124 Philosophy of Storms. 



the dew point would fall about 20°, but the temperature would 

 fall less than 20°, and therefore no vapor would be condensed by 

 such ascent. 



When a cloud begins to form from an ascending column of air, 

 it will be seen to swell out at the top while its base continues on 

 the same level, for the air has to rise to the same height before it 

 becomes cold enough by diminished pressure to begin to condense 

 its vapor into water ; this will cause the base to be flat, even after 

 the cloud has acquired great perpendicular height, and assumed 

 the form of a sugar loaf. Other clouds also for many miles 

 around, formed by other ascending columns, will assume similar 

 appearances, and will moreover have their bases all on the same 

 or nearly the same horizontal level ; and the height of these bases 

 from the surface of the earth will be the greatest about three 

 o'clock, when the dew point and temperature of the air is the 

 greatest distance apart. 



The outspreading of the air in the upper parts of an ascending 

 column will form an annulus all round the cloud, under which 

 the barometer will stand above the mean ; of course the air will 

 descend in the annulus, and increase the velocity of the wind at 

 the surface of the earth towards the centre of the ascending col- 

 umn, while all round on the outside of the annulus there will be 

 a gentle wind outwards. Any general currents of air which may 

 exist at the time, will of course modify these motions from the 

 oblique forces they would occasion. 



The up-moving current of air must of course be entirely sup- 

 plied by the air within the annulus, and that which descends in 

 the annulus itself 



The rapid disturbance of equilibrium, which is produced by 

 one ascending column, will tend to form others in its neighbor- 

 hood ; for the air being pressed outwards from the annulus, or at 

 least retarded on the windward side, will form other ascending 

 columns, and these will form other annuli, and so the process will 

 be continued. 



These ascending columns will have a tendency to approach, 

 and finally unite ; for the air between them must descend, and 

 in descending the temperature of the whole column will increase, 

 for it is known that the air, at great elevation, contains more ca- 

 loric to the pound than the air near the surface of the earth, be- 

 cause it is the upper regions that receive the caloric of elasticity 



