﻿and Fluctuations. 337 



plies that 253 grs, of vapour are withdrawn from the column of 

 air that rests on it, and almost entirely from that part of the 

 column which extends upwards from the ground to a height of 

 10,000 feet. If we suppose the mean temperature throughout 

 this column to be 55° F. (and a few degrees more or less make 

 scarcely any difference), the 253 grs. of vapour have a volume 

 of 89,770 cubic inches ; that is to say, the vapour and the 

 elastic force belonging to it have been withdrawn from the at- 

 mospheric column through a height equivalent to 89,770 inches, 

 or in round numbers 7500 feet. This, from a column 10,000 

 feet high, denotes the withdrawal of three-fourths of the elastic 

 force of the vapour, that is, of a pressure equal to "325 of an 

 inch of the barometric column. 



On the other hand, the heat given off by the condensation of 

 1 pound of water is sufficient to raise 1000 pounds of water, or 

 4200 pounds of air, one degree in temperature. The pound of 

 water, as rain 1 inch deep, covers an area of about 28 square 

 inches; 2V of this heat is therefore applied to the column on 

 the base of 1 inch which we have been considering, or sufficient 

 to raise the temperature of 150 pounds of air 1° F. Now the 

 whole column, to its highest limit, weighs 15 pounds ; so that 

 the liberated heat, if applied to the whole column, is sufficient 

 to raise its temperature 10° F. But there is no reason to sup- 

 pose that it is so applied. We know familiarly that rain cools 

 the air near the surface, which comes within the reach of our 

 daily experience ; and we know that frequently, when rain is 

 falling on low-lying land, we find snow falling on land a little 

 higher; so that we are justified in saying that the heat of con- 

 densation does not strike downwards towards the surface of 

 the earth. It would therefore appear probable that the greater 

 part of this heat is absorbed by the air in the immediate 

 neighbourhood in which the condensation takes place; and 

 supposing that it acts principally on that part of the column 

 which lies between 5000 and 10,000 feet from the ground, it 

 acts on a weight of air which may be considered as one-sixth 

 of the whole, or as 2*5 pounds, and is sufficient to raise its 

 temperature 60° F. It would be useless to carry a calculation 

 such as this any further. The results obtained from data ne- 

 cessarily extremely inexact, are of little numerical value ; but 

 they serve to show dimly the extraordinary amount of the force 

 suddenly let loose high overhead. Heat so applied to a column 

 of air 5000 feet high cannot be compared in its effects with that 

 which causes arise of temperature, even through the same num- 

 ber of degrees, near the surface of the earth, where it acts 

 only on the very lowest strata, and is diffused upwards by a 

 process comparatively slow — so slow, in fact, that the mean 



