INTRODUCTION. IX 



the cold produced by expansion from diminished pressure will con- 

 dense some of this vapor 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 withdrawn.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 tempera- 

 ture of the air before the vapor begins to condense on the outside. 

 The highest temperature at which it will condense, which is varia- 

 ble accordingly 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. 1 (117, 118, 129.) 



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 twenty 

 degrees below the dew point, it would condense about one half its 

 vapor into water, and at forty degrees below, it would condense 

 about three fourths of its vapor into water, &c. This, however, 

 will not be exactly the case from the cold produced by expansion 

 in the upmoving columns ; for the vapor itself grows thinner, and 

 the dew point falls about one quarter of a degree for every hun- 

 dred 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 Fahrenheit, cloud will begin to form ; 

 or, in other words, the bases of all clouds forming by the cold of 

 diminished pressure from upmoving 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. (66, 67, 97). 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 4,800 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 



1 The height of the bases of forming cumuli may be ascertained by the fol- 

 lowing empirical formula : 10300 (^/) = height of base in yards; t being 

 the temperature of the air in degrees of Fahrenheit, and t' the temperature of 

 wet bulb swung briskly in the air. (66, 98.) 



