302 MR J. DALMAHOY ON THE WEIGHT OF AQUEOUS VAPOUR 
At York, during the winter quarters of the years 1832-33, 1833-34, 1834-35, 
a rain-gauge placed on the ground received 17:32 inches of rain, and a similar 
gauge placed at the height of 43-7 feet received only 12°17 inches of rain. 
In this case, according to the hypothesis, a rain-drop, while falling through 
the height of 43-7 feet condensed on its surface a quantity of moisture, the weight 
of which was to the weight of the drop, before the condensation began, in the 
ratio of -42 to 1. 
In order to make a fair comparison between this assumed rate of condensation . 
and the rate deduced from experiment, it would be necessary to know the mean 
temperature of the rain, the mean temperature of the dew-point, the mean size of 
the drops of rain, and the velocity of their fall. 
But as the available data are not sufficient to furnish mean values of these 
quantities, it is necessary, in order to obviate any objection on this ground to the 
proposed mode of comparison, to assume for the temperature of the rain, the 
temperature of the dew-point, and the size of the rain-drops, values the most 
favourable to the hypothesis which the case admits of. 
First, therefore, we are sure that in-assuming the temperature of the rain to 
be 32°, we ascribe to the condensing surface the greatest. cold compatible with its 
being a surface of fluid water. 
Again, since the mean temperature of the air during the winter months of the 
three years of observation was 36°°3, and the mean range of temperature was equal 
to 8°6 degrees, it follows that the mean maximum temperature of the air was 
363 +5 -40°6. The temperature of the dew-point, therefore, cannot possibly be 
assumed as having been higher than 40°°6. 
Further, as Sir Joun Lestir states that the diameters of the drops of rain 
vary from -04 to °25 of an inch; and as it is more favourable to the hypothesis to 
suppose them small than great, it is proposed to assume that the diameter of each 
drop was only :05 of an inch. The weight of a drop of rain of this magnitude is 
0165 of a grain, and the area of its surface is equal to 00786 of a square inch. 
Lastly, since a drop of rain having its diameter equal to -05 of an inch would 
soon attain its terminal velocity, it may be assumed that the drops fell with an 
uniform velocity of 14:6 feet per second, and consequently occupied ae seconds 
in falling through the height of 43°7 feet. 
It now remains to ascertain what rate of condensation is deducible from these 
data, as compared with the rate assumed by the hypothesis. 
In order to do this, let, #”=27, the tension of vapour at 40°6; /’”='2, the 
tension at 32°; m=39°7, the coefficient due to a current of air having a velocity 
of 14°8 feet per second, which is nearly the same as the terminal velocity with 
which the drops of rain have been supposed to fall. 

