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. 



Let/? be the pressure, and t the temperature of the air at 

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

 of the included vapour ; and let //, Q, 9" 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 

 2J and ;/ 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 forp and p' in 

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

 becomes 



P-f" 

 D = /» X log. ^ • . 

 p — i 



Having thus eliminated 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 ass^ign proper mean values to the temperature, the 

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



