28 
BRITISH ASSOCIATION.- 1835 . 
original paper, and may be applied if deemed necessary. Experience 
however has satisfied him that, generally speaking, they may be neg¬ 
lected, as in almost every instance their amount is considerably 
within the inevitable errors of observation. 
The experiments instituted for the purpose of testing the formula, 
and which are detailed in the author’s second paper, were next ex¬ 
plained. The principle of the first of these is as follows: if air, in 
reference to which t, t' and t n (the dew-point) have been accurately 
noted, be raised to any elevated temperature, and the observation be 
repeated in the heated air as far as respects t and t\ we shall have 
two separate sets of observations from which to calculate the point 
of deposition; and as the amount of moisture in the air is not altered 
by the augmentation of temperature it has experienced, both calcu¬ 
lations, provided the formula be correct, should give precisely the 
same result, i. e. the dew-point in the first instance determined by 
observation. Four distinct series of experiments on this plan were 
performed by means of a very simple apparatus, and though the de¬ 
pressions varied from 4 0, 7 to 28°‘5, the calculated dew-points for 
each series were found almost coincident, and the differences between 
these and the observed dew-points were so trifling in amount as to 
be clearly ascribable to unavoidable inaccuracy of observation. 
The next test experiments performed were suggested by the for¬ 
mula itself. If f n — x J-, and f n be supposed equal to 
0, a condition which can only be fulfilled in perfectly dry air, 
dp , . 30 
f ■ = — x an equation from which we deduce d = 87 f’ X — • 
o / Ol) p 
Hence by determining experimentally the depression of the wet 
thermometer in perfectly dry air we shall be enabled to pronounce 
upon the validity of the general method under discussion. In order 
to observe several values of d, air forced from a caoutchouc bag 
was made to pass three times through about two inches of oil of 
vitriol, and then to traverse a tube containing the dry and wet ther¬ 
mometer, and the indications of these instruments w r ere noted down 
as soon as the latter assumed its stationary temperature. Of nine¬ 
teen observations of depression thus obtained, eleven were a little 
greater, and eight a little less than the calculated results. The mean of 
the plus errors of the formula was *28, and of the minus errors *4 of a 
•og __ *40 
degree, so that ■ ~ ^. = — *006 is the mean difference between 
experiment and calculation deducible from the whole. This sin¬ 
gularly close correspondence of theory with experiment is the more 
satisfactory because as the mean pressure for the nineteen experi¬ 
ments was but a little over 30, and as the air was perfectly dry, nei¬ 
ther of the corrections, of which mention has been already made, re¬ 
quired to be applied. 
The most obvious method of testing the formula, or that which 
consists in comparing its results with the dew-points experimentally 
determined, was last noticed. That such criterion may be decisive, 
