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, 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 metalUc 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 one quarter 

 of a degree for every hundred yards of ascent. 



It follows, then, 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, 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 hmidred 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 passes, and should rise to 



Vol. xsxix, No. 1.— April-June, 1840. 16 



