
XVI.—On the Weight of Aqueous Vapour which is condensed on a Cold Surface, 
under given conditions. By James Datmanoy, Esq. 
(Read 3d March 1851.) 
In the accompanying tables are contained the results of some experiments 
respecting the rate at which aqueous vapour condenses on a cold surface. 
These results are not so consistent as could be desired, but having been ob- 
tained by a definite and carefully-conducted process, on may claim at least to 
be received as approximately true. 
In planning the experiments, it was assumed that c=m(f”—/’”), where c is 
the weight of moisture which is condensed on a surface of given area in a given 
time; /” the tension of vapour at the dew-point ; /”” its tension at the tempera- 
ture of the condensing surface; m a coefficient varying with the velocity of the 
current of air. 
It was found, however, in the course of the experiments, that the coefficient m 
was not constant for calm air, as was at first supposed. The results under this state 
of the air indicated that the formula should be changed to c=M (t—?’”) (/”-/”), 
in which ¢ is the temperature of the air; 7” the temperature of the condensing 
surface; M a constant coefficient; and 7”, /’”, and c have their former values. 
The object of the experiments in connection with these formule was to deter- 
mine the values of the coefficients m and M from those of their equivalent ex- 
pressions, amd ee 
agile Caen —7") 
As it would have been difficult, if not impossible, to have kept the condensing 
surface steadily at any low temperature except that of melting ice, the value of 
J” throughout the experiment was equal to 0:2 inch, the tension of vapour at 
32° Fahr. 
In each experiment, therefore, there were only two things to be determined, 
namely, 7” the tension of vapour at the dew-point, and c the weight of condensed 
moisture. 
The value off” was deduced by means of Dr Arsoun’s formula, from the indi- 
cations of the wet and dry bulb thermometers. The data and results connected 
with this quantity will be found in Table I., from the fifth to the eighth column 
inclusive. The instruments used were two standard thermometers, made by 
Messrs ApiE and Son, of Edinburgh. 
The value of ¢ was ascertained by a simple process, which, however, as the 
success of the experiments entirely depended on it, must be described in detail. 
VOL. XX. PART II. 4M 
