38 OF CLOUDS. CHAP. 1. 10. 



rendering the upper atmosphere into which it 

 ascends cooler than it was before, also renders 



ascent to any height in a perfect calm. Yet, as in this case 

 the inertia of the particles of air considerably resist its diffu- 

 sion, so in the opposite one of a brisk current, the vapour, by 

 the same rule, must in some measure be drawn along with 

 the mass into which it enters. 



" 4thly. The quantity of vapour which, under equal pres- 

 sure, can subsist in a given mass of air, will be greater as the 

 common temperature is higher, and vice versa. 



" Aqueous vapour is the only gas contained in the atmosphere 

 which is subject to very sensible variations in quantity. These 

 variations arise from its attraction for caloric being inferior to 

 that of all the others. Hence when a cold body, such as the 

 glass of water in the experiment above quoted, is presented 

 to the atmosphere, the other gases, composing the latter, will 

 only be cooled by it (and that at all known temperatures) ; 

 but the vapour, after being more or less cooled, will begin to 

 be decomposed, its caloric entering the body while the water 

 is left on the surface. 



" The formation of cloud is in all cases t^e remote conse- 

 quence of a decomposition thus effected, except that the caloric 

 escapes, not into a solid or liquid, but into the surrounding 



" Dew is the immediate result of this decomposition. The 

 particles of water constituting it are, singly, invisible, on 

 account of their extreme minuteness. The approach of dew 

 is, nevertheless, discoverable by a dark hazy appearance, 

 verging from purple to faint red, extending from the horizon 

 to a small distance upwards, and most conspicuous over 

 valleys and large pieces of water. 



