362 Formation and Characteristics 



the watery globules, in their descent, meet with a stratum of 

 air of that density which is just equivalent to the weight of 

 the vesicular vapour and the contained atmospheric air to- 

 gether. At this point they will remain suspended till such 

 time as the air contained within the different vesicles becomes 

 of equal density with the surrounding atmosphere." So far, 

 so good ; but, as the parietes of these vesicles are elastic, 

 what is to prevent them from being reduced almost immedi- 

 ately to the temperature of the surrounding strata, then con- 

 tracting, and gradually, I may say rapidly, becoming too heavy 

 for the air to sustain ? 



If I might hazard a conjecture, I should say, that the clouds, 

 so long as they exist as clouds, are always of a higher tem- 

 perature than the medium that supports them ; and, for this 

 reason, these masses of aqueous vapour possess the power of 

 absorbing heat, although the air in which they float scarcely 

 can be said so to do: they may, therefore, maintain their tem- 

 perature by absorbing caloric from the sun's rays during the 

 daytime, and by receiving radiated caloric from the earth 

 during the night. Perhaps, then, a combination of Howard's 

 theory with Mr. White's may explain the matter: the similar 

 electricity of each vesicle may cause repulsion of the sur- 

 rounding ones ; and the superior temperature of the clouds 

 will give them superior lightness. 



Birmingham, May 25. 1836. 



Art. X. Views on the Modes of Formation, and a Notice of the 

 Characteristics, of the Kinds of Cloud, Cumulus and Cirrus, 

 and certain Varieties of these. By W. R. Birt, Esq., Author 

 of Tabulae Anemologictz, or Tables of the Wind, noticed in 

 p. 224. 280. 



I observe in Mr. White's interesting discussion on the 

 principles on which clouds are suspended (p247 — 251.), that 

 Mr. White considers the cumulus to be formed from the 

 cirrus, in consequence of the ascending portions of the aqueous 

 vapour exceeding the evaporation from the upper surface of 

 the stratum of air that holds the vapour in solution. This, he 

 conceives, causes the cirrus gradually to descend to the earth, 

 and, at the same time, proportionably to increase its bulk, so 

 as to form cumulus. 



During the course of my observations (now extending 

 over a period of four years, and which have been taken daily), 

 I have arrived at the conclusion, that the cirrus and cumulus 



