562 



though this has been generally admitted by those who have 

 turned their attention to the subject, I am not aware that 

 any attempt at estimating its exact amount has been as yet 

 made; and as the correction for moisture is frequently of 

 considerable magnitude, and may, in my opinion, be applied 

 with as much accuracy as that for temperature, I have taken 

 the liberty of occupying, for a few moments, the time of the 

 Academy with an explanation of the method which it has oc- 

 curred to me to devise, and with which, from some trials I 

 have made of it, I have every reason to be satisfied. 



Letp be the pressure, and t the temperature of the air at 

 the lower station, t'' the dew point of the air, andy" the force 

 of the included vapour ; and let p', 6, 6" and f" represent the 

 corresponding quantities at the upper station. This being 

 understood, a little consideration will suffice to shew that 

 the presence of the aqueous vapour produces on tlie for- 

 mula a twofold deranging effect. It augments the values of 

 p and p' beyond what they would be in dry air, and it pro- 

 duces an alteration in the length of the column of air between 

 the two stations additional to that which results from the dif- 

 ference between its mean temperature and 32°, or the freez- 

 ing point. The first of these is obviated, or, in other words, 

 the correction for it is made, by substituting for^ and p' in 

 the approximate formula, /?—/" and p' — f", by which it 

 becomes 



P-f" 



Having thus ehminated the effects of the tension of 

 aqueous vapour upon the pressures, we have next to esti- 

 mate the conjoint influence of it and temperature, in elon- 

 gating the pillar of air between the two stations. The theory 

 of mixed gases and vapours enables us to do this, provided 

 we can assign proper mean values to the temperature, the 

 pressure, and the force of vapour of the aerial column in ques- 



