on the Formula for measuring Heights by the Barometer. 229 
logarithmic curves were the same; but, as I have already shewn, 
that, on account of the more rapid decrease of the density of va- 
pour, its modulus must be about a fourth part of the modulus 
of air, the weight of a column of vapour, whose elastic force is/, 
15 1 
will be to that of air, whose elastic force is b, as j X - f 9 or — f 
to b , nearly. Hence, the weight of the column of dry air at the 
lower station, will be to that at the higher, as b — g / to 
Besides the correction I have just explained, there is another 
of perhaps still greater importance, connected with the elastic 
force of vapour, and the dilatation of dry air by moisture, which 
I shall now proceed to examine. It has been long known, that, 
when a portion of water is allowed to pass into the vaporous state, 
under a receiver containing dry air, which is cut off from all com- 
munication with the atmosphere, the elasticity of the vaporised 
air, when a due quantity of water is present, is augmented with 
the increase of temperature, in a much faster ratio than in the 
case of dry air. The first attempt to detect the law of the in- 
crease of the elasticity thence resulting, at least with any thing 
like precision, seems to have been made by General Boy, who 
published the results of his experiments on the subject in the 
67th volume of the Philosophical Transactions ; but, as he was 
altogether ignorant of the law by which the elastic force of va- 
pour is regulated at different temperatures, his experiments, 
though performed with great care, only serve to convince us 
how much manual labour may be expended to no purpose, in 
philosophical researches, when they are not guided by a proper 
discrimination of the various circumstances which may affect 
the result. It is to Dalton and Gay Lussac, but more espe- 
cially to the former, that we are indebted for an accurate invesr 
tigation of the subject. By their experiments, it has been dis- 
tinctly established, that the elasticity of a mixture of air and va- 
pour, so long as the latter retains its elastic condition, is exactly 
equal to the joint elasticity of each of them, taken separately. 
Thus if V represent a certain volumn of dry air, at a particular 
temperature, and under a given pressure 5, while / denotes the 
