for a 

 length of 6 cm. from the closed end. Some cups were still more 

 highly coated, but it was found more advantageous to have a 

 large evaporating surface (both because of reduced error in read- 



THE PLANT WORLD, VOL. 18, NO. 3, 1915 



