On the Theory of the Moist-bulb Hygrometer. By JAMES APJOHN, Ese. M.D. 
M.R.I.A., Professor of Chemistry in the Royal College of Surgeons. 
Read November 24, 1834. 
In the number of the Edinburgh Philosophical Journal just published, and which was 
sent me by a friend, in consequence of its containing a report of the proceedings of 
the late meeting of the British Association for the promotion of science, I find an 
anonymous paper, entitled ‘‘ Observations on the Hygrometer,’”? which induces me 
to take the earliest opportunity of submitting to the Academy the following remarks. 
It is well known to Meteorologists that the Hygrometers of Saussure and De Luc, 
and all others whose indications depend upon variations in the volume or weight of 
the hygroscopic substance employed, have, as philosophical instruments, been in a 
great measure discarded. The direct determination of the Dew-point, as it is techni- 
cally called, is now almost universally practised, either according to the original 
method of Dalton, or by means of the elegant instrument of Professor Daniell. The 
method of the former, however, is much too tedious for practical purposes, and the 
instrument of the latter, though in principle rigorously correct, requires, in order 
to accuracy of result, a quickness of sight, and an adroitness in observation, which 
few can boast of possessing. For the great purposes, therefore, of Meteorology, the 
condensation Hygrometer must be considered as having, as yet, but imperfectly ful- 
filled the high expectations which were entertained of it. 
The Dew-point process was preceded by one of an analogous description, which is 
said to have been first practised by Hutton the Geologist. If the bulb of a thermo- 
meter be kept covered with a thin film of water, its temperature will sink beneath that 
of the atmosphere, (this latter being supposed unsaturated with moisture, ) the maxi- 
mum depression being attained when the heat received in a given time by the film 
from the contiguous air is exactly equal to the caloric of elasticity of the water eva- 
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