194 PHlLnSOPHT OF STORMS. 



leave 6,700 cubic feet of actual expansion of the air in the cloud, for 

 every cubic foot of water generated there by condensed vapor. 



If, therefore, the air on the outside of a cloud cools one degree for 

 every hundred yards in height, while the air in the cloud cools only 

 half that quantity for every hundred yards ; and if every cubic foot of 

 water, condensed from the ascending vapor, expands the air nearly 7000 

 cubic feet, taken in connexion, too, with the fact that the vapor in the 

 cloud has a specific gravity only 5-8 lbs. that of the air outside, it for- 

 cibly explains, on philosophical principles, the cause of the up-moving 

 column being specifically lighter than the surrounding air; the sinking 

 of the barometer, under the base of the cloud ; the immense power and 

 velocity acquired by the condensation of vapor ; and the self-sustaining 

 power of a storm ohce commenced. This last circumstance, in my 

 opinion, is a feature of great merit in Professor Espy's theory. 



This great expansion of the air in the cloud will cause it to spread 

 outwards above in an annulus all around the ascending coluvin. The 

 barometer will, therefoie, not only fall below the mean under the centre 

 of the ascending column, but it will rise above the mean under this an- 

 nulus, outside of the ascending column. And the increased pressure of this 

 annulus will cause the air to rush in towards the centre with greater velocity 

 than itdid before. In consequence ofthe pressure of the atmosphere being 

 greater in the annulus or border of the storm than it is in the centre, the 

 air must descend in the annulus and ascend in the centre of the storm. 

 So long, therefore, as the circumstances are favorable, the process of 

 nimbification must be continued. These favorable circumstances are an 

 elevated 8ew-point, hot air below, and a slow and properly directed cur- 

 rent in the upper regions of the atmosphere. Whenever the dew-point 

 is high, it indicates a large quantity of vapor in,the atmosphere, and con- 

 sequently the existence of a great steam power in the air below, and 

 this, also, diminishes to a considerable extent the specific gravity of that 

 particular body of air. These conditions, even unaided by increased 

 temperature, must create an ascending column ; and should the upper 

 current of the atmosphere, at this time, be moving so slowly and in 

 such a direction as to permit this up-moving column to penetrate it per- 

 pendicularly, or nearly so, then the formation of cloud must continue 

 to go on, particularly if aided by an elevated temperature below. 



Many causes, however, prevent up-moving columns from increasing 

 until rain is the consequence: I. When the complement of the dew- 

 point is very great — 20° or more — clouds can scarcely form ; for as the 

 column must rise twenty hundred yards before cloud can be formed, it is 

 likely either to be dispersed or it acquires the equilibrium of the sur- 



