180 



TABLE XVII. 



FOR COMPARING THE WEIGHT OF A CUBIC FOOT OF DRY AND OF SATURATED AIR. 



THIS table is composed of two tables found in the Greenwich Meteorological 

 Observations for 1842, pages xlvi. and li. ; the first containing the weight of a 

 cubic foot of dry air, under a barometric pressure of 30 inches, at temperatures be- 

 tween and 90 F. ; the bther giving the weight of a cubic foot of saturated air 

 under the same barometric pressure and temperature, together with the excess of the 

 first above the last. 



The weight of a cubic foot of dry air, on which the tables are based, is assumed to 

 be 563 grains Troy, being a mean value, in round numbers, between the determina- 

 tions of Shuckburgh, which is 557.7295 grains, and that of Biot and Arago, 568.7013. 

 The true mean is 563.2154, but 563 is the number used in the calculations. 



The coefficient of the expansion of the air is that of Gay-Lussac, viz. 0.00375 for 

 1 Centigrade, or 0.002083 of its bulk for 1 Fahrenheit. 



Use of the Table. 



This table shows the amount of buoyancy imparted to the air by the addition of 

 moisture ; and from it, the temperature and the relative humidity of the air being 

 known, the weight of a cubic foot of air, in the actual condition of the atmosphere at 

 the time of an observation, can be deduced. 



It suffices to take in the fourth column, headed " Excess," the quantity corre- 

 sponding to the temperature of the air in the first, multiply it into the given Relative 

 Humidity, and subtract the product from the number in the second column. The 

 result will be the weight of a cubic foot of air at the existing temperature and 

 moisture, under a barometric pressure of 30 inches. 



This result will be reduced to its true value, under the barometric pressure given 



Height of Barometer 

 by the observation, by multiplying it by - *. r^ 



Example. 



The temperature of the air is 60 F. ; the relative humidity, 0.852 ; the barom- 

 eter reads 29 inches. 



The table gives, for temperature of air, 60 ; excess, 3.35 x 0.852 2.85, which, 

 subtracted from 531.91 in the second column, = 529.12, weight of a cubic foot 



f)O ij,,.]| 



of air under 30 inches of pressure ; and 529.12 x ~ = 511.48, the weight of 



a cubic foot of air in the given conditions of temperature, moisture, and barometric 

 pressure. 



B 144 



