82 



Daniell on the Correction 



In this situation, therefore, the operation stands: the column 

 of mercury which is the measure apphed, is rendered an inva- 

 riable standard of comparison, by being brought by an easy cal- 

 culation to a known density ; the altitude measured is then in 

 proportion to the specific gravity of the air. 



But heat is not the only agent which alters the specific gravity 

 of air, the admixture of aqueous vapour, it is well known, also 

 produces very important changes. It did not, as I have shewn, 

 escape the observation of General Roy, that air, in contact with 

 water, expanded much more than dry air, and from well-con- 

 ducted experiments he ascertained that the expansion was greater, 

 for equal increments, as the temperature rose. From the mean 

 results which he obtained, the following increasing rates of 

 expansion were derived : 



l(fOO parts of air in contact with water, and under a pres- 

 sure of 32.18 inchesexpanded for each degree, 



From 



Hence we have the progressive scale of expansion as 

 follows : 



