§2 REPORT—1840. 
of the specific gravity of atmospheric air, its equilibrium is unstable, 
and up-moving columns or streams will be formed. As these columns 
rise, their upper parts will come under less pressure, and the air will 
therefore expand ; as it expands, it will grow colder about one degree 
and a quarter for every hundred yards of its ascent, as is demon- 
strated by experiments with the Nephelescope. The ascending columns 
will carry up with them the aqueous vapour which they contain, and, 
if they rise high enough, the cold produced by expansion from dimi- 
nished pressure will condense some of this vapour into cloud; for it is 
known that cloud is formed in the receiver of an air-pump when the 
air is suddenly withdrawn. The distance or height to which the air 
will have to ascend before it will become cold enough to begin to form 
cloud, is a variable quantity, depending on the number of degrees 
which the dew-point is below the temperature of the air; and this 
height may be known at any time by observing how many degrees a 
thin metallic tumbler of water must be cooled down below the tempe- 
rature of the air before the vapour begins to condense on the outside. 
The difference between the dew-point and the temperature of the air in 
degrees is called (by Mr. Espy) the complement of the dew-point*. 
“ As the temperature of the air sinks about one degree and a quarter 
for every hundred yards of ascent, and the dew-point sinks about a 
quarter of a degree, it follows that as soon as the column rises as many 
hundred yards as the complement of the dew-point contains degrees 
of Fahrenheit, cloud will begin to form; or, in other words, the bases 
of all clouds forming by the cold of diminished pressure from up- 
moving columns of air, will be about as many hundred yards high as 
the dew-point in degrees is below the temperature of the air at the time. 
If the temperature of the ascending column should be ten degrees 
above that of the air through which it passes, and should rise to the 
height of 4800 feet before it begins to form cloud, the whole column 
would then be 100 feet of air lighter than surrounding columns; and 
if the column should be very narrow, its velocity of upward motion 
would follow the laws. of spouting fluids, which would be eight times 
the square root of 100 feet a second, that is, 80 feet a second, and the 
barometer in the centre of the column at its base would fall about the 
ninth of an inch. As soon as cloud begins to form, the caloric of 
elasticity of the vapour or steam is given out into the air in contact 
with the little particles of water formed by the condensation of the 
vapour. This will prevent the air in its further progress upwards 
from cooling so fast as it did up to that point ; and, from experiments 
with the Nephelescope, it is found to cool only about one-half as much 
above the base of the cloud as below; that is, about five-eighths of a 
degree for one hundred yards of ascent, when the dew-point is about 
* The height of the bases of forming cumuli may be ascertained by the fol- 
t—t! 
lowing empirical formula: 103 (=) = height of base in 100 yards; tbeing 
the temperature of the air in degrees of Fahrenheit, and ¢! the temperature of 
the wet bulb seoung briskly in the air. 
each. dak ie 
