304 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



standard evaporating surface) may be readily found by employ- 

 ing a table of aqueous vapor-pressures and the formula for rt 

 given above. 



It will be seen at once from the formula that 



p 



Rtt = RtX — 



Now, Rt (which is Ri^ when 20° is taken as standard temperature) 



is a characteristic constant for any slip of paper, to be found 



by looking up the shp, by its number, in the standardization 



record. Ft is 17.406 mm.,'^ a^^j j>^, ^n^y be found from a table 



of vapor-pressures, being the vapor pressure corresponding to 



the field temperature with which the leaf tests are being made. 



As in other similar cases, it is convenient to calculate the 

 p 



ratio — for all the temperatures for which it may be needed 



and to arrange these values in the form of a table for field use. 

 Our table 2 is such a table of ratios, for all temperatures from 

 0° to 45.9°C., by increments of 0.1°. From the nature of their 

 derivation, each of the values here given is a coefficient of cor- 

 rection by which must be multiplied the characteristic time 

 period at 20° {Ri)i for any slip of paper, in order to obtain the 

 corresponding time period for the same slip of paper (on the 

 standard evaporating surface) at the temperature to which the 

 coefficient refers. Following the usual custom in such cases, 

 whole degrees of temperature are shown in the first column of 

 table 2 and the added tenths are shown at the heads of the 

 other coluimis. Thus, the coefficient for 15.0° is 1.37, that for 

 15.4° is 1.34, that for 20.0° is 1.000, that for 21.6° is 0.91, etc. 

 The vapor-tensions for these calculations have been taken from 

 Broch's table, as given in the Smithsonian meteorological tables,'^ 

 and the values are slightly different from those employed for 

 our table 1. 



12 From Van Nostrand's chemical annual, 1909. 



** Smithsonian meteorological tables, 3rd revised edition. Smithsonian 

 Institution, Washington, 1907. Table 43, which is from "Broch, Trav. Mem. 

 Bur. Internat. Poids et Mesures 1: A19-39." 



