HYGROMETRICAL TABLES. 



lvii 



with greater accuracy. For all important calculations except those at low 

 temperatures the values of <5 in the Table on page lvi should be employed. 



e is the pressure of saturated aqueous vapor at temperature t, taken 

 from Tables 71 and 72. 



a is the coefficient of expansion of air for i° C: a = 0.003670. 



/ is the temperature in Centigrade degrees. 



Whence we have 



W (grams) = 1. 05821 X — ; ■• 



v& ° 1 + 0.003670 / 



Table 74 is computed from this formula and gives the weight of satu- 

 rated vapor in grams in a cubic meter for dew-points from — 29 to + 40. 9 

 C, the intervals from 6° to 40°9 C, being o?i C. The tabular values are 

 given to three decimals. 



The weight W 1 of a cubic foot of saturated vapor is obtained by convert- 

 ing the foregoing constants into English measures. 



The weight of a cubic foot of dry air at temperature 32°F. and at a 

 pressure of 760 mm. or 29.921 inches is 



. . . 1292.78 X 15.43235 

 oi (grains) = / ?. = 564-94- 



& (3.280833) 3 



We have therefore, 



Wi (grains) = X — : 37 = 11.7459 , t- 5^ 



v& ' 29.921 1 + ai (/1 - 32 ) '^ oy 1 + 0.002039 (ti -32 ) 



The temperature t x is expressed in degrees Fahrenheit; the vapor 

 pressure e lf expressed in inches, is obtained from Tables 69 and 70. 



Table 73 gives the weight of saturated aqueous vapor in grains per 

 cubic foot for dew points given to every degree from — 30 to + 20 , to 

 each half degree from + 20 to + 70 , and for every 0.2 from 70°o to 

 H9°8 F, the values being computed to the thousandth of a grain. 



REDUCTION OF OBSERVATIONS WITH THE PSYCHROMETER AND 

 • DETERMINATION OF RELATIVE HUMIDITY. 



The psychrometric formula derived by Maxwell, Stefan, August, 

 Pegnault and others is, in its simplest form, 



e=e'-AB (t-t'), 

 in which t = Air temperature. 



/' = Temperature of the wet-bulb thermometer. 

 e = Pressure of aqueous vapor in the air. 

 e' = Vapor pressure, saturated, at temperature t' . 

 B = Barometric pressure. 



A = A quantity which, for the same instrument and for certain 

 conditions, is a constant, or a function depending in a 

 small measure on /'. 



