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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/, /' 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 (I.) should 

 be multiplied, is 



P +P . 



P+P'fJ-f 



This will convert the formula (I.) into the following : — 



H = 10000/^- (l + ® ) f-—l±PL-J) log ly (II.) 

 V 493/ \p+p'-f-f)' & P 



This may be considered as the statical barometric formula, and 

 in it account is taken of both moisture and temperature. It is 

 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 time 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. 



