84 Daniell on the Correction 



This, upon the whole, exhibits a very surprising and grati- 

 fying degree of accordance, especially in the upper part, and con- 

 sidering the disadvantages under which the General laboured, 

 does infinite honour to his accuracy and perseverance. Little 

 more was wanting to have enabled him to complete the solution 

 of the problem, towards which he contributed so many steps^ 

 than an instrument to measure the quantity of the agent, the 

 effects of which he so well appreciated. 



But the expansion which vapour causes in air, is not precisely 

 similar to that occasioned by heat, for while it dilates its parts, 

 it adds its own weight to the mixture. Let it be required to know 

 the specific gravity of air at 32° saturated with vapour, compared 

 with dry air at the same temperature. Call the latter 1.00000. 

 The quantity of expansion will be .00666, which deducted from 

 1.00000 leaves .99334. Now the weight of a cubic foot of air, 

 under the conditions above named, is 563.414 grains, and the 

 weight of a cubic foot of vapour at 32° is about 2.317 grains, 

 which the former being 1.00000, will be nearly .00411, and 

 which added to the .99334 before obtained, will give .99745 

 for the specific gravity sought. 



Upon this principle I have constructed the following table, 

 by means of which, the specific gravity of any mixture of at- 

 mospheric air and aqueous vapour from 0°to 90° may readily be 

 found with sufficient precision. I have made air under a pres- 

 sure of 30 inches of mercury, and at the temperature of 32°, 

 the standard of comparison. The first column contains the de- 

 grees of Fahrenheit's thermometer — the second shews the quan- 

 tity due to each degree of heat, to be subtracted or added ac- 

 cording as the temperature is above or below the standard — 

 the third exhibits the expansion of volume occasioned by va- 

 pour of the respective degrees of elasticity appropriate to the 

 several degrees of heat, and is always to be subtracted — the 

 fourth is the correction to be applied for the weight of the va- 

 pour, and is constantly to be added, — and the fifth is the correct 

 specific gravity, supposing the air saturated with moisture at the 

 given temperature. 



