Ixviii INTRODUCTION. 



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

 temperatures the values of d in the Table on page Ixvii should be employed. 



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

 from Tables 76 and yy. 



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



/ is the temperature in Centigrade degrees. 



Whence we have 



W (grams) =^ 1.0^821 x 



^^ ^ 1+0.003670^ 



Table 81 is computed from this formula and gives the weight of satu- 

 rated vapor in grams in a cubic meter for dew-points from —70° to +40°9 C, 

 the intervals from —35° to 40°9 C, being o°i C. The tabular values are 

 given to three decimals for temperatures above — 4i°5, and to four decimal 

 places for temperatures below —41? 5. 



The weight IVi 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 2)~° P- and at a 

 pressure of 760 mm. or 29.921 inches is 



? / • N 1292.78 X I S-4.323q , ^ 

 S, (grams) = (3.280833)^ "^ ^^-^•^4- 

 We have therefore, 



IV, (grams) = X — — ^ =^ II-7459 



29.921 i+ai(^-32°) ■'"'^^ 1+0.002039 (fi- 32°) 



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

 pressure e,, expressed in inches, is obtained from Tables 74 and 75. 



Table 80 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 o°2 from 7o°o to 

 Ii9?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, Reg- 

 nault and others is, in its simplest form, 



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

 in which / = Air temperature. 



^' = Temperature of the wet-bulb thermometer. 

 e= Pressure of aqueous vapor in the air. 

 e'= Vapor pressure, saturated, at temperature /'. 

 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 f'. 



^ The latest adopted value of S = 1.2028 makes this factor 1.05822, and in a few cases, 

 especially at high temperatures, increases IV by O.OOI over the values given in Tables 81 

 and 80. ^564-95 with 5=1.2928. 3 11. 7461 with 5=1.2928. 



