AQUEOUS ATMOSPHERIC PHENOMENA. 



463 



the production of cloud-land, may be fulfilled in one stratum of the atmosphere and not in 

 another, and hence the frequent diversity in the appearance of the sky, the clear blue 

 fields and patches of ether alternating with visible vaporous structures. The clouds are 

 supposed to consist of vesicular vapours, or minute globules of water filled with air, but 

 there is great difficulty, even with the aid of this view of their structure, most probably 

 correct, in explaining their suspension aloft, for the globules must be specifically heavier 

 than the air by which they are upborne. The theory of ascending currents of heated air 

 has been proposed by M. Gay Lussac to account for their position ; and the retention of 

 solar heat in the clouds themselves, buoying them up, and causing them to float, by 

 M. Fresnel ; but this is a point respecting which we are left without the guidance of any 

 positive data. The clouds float at different elevations, but the higher we ascend, the drier 

 the atmosphere is found, and the less loaded with vapours. " We shall not err much," 

 soys Mr. Leslie, "if we estimate the position of extreme humidity at the height of two 

 miles at the pole, and four miles and a half under the equator, or a mile and a half beyond 

 the limit of congelation." Dr. Dalton asserts that small fleecy patches of cloud are 

 frequently from three to five miles in height, and such have been observed sailing above 

 the most elevated peaks of the Andes, which rise 25,000 feet above the level of the sea ; 

 but other authorities claim for some visible clouds a still greater elevation. The height 

 varies at different seasons of the year, and there is little doubt that it is much more 

 frequently below than above a mile. Dalton gives a table from observations made by 

 Mr. Crosthwaite of Keswick, who fixed marks on the side of Skiddaw, a mountain 1050 

 yards high, by which he was able to ascertain by inspection the height of the clouds when 

 they did not exceed that of the mountain. During five years he conducted observations, 

 three times each day, excepting a few intermissions which amounted only to missing less 

 than a week per year. The table gives the number of times, in the respective months, 

 that the clouds were at the height stated. The last column gives the number of times in 

 which either the clouds were above Skiddaw, or there were no clouds at all. 



It thus appears, that, for 12 times that the clouds were from 200 to 300 yards high, in the 

 month of January during the five years, there were 36 times in which they were from 

 1000 to 1050 yards high ; and for twice that they were at the former elevation in the 

 month of June, there were 34 times in which they were at the latter. 



The forms assumed by the clouds are so infinitely diversified, as to render it apparently 

 hopeless to attempt their arrangment in a few general modifications. But a classification 

 has been made with some success, which reduces these varied aerial objects into seven 

 genera, each of which is susceptible of such perspicuous description as to be readily 

 recognised, and referred to its appropriate class and name. Mr. Luke Howard's ingenious 



