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column of air above 32°; p, p’, the corrected barometric heights 
for the temperature 32°, at the lower and upper stations re- 
spectively; and H, the height of the mountain in fathoms.* 
If aqueous vapour constitute part of the column, and the mix- 
ture of dry air and vapour be in a state of equilibrium, then 
the column of air calculated by (I.) will be too short, and must 
be increased by the expansion due to the quantity of vapour 
in the column. If f, f’ denote the elastic force of the vapour 
at the lower and upper stations respectively, it is easy to show 
that the hygrometric coefficient, by which equation (1.) should 
be multiplied, is 
PAP: > 9: 
Ben sit Sa, 
This will convert the formula (I.) into the following :— 
e \ ( ptp P 
isl = 10000 fath- 1 - —— Sian ee lo aa ME 
( Por Tao ve SD el) 
This may be considered as the statical barometric formula, and 
in it account is taken of both moisture and temperature. Itis 
certain that if the air be in a state of equilibrium, this formula 
will represent with accuracy the difference of level between the 
two stations. 
If vapour or any other gas be suddenly introduced into a 
portion of a vertical column of air, and it requires ¢ime for the 
air to expand in consequence, the barometric pressure observed 
at any such point will be too great by a quantity, which at its 
maximum will be the elastic force of the vapour introduced. 
If we suppose therefore that the true barometric pressure at 
any point is p —f, this is equivalent to supposing that the intro- 
duced vapour has not yet commenced to expand the column. 
This supposition may be considered as belonging to the state 
of incipient motion. If we suppose that the expansion of the 
air, consequent on the introduction of the vapour, has ceased, 
* The exact coefficient for the latitude of Dublin is 10008 fath. 2 ft. 
