HYGROMETRICAL TABLES. llX 



With this value the psychrometric formula in metric measures becomes 



e = e' — 0.000660 B (t — t') (1 + 0.00115 /')• 

 Expressed in English measures, the formula is 



e = e' - 0.000367 B (t - /') [1 + 0.00064 W - 3 2 °)1 



= e'- 0.000367 B(t- t') (1 + ^f) 



in which e = Vapor pressure in inches. 



e' = Pressure of saturated aqueous vapor at temperature /'. 

 / = Temperature of the air in Fahrenheit degrees. 

 t' = Temperature of the wet-bulb thermometer in Fahrenheit 



degrees. 

 B = Barometric pressure in inches. 



TABLE 75. 



Table 75. Reduction of Psychrometric Observations — English measures. 

 Values of e = e' — 0.000367 B (t — t') I 1 H — J 



This table provides for computing the vapor pressure, e, from obser- 

 vations of ventilated wet- and dry-bulb Fahrenheit thermometers. From 

 the vapor pressure thus computed the dew-point and relative humidity of 

 the atmosphere may be obtained. 



The tabular values of the vapor pressure, e, are computed for degree 

 intervals of /' from — 20 to + 1 io° F. Below + io° the interval for t — t' 

 is o°2, and above io° the interval is i°. The computation has been made for 

 B = 30.0 inches, but at the bottom, and usually, also, at the top of each 

 page of the table is given a correction, Ae X AB, computed for B = 29.0 

 inches or AB = 1 inch, and for the value of /' indicated. The correction is a 

 linear function of AB. For atmospheric pressures less than 30.0 inches, it 

 is to be added to the tabular values of e, while for atmospheric pressures 

 greater than 30.0 inches it is to be subtracted. 



The values of e are given to 0.000 1 inch for t' less than io°, and to 0.00 1 

 inch for /' greater than io°. 



Examples : 



1. Given, t = 84^3; /' = 667, and 5=30.00 inches. With /' = 667 

 and t— t' = I7°6 as arguments, Table 75 gives for e the value 

 0.462 inch. On page 174, for t— t' = 0.0 it is seen that a vapor pres- 

 ure of 0.462 inch corresponds to a temperature t' — t = 57°, which 

 is the saturation, or dew-point temperature for the data given. 



2. Given, t = 34^5; t' = 20/4; B = 22.3 inches. With t' = 294 and 

 t — t' = 5°i as arguments, Table 75 gives for e the value 0.104. 



AB = 30.0 — 22.3 = 7.7, and Ae X AB = 0.0018 X 7.7 = 0.014. 

 Correct value of e =0.118 inch 



