Philosophy of Storms. 121 
The ascending columns will carry up with them the aqueous 
vapor which they contain, and if they rise high enough, the cold 
produced by expansion from diminished pressure, will condense 
some of this vapor into cloud ; for it is known that cloud is form- 
ed in the receiver of an air-pump when the air is suddenly with- 
drawn. 
The distance or height to which the air will have to ascend 
before it will become cold enough to begin to form cloud, isa 
variable quantity depending on the number of degrees sehich the 
dew point is below the temperature of the air; and this height 
may be known at any time by observing hove many degrees a 
thin metallic tumbler of water must be cooled down below the 
temperature of the air, before the vapor begins to condense on the 
outside. The highest temperature at which it will condense, 
which is variable according as there is more or less vapor in the 
air, is called the ‘dew point,” and the difference between the 
dew point and the temperature of the air in degrees is called the 
complement of the dew point. 
It is manifest that if the air at the surface of the earth should 
at any time be cooled down a little below the dew point, it would 
form a fog by condensing a small portion of its transparent vapor 
into little fine particles of water, and if it should be cooled 20° 
below the dew point, it would condense about one half its vapor 
into water, and at 40° below, it would condense about three 
fourths of its vapor into water, &c. 
This, however, will not be exactly the case from the cold pro- 
duced by expansion in the up-moving columns; for the vapor 
itself grows. thinner, and the dew point falls about picid 
of a degree for every hundred yards of ascent, 
It follows, then, as the temperature of  ageptiar sei one 
degree and a quarter for every hundred yards of ascent, and the 
dew point sinks about a quarter of a degree, that as soon as the 
column rises as many hundred yards as the complement of the 
dew point contains degrees of Fahr. 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 is below the tem- 
perature of the air at the time. 
If the temperature of the ascending column should be 10° 
above that of the air through which it aaa and should rise to 
Vol. xxx1x, No. 1.—April-June, 1840. 
