Si On the Hygrometer by Evaporation, 



and ,r the unknown tension of the aqueous vapour : then the 

 weight of a cubic foot of the atmospheric air will, in these cir- 

 cumstances, be equal to the weight of a cubic foot of dry air 

 under the pressure h—x^ and the weight of a cubic of vapour 

 of the tension .r, the common temperature being t. 



Let "tO stand for the weight of a cubic foot of vapour at the 

 temperature zero, the tension, as determined by Dalton, being 

 i of an inch of the barometric column : then the weight of a 

 cubic foot of dry air at the same temperature and pressure will 

 be |- X tu * ; and consequently the weight of a cubic foot of 

 the same air at the same temperature and under the pressure 

 30 inches, will be 5 X 30 x f x it), or 24<0 x tt\ 



Now, 24<0 X "w being the weight of a cubic foot of dry air 

 at the teinperatui'e zero and pressure 30 inches, tlie weight of 

 the same volume when the temperature is t, and the pressure 

 i— .r, will be 



h—x 1 



240 X tt X — —- X 



30 . I+ott' 



where vi — -003 75, or the expansion for one centesimal degree: 

 and the weight of a cubic foot of vapour of the temperature t, 



and tension x\ will be Stu x ~- — • Wherefore the weight of 



1 -f WIT ° 



a cubic foot of atmospheric air originally considered will be 

 thus expressed^ viz. 



24'Ow X -^ X -— 1- 5 to X 



30 l-j-niT l+»iT 



Let a stand for the specific heat of air, and s for that of 

 vapour : tlien we get 



j 24-Ow) X — ^ X -^ + 5 w X -4^ \ X St 



( l + mr 30 1-|-tot) 



for the weight of water that would be raised one degree of the 

 thermometer, by the heat extricated in cooling the mixed mass 

 of air and vapour a number of degrees expressed by St. 

 Again, put X for the latent heat of steam : then 



\ 



240 X — X — — -f 5WJ X 



l-(-??lT 30 l+7?ir ) A. 



will be the weight of steam due to the above-mentioned quan- 

 tity of heat ; or the weight of vapour that contains in a latent 

 state all the heat extricated in cooling the mass of air to the 

 temperature t — St. 



^^ hcrcforc we have the following weight of dry air and va- 



* I3iot"s Trn'xti- dc Fhyw^ur^ vol. i. pp. ?:)6, 297. 



pour 



J 



