60 



ON THE ORIGIN AND NATURE OF 



lighten the air in the locality, to an extent that shall create a 

 rapidly ascending current. That much latent heat is however 

 really given out and made sensible, raising the temperature in 

 the part where vapour is converted into water, is well known 

 and universally admitted. It is familiarly experienced when 

 steam is condensed in our steam engines, and I have explained 

 it more fully in papers formerly read to this Society. 



It is not however often that the whole of the vapour that is 

 in the atmosphere is condensed into water, or even so much 

 of it as there commonly is in the condensor of the steam 

 engine, seeing that the heat liberated in the atmosphere 

 warms the part and the air that is in it, and thus stops or 

 checks the condensation that is taking place. And it is not 

 until the gases are made lighter through being warmed, and 

 that the remaining vapour which is mingled with them is 

 carried successively to greater elevatians, tliat the whole or 

 nearly the whole of the vapour existing in any locality i» 

 condensed. 



When dry air, that is, air entirely without vapour, is taken 

 to a height of, say one hundred yards, the expansion that is 

 consequent on the diminished incumbent pressure at that 

 height, cools it say 1° Fahrenheit ; but if the air be satu- 

 rated with vapour, some portion of the vapour will be con- 

 densed by the cold produced through expansion of the air, 

 and the result will be that the mixed mass of air and vapour 

 will be found to be cooled, not to the full degree of dry air 

 belonging to the elevation, but only to about one half of it; 

 the warming influence of condensation of a part of the vapour 

 having counteracted the cooling effect of expansion of the 

 gases, to the extent of the other half. The mixed mass being 

 however half a degree warmer than the adjoining cold air, is 

 forced up in the atmospheric space by the colder and heavier 

 air. On reaching the height of two hundred yards, incum- 

 bent atmospheric pressure is sufficiently reduced to cool pure 

 fdv 2°, but condensation of vapour counteracts this cooling to 



