. a 

WHICH IS CONDENSED ON A COLD SURFACE. 301 
city of it was regulated by the number of revolutions made by the handle of the 
instrument in the course of ten seconds. The results correspond respectively to 
5, 10, 18, and 25 of these revolutions, or on a rough estimate to actual velocities 
of 4:12, 8:24, 14°8, and 20:6 feet per second. 
With such an instrument it was difficult to maintain a rapid current at a 
uniform rate, and hence, probably, the reason why the results due to five and ten 
revolutions of the handle are less discordant than those corresponding to eighteen 
_ and twenty-five revolutions. 
« c c 
The second table contains the values of M= Gypsy) and of T= Gh 
the former corresponding to the experiments in the thirteenth column, and the 
latter to the experiments in the fourteenth, fifteenth, sixteenth, and seventeenth 
columns of the first table. The results in column twelfth, for reasons which will 
be obvious from the remarks on that column, are omitted in the second table. 
Taking the mean of the numbers in each column of this table, and changing 
the unit of surface from 11:8 to 100 square inches, and the unit of time from five 
minutes to one minute, it appears that the value of M for calm air is 0°12, and 
that the values of m for velocities of 4°12, 8:24, 14:8, and 20°6 feet per second are 
respectively 18°3, 26-5, 39-7, and 44:6. 
These results may be useful in various meteorological investigations; but at 
present it is proposed to apply them only to one question connected with the 
theory of rain. 
The admirable observations of Professor Puiiiies and Mr Gray, published in 
the Reports* of the British Association, shew that at York there fell in the course 
of three years, into a rain-gauge placed on the ground, 65-430 inches of rain ; into 
a gauge placed at the height of 43-7 feet, 52°169 inches; and into a gauge at the 
height of 213 feet, 38-972 inches. 
Professor Puitures has proposed, in explanation of these anomalous results, 
an hypothesis which may be thus enunciated. As rain falls from a considerable 
height in the atmosphere, its temperature is less than that of the dew-point of 
the vapour through which it passes in the course of its descent; and this gives 
rise to a continuous deposition of moisture on the surface of each rain-drop suffi- 
cient, in the aggregate, to account for the difference between the quantities of 
rain received in the higher and lower gauges. 
This ingenious explanation has been so ably advocated by its proposer, and 
by other highly competent judges, that itis not without hesitation I venture to 
object to it, on the ground that the rate of condensation which it assumes, when 
___ compared with the rate deduced from experiment, is too great. 
I shall now endeavour to offer some proof in support of this conclusion. 
* Vol. ii., p. 401; vol. ii., p. 560; vol. iv., p, 171. 
