68 BURTON EDWARD LIVINGSTON 



ing and because the larger surfaces remain without serious al- 

 teration for a much longer time) , and most of the cups used and 

 distributed for the last five years have had uncoated portions 

 8 cm. long. Thus the evaporating cylinder has the last-named 

 length, an outside diameter of about 2.5 cm. and a hemispherical 

 closed end, the whole being continuous below with the non- 

 porous remainder of the cup. 



Other sizes and shapes of cup have been described from time 

 to time and some of these have been furnished to the market. 

 Yapp's 16 and Transeau's 17 cups differ from the usual one in minor 

 points, but neither of these has come into very general use. The 

 present writer has been able to procure, especially, two other 

 cups, which have been placed upon the market. One of these, 

 the insoluble cup, is of about the same form as the regular one, 

 somewhat smaller, and made of a much more resistant material. 

 In future this will have the same size and form as the regular cup. 

 It has a glazed base, making shellac unnecessary. The other 

 cup mentioned is termed "thin-walled." It is a straight cyl- 

 inder 14 cm. long, 1.8 cm. in diameter, closed above by a some- 

 what flattened top and glazed at the base to a height of 2.5 cm. 

 This was obtained largely for its very thin wall, about a milli- 

 meter in thickness, which suits it to certain comparative studies 

 which need not be discussed here. Both of these special cups are 

 now obtainable in pure white, though earlier lots of the insoluble 

 form were slightly yellowish in color. The two smaller forms 

 were designed for special uses. 



The Standardized Cup 



The main feature which has given the porous cup atmometer 

 its present importance as an instrument of research is that it can 

 be standardized and that standardization may be repeated as 

 often as is desired. The cups are standardized by operating 

 them with pure water, along with a cup of previous standardiza- 



10 Yapp, R. H., On stratification in the vegetation of a marsh and its relation 

 to evaporation and temperature. Ann. Bot. 23: 275-319, 1909. 



17 Transeau, E. N., A simple vaporimeter. Bot. Gaz. 49: 459-460, 1910. 



