234 
On Revolving Storms. By JosepH Joun Murpuy, Esq.* 
When the temperature of the air is in a state of con- 
vective equilibrium, or, in other words, when the difference 
between the temperatures of any two strata is that which is 
due to the difference of pressures, it is obvious that any de- 
crease in the specific gravity of the lower stratum, whether 
from expansion by heating, or by mixture with watery 
vapour, which is lighter than air, will give that stratum a 
tendency to rise; but as an entire stratum cannot rise in 
mass, the air will flow upwards in vertical columns, the 
place of which may be determined by the smallest inequality 
in the ground, or motion on it. If the ascending force is 
powerful and the ground very dry, the ascending columns of 
air will be marked by clouds of dust or sand, and it is thus 
that the ‘ dust-storms” of Western India, and the “ sand- 
spouts” of the African desert, are produced. 
Professor Hennesy has observed that such vertical circu- 
lation is shown to exist in our climate by the rapid fluctuation 
of a thermometer exposed to the free air in warm sunshiny 
weather; and cumulous cloud is produced when the watery 
vapour contained in such an ascending column is condensed 
by the cold of diminished pressure. 
When condensation takes place in an ascending column, 
the cooling from diminished pressure will be retarded by the — 
heat liberated in condensation, and the ascending column, 
at any altitude higher than that at which condensation has 
begun, will be warmer than the surrounding air on the same ~ 
level, supposing, as at first, that the temperatures of the 
surrounding air are in a state of convective equilibrium. . 
When unity of weight of vapour is condensed, heat 
enough is liberated to raise the temperature of an equal 
weight of water by 1178° Fahr., minus the temperature at 
which condensation takes place. To estimate the effect of | 
this in heating air, the quantity must be divided by ‘2377, 
which is the specific heat of air at constant pressure ; and 
to estimate the effect of the condensation of unity of volume 
* Communicated to the Belfast Natural History and Philosophical Society, 
29th October 1862 and 7 January 1868. | 
