SUPPLEMENTARY REPORT ON METEOROLOGY. 97 
air under the circumstances* (which is the true expression for 
dampness), is at once obtainable from a table which shows the 
maximum force of vapour for each degree, and the number of 
grains in one cubic foot of space. But unfortunately the ex- 
periment is always a troublesome, sometimes an impracticable, 
and sometimes a fallacious one. Mr. Harcourt} has pointed out 
various mechanical circumstances which affect the appearance 
of a film of dew. To reduce a hygrometric observation to an 
observation simply, and not to an experiment, is the object of 
the moist-bulb problem, in which the refrigeration of a wetted 
surface becomes the index of the dryness. The theory is more 
troublesome, but the observation has every requisite of sim- 
plicity and consistency f. 
173. The moist-bulb problem was especially pointed out as 
one deserving careful solution in the recommendations of the first 
meeting of the British Association at York§, and they have 
been responded to in more than one quarter, so that we may 
now consider the moist-bulb problem as practically solved. 
174. Even at that time the solution had taken a simple and 
exact form in Germany, and for the labours of Auguste|l, 
Bohnenberger§ and Kamtz**, the British Association can- 
not probably claim any merit. The works of the two former 
are, | am sorry to say, still as unknown to me (by actual in- 
spection), as when I wrote my former report. 
175. Athermometer having a thin film of water surrounding 
it will take a temperature depending on the following circum- 
stances :—The air in contact (whether it move quickly or slowly) 
gives to the film of water, which is converted into vapour suf- 
ficient in quantity to saturate the space which the air occupies, 
just enough of heat to vaporize that water, and the reduction 
of temperature will be accordingly. Thus, if the air (or space) 
be very dry, it will take up much vapour, but that vapour must 
have combined with much heat in order to change its state from 
water, and the temperature of the air in the (now) saturated 
* When we speak of vapour contained in air, of course we are not to be 
understood to infer any combination between them. 
+ Phil. Mag., 3rd Series, vii. 409, and British Association, Fifth Report, 
Sections, p. 54. 
{ This method was the invention of Dr. Hutton, of Edinburgh, which 
M. Kupffer has erroneously attributed to Auguste (Instructions, &., p.32), and 
Mr. Prinsep to Leslie (Journal of Asiatic Society of Bengal, 1836, p. 399.). 
§ Original edition, p. 49. These recommendations were drawn up by the 
author of the present report. 
|| Uber die Fortschritte der Hygrometrie in der neuesten Zeit. Berlin 
1830. 
q In the second volume of the Tubingen Nat. Hist. Society’s Memoirs. 
** Lehrbuch der Meteorologie, i. 
VOL. Ix. 1840. H 
