lx INTRODUCTION. 



For t— t' = o°o a vapor pressure of o. 1 1 8 inch corresponds to a tem- 

 perature t' — t= 23 (see page 174), which is the saturation or dew- 

 point temperature for the data given. 



Table 76. Relative humidity — Temperature Fahrenheit. 



The table gives the vapor pressure corresponding to air temperatures 

 from — 30 to +120 at degree intervals (side argument) and for percen- 

 tages of saturation at 10 per cent intervals (top argument). It is computed 

 from the formula 



e = e s X relative humidity, 

 where e s is the saturation vapor pressure at the given air temperature. Below 

 a temperature of 20 the values of e are given to 0.0001 inch; above 20 

 they are given to 0.001 inch. 



Examples: 



1. In dew-point example 1, above, the computed vapor pressure is 

 0.462 inch. Entering Table 76 with air temperature 84 3 as side 

 argument, we obtain vapor pressure 



0.356 inch = relative humidity 30 



and 



90 

 0.462 inch — 0.356 inch = 0.106 inch = — = 9 



therefore, vapor pressure — 



0.462 inch with t = 84^ F. = " " 39 



2. In dew-point example 2, above, the computed vapor pressure is 

 0.1 18 inch. Entering Table 76 with air temperature 34°5 as side 

 argument, we obtain, vapor pressure 



0.100 inch = relative humidity 50 



and 



90 

 0.1 18 inch — 0.100 inch =0.018 inch = — =9 



10 



therefore, vapor pressure — 



o. 1 18 inch with /= 34.5 F. = " 59 



Reduction of Psychrometric Observations — Metric measures. 

 Table 77. Values of e = e' — 0.000660 B (t — t') (1 + 0.00115 t') 



This table provides for computing the vapor pressure from observations 

 of ventilated wet- and dry-bulb Centigrade thermometers. "From the vapor 

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

 mosphere may be obtained. 



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

 intervals of t' from — 30 to + 45 C. Below — 5°0 the interval for / — t 1 



