24 



ON THE SBPARATE PRESSURES OF THE AQUEOUS AND 



inch, though the cause of that fall may have been in energetic 

 operation over a very extensive area, producing disturbances 

 of the most formidable character in the upper regions. 



It is, however, probable, * that very great falls of the 

 barometer take place only when condensation occurs within 

 an atmosphere which is, in the part, below the freezing point, 

 and that the greatest falls occur when the air is the most 

 below freezing. Suppose, for instance, the air at the surface 

 of the earth to be near the freezing point, and to be pretty 

 fully saturated with vapour, when, from some cause, con- 

 densation begins near the surface, and an ascending column 

 is formed: the column would, in its ascent, soon pass into 

 air that was below the freezing point, and when it attained 

 the height of 1000 feet, it would have a temperature 10° 

 above the adjoining cold air at the same level, and would 

 be pressed up by it with a force corresponding with the 

 difference of the temperature of the two masses at the same 

 level. But the ascending column is itself cooled by the 

 reduction of incumbent pressure ; when, however, it reached 

 32°, a part of the water that had been produced in the column 

 would be frozen, which would arrest the descent of the 

 temperature and keep it at 32°. The congelation would now 

 liberate fresh heat, to act upon the air and further lighten 

 and raise the column ; and this double process of conden- 

 sation of vapour and congelation of water would continue, and 

 the temperature in the column would remain for some time 

 at 32^, as long as fresh vapour for condensation was supplied 

 from the lower air. The column might thus continue 

 ascending like a rocket, carrying within itself the cause of 

 its ascent, until it reached a height of 2000, or 4000, or 

 6000 yards, and there would be a difference in the tem- 

 perature of the ascending column, and of the adjoining air, at 

 equal heights, of 10° at 1000 yards, 20^ at 2000 yards, 

 30'" at 3000 yards, and so on, 10° for every additional 1000 

 yards of elevation. 



