130 



TABLE X. 



WEIGHT OF VAPOR, IN GRAINS TROY, 



CONTAINED IN A CUBIC FOOT OF SATURATED AIR, UNDER A BAROMETRIC PRESSURE OF 

 30 ENGLISH INCHES, AT TEMPERATURES BETWEEN AND 105 FAHRENHEIT. 



THE weight of a litre of dry air at the temperature of zero Centigrade, or 32 

 Fahrenheit, and under a barometric pressure of 760 millimetres, as determined by 

 the experiments of Regnault (Memoires de Vlnstitut, Tom. XXI. p. 157), and corrected 

 for a slight error of computation (see above, p. 38), is 1.293223 grammes. The co- 

 efficient of expansion of the air, according to the same physicist, is 0.00367 for 1 

 Centigrade ; and the theoretic density of vapor is nearly 0.622, or -|, of that of the air 

 at the same temperature and pressure. From these elements the weight of the vapor 

 contained in a determined volume of air, the temperature and humidity of which are 

 known, can be deduced. 



Reducing these values to English measures, 1 litre being = 61.02705 cubic inches, 

 and 1 gramme = 15.43208 grains Troy, we have 



1.293223 grammes = 19.9571208 grains, 

 and 



61.027051 cubic inches : 19.9571208 grains : : 1 cubic inch : 0.32702 grain. 



Therefore, the weight of a cubic foot of dry air, at 32 Fahrenheit, under a pressure of 

 760 millimetres, or 29.922 English inches, is = 0.32702 grain X 1728 = 565.0923 

 grains Troy. Under a barometric pressure of 30 inches, it becomes 



22 X 5 65.0923 = 566.5654 grains. 



The coefficient for the expansion of the air becomes 0.0020361 of its bulk for 1 

 Fahrenheit. 



Now, if we call 



t the temperature of the air ; 



W = the weight of vapor in a saturated air at the temperature t ; 

 F = the maximum of the force of vapor due to the temperature t, as given 

 in the tables ; 



then the weight of the vapor contained in a cubic foot of saturated air is given by the 

 formula 



w = 622 _ 566.5654 grains ^ F_ . 



1 -h 0.002036 X (t 32) 30 ' 



from which the values in Table X. have been computed. The forces of vapor due to 

 the temperatures in the first column are those of Regnault, as given in Table VI. 



It is evident, that, in order to find the weight of the vapor contained in the air 

 at any state of humidity and pressure, it suffices to substitute for the normal values of 

 -- the force of vapor and the barometric pressure given by the observation. 



B 94 



