1 825.] Bd/rometrical Measurement of Heights, 10 



equal volume of the dry air of the compound would Weigh 



i^ of 1 , or * 0-60 



The cubic foot of vapour supporting the remaining pres- 

 sure (10 inches) would weigh ^ of ^f of 1, or , 025 



Sum.... ». 0-85 



The densities being as the weights, the moist air would be 

 specifically lighter than dry air under the same observed pres- 

 sure in the ratio of 85 to 100. 



(46.) Were the density of aqueous vapour equal to that of dry 

 air, the density of the mixture would not differ from that of dry 

 air alone, observed under the same pressure ; but being |. 

 lighter, we must reduce in the same ratio what would have been 

 on the former supposition its proportion of the total weight of 

 the compound, equal to its proportion of the elasticity of the 

 whole (or -i-2- of 1). 



Example, — Density at 25 inches , , I'OO 

 Deduct f of -12. of 1. . . 0- 15 



0*85 as before. ^ 



But this method of expressing the ratio of density of dry to 

 moist air has the defect of not exhibiting the value of their dif- 

 ference in the most striking point of view, and would intolerably 

 encumber and embarrass our calculations. We shall find it 

 more convenient and intelHgible to indicate at what (superior) 

 temperature the density of dry air would be equivalent (or be 

 reduced) to that of moist air observed under the same pressure. 



To render the proposed method the more intelhgible, we shall 

 confine our calculations in the first instance to air in a state of 

 complete saturation. Suppose, for example, that we had 480 

 volumes of dry air of a density, which we will call 480, at the 

 temperature of 32° F. when supporting the pressure of 30 inches 

 of mercury ; required to know at what other (superior) tempera- 

 ture the density would be equal to that of saturated air support- 

 ing thie saiiie pressure, and existing in a temperature of 90° F. ? 



Calculation, 



at 32-00° F. volume 480-00 Density 480-000 



Dry air ^ 90-00 538-00 428-253 



99-78 547-78 420-598 



"'1 



Saturated air at 90°. Dew-point 90°. (Force of the vapour 

 1-43 inch.) Density per formula 420-598 (or 428*253 minus f 

 of^c^^ of 428-253). 



Hence the density of saturated air observed at a temperature 

 of 90° F. and tinder the pressure of 30 inches of mercury, is 

 j)reciBely equal to that of diy air under the same pressure, and 



Nei9 Series, vol, x. r 



