MEASURING OF AIR. 439 



TABLE III. 



H'eifjht of moisture present. 

 Test Humidity Grs/cu.ft. Lbs/cu.ft. 



In order to calctilate the w^eight of dry air ])er cul)ic foot, 

 it must be remembered tliat the atmospheric i)ressure is made 

 up of the pressure due to the ch-y air /^liis that of the water 

 vapour present and that it is only the former that can be applied 

 to the state equation of dry air. For this calculation the follow- 

 ing physical constants have been taken : 



Weight of I cubic foot of dry air at 32° F. is 0.08073 lbs. 

 under a pressure of 14.696 pounds per scjuare inch and the 

 standard value of gravity. 



Hence the weight of i cub. ft. -— 2.699 P ^■ 



wdiere p is the pressure of the dry air in Ibs/sq inch. 



and 7" the absolute temperature in degrees Fahrenheit. 



TABLE I\'. 

 }J'ei(jht of -tcorking air. 



In the second column of the above Taljle pa is the observed 

 barometric pressure (see Table 11) and / is the i)artial pressure 

 due to the water vapour (Table II). 



The correct value of R in the state equation of pz' = R.T. 

 can now be calctdated. In English measures : — 



p = 14.696 Ibs/sq.in. = 14,696 X 144 Ibs/sq.ft. 



T = 491.4 



and since the weight of a cub. ft. of dry air =: 0.08073 



pounds 

 z' = 1/0.08073 L^nbic feet per pound. 



Hence R = 14.696 X 144 49^-4 X 0.08073 — 53.36) 



