256 Mr Anderson’s Corrections for the Effects of Humidity 
feet of the mean heat of the column above or below the stand- 
ard temperature is applied. 
The two corrections for the influence of humidity which I 
have suggested, the first depending upon the pressure of vapour, 
and the second upon the dilatation which that vapour produces 
upon dry air, will, I am persuaded, reconcile, in a very satisfac- 
tory manner, the discordant results which have hitherto been 
obtained from barometrical measurements, even when conducted 
by persons of undoubted accuracy and skill. Their importance 
will best appear, by stating, that, in all cases, the heights de- 
duced by the formula, modified, as I have proposed, by correc- 
tions for humidity, coincide, as nearly as can be expected, with 
the heights obtained by levelling, or accurate geometrical mea- 
surement ; whereas, the results obtained from the formula of De 
Luc, improved as it has been by the labours of Sir G. Shuck- 
burgh and General Roy, or even those furnished by the more 
refined formula of Laplace, sometimes differ 40 or 50 feet from 
the truth, in a diflTerence of elevation of 1000 feet. At the 
same time, I readily admit, that, in this country (and perhaps 
the remark may be applied to the greater, part of Europe), when 
the air is in a mean state of humidity, the increase of pressure 
upon the mercurial column, arising from the mere weight of va- 
pour, and the additional increase of pressure at the upper sta- 
tion, produced by the elongation of the aerial column by mois- 
ture, generally balance each other so exactly, as to render the 
results deduced by these formula, as they now stand, extremely 
correct. 
Having thus shewn the necessity of applying to the common 
j^rmida two corrections for the effects of humidity, I shall now 
proceed to point out the manner in which these corrections are 
to be determined and applied. 
In the^^r^^ place, the absolute elasticity of the atmospheric 
vapour, at each station, must be accurately ascertained, either 
by reducing the indications of a well adjusted hygrometer to 
the corresponding tension of vapour, or by determining the point 
of deposition by actual experiment, agreeably to the method re- 
commended for that purpose by Mr Dalton. The first of these 
methods, it must be confessed, is exceedingly troublesome, and 
liable to a good deal of uncertainty ; as the law which connects 
