250 Principles on which Clouds are suspended, 



will descend in the form of rain, I think I am borne out in 

 this opinion by the well-known fact, that hailstones, which are 

 evidently derived from the same source, are usually found to 

 be hollow in the centre ; and, also, that the low specific gravity 

 of large flakes of snow, which descend so slowly and majestic- 

 ally through the atmosphere, is to be attributed to the same 

 cause. Taking the term vesicular vapour in its common ac^ 

 ceptation, as applied to the moisture of clouds and fogs, it 

 seems scarcely possible to conceive any disengagement of 

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

 a certain portion of air to be intermixed with it. 



I will now endeavour to put this theory to the test, by 

 applying it to one or two of the principal modifications of 

 clouds, as laid down in Howard's nomenclature. And, first, 

 the cirrus, or that species of cloud which has the greatest 

 variety of extent and direction; sometimes remaining almost 

 stationary for many hours, and at other times undergoing 

 exceedingly rapid changes; so that the cirrus cloud has been 

 denominated the " Proteus of the skies." After some con- 

 tinuance of clear weather, the cirrus is frequently the first cloud 

 to be seen; appearing like fine white threads, penciled, as 

 Mr. Howard has it, on the clear blue sky. These fine thread-- 

 like pencilings, if I may so denominate them, occupy the 

 highest station in the atmosphere; and are, I conceive, imme- 

 diately formed from the setting free of the vesicular vapour ; 

 and, hence, they occupy that precise line of elevation at which 

 the quantity of moisture held in an aeriform state is at its 

 minimum. They therefore often remain, in the summer time, 

 for a long time without having any apparent increase or dimi- 

 nution ; because, while the evaporation is continually going on 

 from the upper surface, and thereby tending to diminish the 

 clouds, the ascending portions of air keep continually supply- 

 ing on the lower surface, so that no actual diminution takes 

 place ; but, when the ascending portions exceed the evaporation 

 from the upper surface, then the clouds begin to descend, or 

 gravitate slowly towards the earth, and, at the same time, pro- 

 portionably increasing their bulk ; and thus form, in the 

 second place, that modification of cloud, which Howard terms 

 cumulus * ; which, also, will be found to occupy that par- 

 ticular line of elevation at which the density of the atmo- 

 sphere is exactly equivalent, as in the former case, to the 

 weight of the vesicular vapour, containing portions of atmo- 

 spheric air mixed with it. 



I do not submit these observations to your scientific readers 



* It is very interesting to witness the inosculation of different modifica- 

 tions of clouds. 



