Daniell on a tiew Hygrometer. 309 



cubic foot of water at its maximum of density, weighs 

 437102.4946 grains. The weight, therefore, of a cubic foot of 

 steam at the above temperature and pressure, is *^^l''yl'*''*^ or 

 257.1191 grains. From hence we may find the weight of an 

 equal bulk of vapour of the same temperature, under any other 

 given pressure, suppose 0.524 I for the volume being in in- 

 verse proportion to the pressure 



Ins. Ins. Gri. Grs. 



30. : 0.524: : 257.119 : 4.491 

 the weight required. 



Having now obtained the weight of a cubic-foot of vapour, at 

 a pressure of 0.524, and at a temperature of 212°, we may 

 proceed to find its weight under the same pressure, at any other 

 temperature, suppose 60°. The gasses, it will be remem- 

 bered, expand ■:^ part of their volume for every accession of 

 heat, equal to 1° of Fahrenheit's scale ; therefore reckoning as 

 unity a volume of gas at 0°, its volume at 60° is to its volume 

 at 212oas H-/8% 's to 1 +-H§, or; : 1.125 : ,1.441, there- 

 fore the density and weight being in inverse proportion to the 

 volume 



Ins. In«. Grs. Grs. 



1.125 : 1.441 : : 4.491 : 5.628 

 the weight of a cubic-foot of vapour at the temperature of 60°, 

 and a pressure of 0.524 inches. 



It must be further remembered, that it has been proved by Mr. 

 Dalton, that as much vapour of determined temperature is 

 formed in a given bulk of air, as in a vacuum of equal space ; 

 therefore the above result gives the weight of vapour, which 

 can exist in a cubic foot of the air at the temperature of 60°. 

 The fourth column of the table contains the proportionate ex- 

 pansion for the corresponding degrees. 



The calculations for these several series have been made to 

 the third place of decimals, which will be sufficiently accurate 

 for all common purposes. The manner of using the table will 

 be best understood from an example. 



Let the temperature of the atmosphere be 70" ; and the point 

 of condensation, as found by the hygrometer, 55°; the pressure 

 of the vapour under these circumstances, is immediately found 

 opposite to the degree of its constituent heat 55° 0.443. To 

 find its weight, we proceed thu:> ; — supposing as much as pes- 



