1818.] On the Suspension of Clouds. 105- 



Let us suppose then an ascending* portion of air to have 

 arrived at such a height that its diminished temperature no 

 longer allows of its retaining the whole of the moisture it had 

 dissolved when near the surface : the consequence will be, that 

 a portion of water is disengaged in the state of what is termed 

 vesicular vapour, that is, holding intermixed with it a portion of 

 atmospheric air. Now as the air is considerably rarer at the 

 height of two or three miles (the usual elevation of the clouds) 

 than nearer the surface, it may easily happen, that the watery 

 vesicles in the progress of their descent meet at length with a 

 stratum of air of such density as to be just equivalent to the 

 weight of the vesicular vapour joined with that of the air inter- 

 mixed with it ; and here they will remain suspended, until the 

 air within becoming of nearly equal density with the atmosphere 

 without, or the particles coalescing into larger drops, the whole 

 becomes too heavy for the atmosphere to support. 



Nor is this a mere hypothesis ; the presence of intermixed air 

 seems confirmed by the fact, that hail-stones, which are 

 evidently derived from this source, always are found hollow in 

 the centre, and the flakes of snow which fall so slowly through 

 the atmosphere owe their low sp. gr. to the same cause. 



Indeed it is scarcely possible to conceive a disengagement of 

 moisture to take place, without, at the same time, supposing a 

 a certain portion of air to be intermixed with it ; and, accordingly, 

 this idea seems to be very general, judging from the term 

 " vesicular vapour," in common use, as applied to the moisture 

 of fogs and clouds. 



To apply this theory to particulars ; two of the principal modi- 

 fications of clouds, according to Mr. Howard's nomenclature, 

 are the cirrus and the cumulus ; the cirrus, including those 

 light fleecy clouds which occupy the higher portions of the 

 atmosphere ; the cumulus, those denser and less elevated clouds 

 which increase from above in convex or conical heaps. 



The cirri would seem to be formed immediately from the dis- 

 engagement of the vesicular vapour, and, therefore, to occupy 

 that line of elevation at which the quantity of moisture held in an 



Rat? of evaporation in fmur - 

 Milei above the surface. quence of toe dimiui.hed AKiuity of air for moisture. 



pressure. 



6 128 2 



5 64 4 



4 32 8 



3 16 16 Medium point 



2 S 32 



1 4 64 



2 128 



Ilenee the smallest quantity of moisture will be contained in the atmosphere at the 

 medium point. 



* I need not here insist upon the beautiful provision made by nature for pre- 

 serving a constant circulation in the atmosphere, by causing every accession of 

 h«<tt to be accompanied by a drcrea.-c iu density. 



