TRANSACTIONS OF THE SECTIONS. 33 
70°. If the dew-point is higher, it cools a little less, and if the dew- 
point is lower, it cools a little more, than five-eighths of a degree in 
ascending one hundred yards. 
«Now it has been ascertained by aéronauts and travellers on moun- 
tains, that the atmosphere itself is about one degree colder for every 
hundred yards in height above the surface of the sea; therefore, as the 
air in the cloud above its base is only five-eighths of a degree colder 
for every hundred yards in height, it follows, that when the cloud is 
of great perpendicular height above its base, its top must be much 
warmer than the atmosphere at that height, and consequently much 
lighter. Indeed, the specific gravity of a cloud of any height, com- 
pared to that of the surrounding air at the same elevation, may be 
calculated, when the dew-point is given; for its temperature is known 
by experiments with the Nephelescope, and the quantity of vapour 
condensed by the cold of diminished pressure at every point in its 
upward motion, and of course the quantity of caloric of elasticity 
given out by this condensation is known, and also the effect this 
caloric has in expanding the air receiving it, beyond the volume it 
would have if no caloric of elasticity was evolved in the condensation 
of the vapour. For example, according to the experiments of Prof. 
_ W.R. Johnson, of Philadelphia, a pound of steam, at the temperature 
of 212°, contains 1030° of caloric of elasticity ; and if the sum of the 
latent and sensible caloric of steam is the same at all temperatures, it 
follows, that a pound of steam being condensed in 1210 pounds of 
water at 32° would heat this water up one degree ; and, as the specific 
caloric of air is only 0-267, if a pound of vapour should be condensed 
in 1210 pounds of air, it would heat that air nearly 4°, or, which is 
the same thing, it would heat 100 pounds of air about 45°. And in 
all these cases it would expand the air about 8000 times the bulk of 
water generated; that is, 8000 cubic feet for every cubic foot of 
water formed out of the condensed vapour. And as it requires 
between 1300 and 1400 cubic feet of vapour, at the ordinary tem- 
peratures of the atmosphere, to make one cubic foot of water—if this 
quantity be subtracted from 8000 it will leave upwards of 6600 cubic 
feet of actual expansion of the air in the cloud for every cubic foot of 
water generated there by condensed vapour. 
“This great expansion of the air in the forming cloud will cause the 
air to spread outwards in all directions above, causing the barometer 
to rise on the outside of the cloud, above the mean, and to fall below 
the mean under the middle of the cloud as much as it is known to do 
in the midst of great storms. For example, if the dew-point should 
be very high, say 78°, then the quantity of vapour in the air would 
be about one-fiftieth of its whole weight; and if the up-moving column 
should rise high enough to condense one-half its vapour into cloud, it 
would heat the air containing it 45°, and the air so heated would occupy 
qi’s More space than it would be if it was not so heated. And if we 
assume a case within the bounds of nature, and suppose the cloud and 
the column under the cloud to occupy three-fourths of the whole 
weight of the atmosphere, or, in other words, if we suppose the top 
1840. D 
