34 REPORT—1840. 
of the cloud to reach a height where the barometer would stand 
at 74 inches, and the mean temperature of the whole column 40° 
warmer than the surrounding air, then would the barometer fall under 
the cloud at the surface of the earth, ;4%, of 22°5, or a little more than 
2 inches. 
“Though the air may be driven up by the ascending column much 
higher than the point assumed in the last article, the cloud will cease 
to form at greater heights, because the dew-point, at these great 
elevations, falls by a further ascent as rapidly as the temperature—and 
at greater elevations, it will even fall more rapidly. If, for instance, 
the air should rise from where the barometer stands at 6 inches to 
where it stands at 3 inches, the dew-point would fall about 20°, but 
the temperature would fall less than 20°, and therefore no vapour 
would be condensed by such ascent. 
“When a cloud begins to form from an ascending column of air, it 
will be seen to swell out at the top while its base continues on the 
same level, for the air has to rise to the same height before it becomes 
cold enough, by diminished pressure, to begin to condense its vapour 
into water; this will cause the base to be flat, even after the cloud has 
acquired great perpendicular height, and assumed the form of a 
sugarloaf. Other clouds also for many miles around, formed by other 
ascending columns, will assume similar appearances, and will moreover 
have their bases all on the same or nearly the same horizontal level ; 
and the height of these bases from the surface of the earth, will be the 
greatest about 3 o’clock, when the dew-point and temperature of the 
air are the greatest distance apart. The outspreading of the air in 
the upper parts of an ascending column will form an annulus all round 
the cloud, under which the barometer will stand above the mean; of 
course the air will descend in the annulus, and increase the velocity of 
the wind at the surface of the earth, towards the centre of the ascend- 
ing column, while all round on the outside of the annulus there will 
be a gentle wind outwards. Any general currents of air, which may 
exist at the time, will of course modify these motions, from the oblique 
forces they would occasion. The up-moving current of air must of 
course be entirely supplied by the air within the annulus, and that which 
descends in the annulus itself. The rapid disturbance of equilibrium, 
which is produced by one ascending column, will tend to form others 
in its neighbourhood ; for the air being pressed outwards from the 
annulus, or at least retarded on the windward side, will form other 
ascending columns, and these will form other annuli, and so the process 
will be continued. These ascending columns will have a tendency to 
approach, and finally unite; for the air between them must descend, 
and in descending the temperature of the whole column will increase, 
for it is known that the air, at great elevations, contains.more caloric 
to the pound than the air near the surface of the earth, because it is 
the upper regions that receive the caloric of elasticity, given out in 
the condensation of vapour into clouds. Therefore, when the air has 
descended some time in the middle, between two ascending columns, 
the barometer will fall a little, or at least not stand so high above the 
