On the Theory of the Moist-bulb Hygrometer. By JAMES APJOHN, Esa. M.D. 
M.R.I.A., Professor of Chemistry in the Royal College of Surgeons. 
(Continued. ) 
Read April 27, 1335. 
Ar the meeting of the Academy held in November last, I was permitted to read a 
short memoir on the subject of a formula, at which I had a considerable time pre- 
viously arrived, for inferring the Dew-point from the indications of the Moist-bulb 
Hygrometer. This formula was deduced altogether from general considerations ; 
and, though satisfied, from some hasty observations of my own, that it represented 
facts with considerable accuracy, I was not, at the time, in possession of evidence 
which could be considered as establishing this important point in an unequivocal 
manner. ‘The table which is subjoined to my paper undoubtedly shows, that, within 
certain limits, my formula is in accordance with experiment ; but the observed de- 
pressions in the‘table are, generally speaking, so small, that a formula in itself incor- 
rect might, it must be admitted, yield results which would deviate from the observed 
dew-points by quantities not exceeding the possible errors of observation. Berzelius, 
for example, states (Traité de Chimie, tom. viii. p. 254,) that, from the experiments 
of August, Bohnenberger, and others, it appeared that the temperature of a thermo- 
meter with moistened bulb was an arithmetic mean between that of the air and the 
dew-point ; and this rule, which would make ¢’=2# —?, though utterly erroneous, 
would apply to the table appended to my paper, nearly as well as the formula I have 
deduced. The validity, therefore, of my method required to be more rigorously 
tested ; and having been for some time engaged in experimental researches, instituted 
with this object, which have led to interesting, and to me most satisfactory results, I 
am anxious to submit them, with as little delay as possible, to the judgment of the 
Academy. 
VOL. XVII. oi 
