of the Moist-bulb Hygrometer. 285 
fourth the pressures, and those in the fifth the dew-points experimentally determined 
by the method of Dalton :— 
(1) (2) (3) (4) (5) (6) 
t t Pp d p t’ ob. t’ cale. 
G72 << Oe eemlOie weeded Ot moDal ss. JOOkO 
56.4555. NAS oe Ss C.VNIK. feS0.02) ANSeS .2i09 4254 
The numbers in column (6) are the dew-points calculated by my formula; and 
while there is an almost exact correspondence between the first and the result of ex- 
periment, the second, it will be seen, is higher than the observed temperature of 
deposition by nearly three degrees. There is here, however, obviously some mistake. 
It is impossible that, with the recorded temperatures of air and hygrometer, the dew- 
point could have been so low ; and this conclusion I do Pot at present draw from my 
theoretical views, for that would be to subject myself to the imputation of arguing in 
a circle ; but from the following observation, made by me with great care on the 22d 
of March :— 
d t’ ob. 
56 50 6 44 
Here the temperatures ¢ and ¢ differ from those taken from the Encyclopedia only 
by about half a degree ; and nevertheless the observed dew-point 44 is higher than 
39.5 by 4.5 degrees. [rom these observations, therefore, I am, I conceive, entitled 
to conclude—Ist, that the series in which the depression amounts to 15°.2, being 
in exact accordance with my formula, lends it some degree of support; and 2dly, that 
my method cannot be considered as impugned by the other’series, inasmuch as this is 
in some particular manifestly incorrect. But it is time to enter upon the experimen- 
tal tests to which I have resorted. 
If air, in reference to which ¢, ¢ and ¢” have been accurately noted, be raised to 
any elevated temperature, and the observation be repeated in the heated air, as far as 
respects ¢ and ¢’, we will have two* separate sets of observations, from which to cal- 
culate 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 calculations, provided 
our formula be correct, should give precisely the same result; 7. e. the dew-point in 
the first instance determined by observation. Such is the principle of the test expe- 
riments which I first performed. ‘The air- was heated by urging it in a continued 
stream, by means of a double bellows, through the worm of a small still—such as are 
for sale in the Opticians’ shops—the worm-tub being filled with water of the desired 
temperature ; and, in order to the necessary observations, in a glass tube connected 
by a cork with the upper extremity of the worm, a couple of small thermometers were 
* Any number of observations, having reference to the same dew-point, may, it is obvious, be thus obtained. 
