AEROSTATICS. 237 



This formula is deduced from DAL TON'S Experiments, 

 Manchester Memoirs, vol. v. p* 555., &c. and nearly 

 coincides with the method which he used for inter- 

 polating between the numbers determined by obser- 

 vation : x is positive when the heat is greater than 

 212, negative when less. 



For the purpose of calculation, it is best to take the 

 logarithm of the above expression ; k is then 



Iog/=log30 + ^f log (1.250 -.015 x || 



Hence, when x is known, f may be found, which 

 measures both the compression on the surface of 

 the water, and the elastic force of the steam. The 

 mean weight of the atmosphere is here understood 

 to be equal to 30 inches of mercury, the mean 

 height of the barometer at the level of the sea. In 

 the construction of thermometers, care should be 

 taken to mark the boiling point, or 12^ only when 

 the barometer stands at 30 inches. 



The above formula will serve to calculate the amount 

 of the corrections to be made, when the barometer 

 stands at any other height. The same may be de- 

 termined from Mr DALTON'S Table, ubi supra^ 

 p. 561, or simply, as the variations are but small i 

 by allowing at the rate of 1 for six tenths of an 

 inch in the height of tlie barometer. 



332. It appears, from the experiments of 

 DALTON, that the above formula applies to the 

 steam produced from all liquids ; providing x be 



reckonedi 



