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300 MR J. DALMAHOY ON THE WEIGHT OF AQUEOUS VAPOUR 
The aqueous vapour was condensed on the inner surface of a nearly cylindrical 
copper vessel, the depth of which was 0°5 of an inch; the diameter at the bottom 
3 inches, the diameter at the mouth 3:075 inches; consequently the inner area 
was equal to 11°8 square inches. This vessel had a thin copper lid, the catch of 
which adhered to the inner surface, and so did not interfere with the thorough 
removal of moisture from the outer surface. 
The temperature of the condensing surface, it has been already remarked, was 
uniformly 32° Fahr. This temperature was maintained by filling with pounded 
ice a cylindrical copper box, and burying in it the condensing vessel, all except 
its upper edge, which was made to fit into, and slightly project above, a circular 
opening in the top of the box. 
The following are the steps of the process by which the weight of the moisture 
condensed on the cold surface was ascertained. 
The condensing vessel and its lid were first carefully dried and then weighed 
in a balance, which was sensible to ‘01 of a grain, when each scale was loaded 
with 800 grains. The vessel, with its lid closely applied, was now placed among 
ice, as before described, and allowed to cool for about five minutes. The lid was 
then removed, and the condensing surface exposed either to a current of air of 
known velocity, or to calm air. After the exact interval of five minutes the lid 
was replaced, and the vessel was taken out from among the ice; its temperature 
was then raised above the dew-point by the heat of the hand, while its outer surface 
was carefully dried. Lastly, it was again weighed, and the excess of this above 
the former result was equal to the weight of moisture which had been condensed. 
The results thus obtained will be found in Table L, one column twelfth to 
column seventeenth inclusive. 
It appears from column twelfth, that when the air was calm, and the mouth 
of the condensing vessel was directed upwards, the quantity of condensed vapour 
was so small and variable as to render it probable that, under this arrangement, 
there would have been no sensible condensation at all, had it not been for those 
irregular currents, which are known to prevail even in air apparently calm. 
Column thirteenth shews that when the air was calm, and the mouth of the 
vessel was turned downwards, much more vapour was condensed than when the 
mouth was upwards. This was evidently the effect of a current produced by contact 
of the warm air with the cold condensing surface, and which, for obvious reasons, 
could not take place when the mouth of the vessel was directed upwards. The 
effect of this current on the rate of condensation seemed to vary nearly as ¢—2”, 
at least it would be difficult, except on this assumption, to reconcile the four last 
with the preceding results in the column. 
The last four columns exhibit the results of experiments, in which the con- 
densing surface was exposed to an artificial current of air. 
The current was produced by means of a common fire-blower, and the velo- 

