TRANSACTIONS OF THE SECTIONS. 35 
mean as it does on the outside of the two clouds, and so the columns 
will be pressed towards each other. If one of two neighbouring 
columns should be greatly higher than the other, its annulus may 
overlap the smaller one, and of course the current under the smaller 
cloud will be inverted, and the cloud which may have been formed 
over the column thus forced to descend will soon disappear; for as it 
is forced downwards by the overlapping annulus of the more lofty 
column, it will come under great pressure, and its temperature will be 
thus increased ; and it is manifest, that as soon as its top descends as 
low as its base, it will have entirely disappeared ; and in the mean time 
the larger cloud will have greatly increased. 
“ As the air above the cloud formed by an ascending column is forced 
upwards, if it contains much aqueous vapour, a thin film of cloud will 
be formed in it by the cold of diminished pressure, entirely distinct 
from the great dense cumulus below; but as the cumulus rises faster 
than the air above it (for some of the air will roll off), the thin film 
and the top of the cumulus will come in contact; and sometimes a 
second film or cap may be formed in the same way, and perhaps a 
third and fourth. When these caps form, there will probably be rain, 
as their formation indicates a high degree of saturation in the upper 
air. 
“When the complement of the dew-point is very great (twenty 
degrees and more), clouds can scarcely form ; for up-moving columns 
will generally either come to an equilibrium with the surrounding air, 
or be dispersed before they rise twenty hundred yards, which they must 
do in this case before they form clouds. Sometimes, however, masses 
of air will rise high enough to form clouds; but they are generally 
detached from any up-moving column underneath, and of course 
cannot then form cumuli with flat bases ; such clouds will be seen to 
dissolve as soon as they form, and even while forming they will gene- 
rally appear ragged, thin, and irregular. Moreover, if the ground 
should be colder during the day than the air in contact with it, as 
sometimes happens after a continuance of very cold weather, then as the 
air touching the cold earth will be colder than the stratum above it, 
ascending columns cannot exist, and of course no cumuli can be 
formed on that day, even though the air may be saturated with vapour 
to such a degree as to condense a portion of it on cold bodies at the 
surface of the earth. Neither can clouds form of any great size, when 
there are cross currents of air sufficiently strong to break in two an 
ascending current, for the ascensional power of the up-moving current 
will thus be weakened and destroyed. This is one means contrived by 
nature to prevent up-moving columns from always increasing until 
rain would follow. Without some such contrivance it is probable that 
every up-moving column which should begin to form cloud when the 
dew-point is favourable, would produce rain; for as soon as cloud 
forms, the up-moving power is rapidly increased by the evolution of 
the caloric of elasticity.” 
_ Among the consequences of his theory, Mr. Espy describes in what 
manner it happens that rain does not fall on the leeward side of very 
D2 
