of Barometrical Mensuration. 83 



To compare this with the results of modern experiments, we 

 must have recourse to calculation. The accurate experiments 

 of Mr. Dalton will furnish the data. A volume of diy air of 

 given elasticity being mixed with vapour, also of known elasti- 

 city, will have its volume increased in proportion to the elasti- 

 city of the mixture. Thus, a cubic foot of air of the tempe- 

 rature of 212° which would support a column of mercury of 

 30 inches, being mixed with a cubic foot of vapour of 212° 

 also of the elasticity of 30 inches, would occupy a space of two 

 cubic feet. 



For 30 inches : 60 inches : ; 1 : 2. 



Now from the experiments of Mr. Dalton, we are acquainted 

 with the force of vapour for every degree of a very long range 

 of the thermometric scale, and if we assume the volume of dry 

 air at 0° temperature, and 30 inches* pressure, as 1.00000, the 

 calculation is sufficiently simple. For example, let it be re- 

 quired to know the expansion which would take place in air in 

 contact with water by a rise from 0° to 32°. The force of vapour 

 of that temperature, according to Mr. Dalton's table, is 0.200 

 inches, therefore 



30.000 : 30.200 :: 1.00000 : 1.00666. 



This is the expansion which would arise from vapour only : 

 to this we must add the expansion which would take place 

 from the addition of heat, viz. ^l^th part for every degree. 



Now .002083 X 32«' = .066656, which added to 1.00666, 

 makes the total expansion 1.07332. 



In this manner I have calculated the following scale, corre- 

 sponding with General Roy's : 







32 



52 



72 



92 



112 



132 



152 



172 



192 



100000 

 107332 

 112169 

 117565 

 123966 

 132266 

 142832 

 157699 

 178266 

 206199 



212 .. . 244166 

 G2 



