22 : ANNIVERSARY ADDRESS. 
idea of what they are from a consideration of the natural conditions 
under which rain is deposited. I am not going to ask you to fol 
low me through the elaborate investigation of this question by Sit _ 
W. Thompson, M. Peslin, Dr. Hann, and others. It will be sufi 
cient to say that they have proved that the principal cause in the 
formation of rain is the ascent of saturated or nearly saturated ait, 
and that the rain caused by the mixing of two currents of ait 
bears a very small proportion to the whole. Their investigations 
have further taught us that air as it rises, whether from the 
effect of heat or up-draught, loses 1° of temperature for every 180 
feet which it ascends; 
reached, a cloud is formed, and the latent heat given out by the 
condensed vapour warms the air so much that it has to rise 286 
feet to lose 1° of temperature. Its upward velocity is therefore 
accelerated, and its moisture rapidly precipitated ; and this must g? 
on until it loses the excess of moisture and reaches the temperatwr® 
_ of the surrounding air, These are facts which have an important 
bearing upon our inquiry, and these laws may be seen in operation 
any calm fine day in the formation of cumulus clouds. Where the 
sun acts upon moist earth or water, it causes, first heat, then ev 
poration and an upward motion of the moist air, which, when # 
reaches the altitude and the temperature of the dew-point, co® 
denses into a cumulus cloud ; the central parts, heated by the hes 
given out by condensation, rush upwards, rolling masses of cloud 
out of the top as the condensation increases—the extent of the 
cloud forming a measure of the activity of the forces which gav? 
rise to it. 
A curious and instructive instance of this phenomenon in nature — 
is found in the equatorial region of calms. Here the sun almost 
invariably rises in a cloudless sky, which remains clear until about 
noon, when heavy masses of cloud begin to collect and, rapidly 
inereasing, form a dense black covering from which rain ee 
down in torrents ; s . 
but if, as it ascends, the dew-point ® — 
