gg T«*: IJATURAL BXSTORV OF CteUjBS. 



TItc Nimbuf. A dense cloud, spreading out ialo » crowo 

 of cirrus, and parsing beneath into a shower. 



Of the Cirrus, 



Cirrus da» This is always the least dense, and commonly the most 



jcribtd, el'^uted modification. It is sometimes spread horizontally 



through a vast extent of atraosphert ; the whole breadt.i of 

 the sky being insufficient to show where it terminates. In this 

 case, its parallel bars appear, by an optical deception, to 

 converge in opposite points of the horizon. At others, it is 

 exhibited in unconnected perpendicular bund-les, of the mo£t 

 minute size. Between these extremes, it may be traced in 



Its fonnation. every degree of extent and inclination to the horizon. In a 

 sei^ene sky the cirrus is first indica eri by a few threads, pen- 

 cilled in white, on the .izi.re tV'Ound. Its iucrease takes 

 place in variows ways, and may be comptired sometimes to 



\ vegetation, more often to crystallization. 7 hus, I. Parallel 



thrfcads are added to each other horizontally, and occasion- 

 aUy other strata of the same, crossing- the first at right or 

 oblique ang"!es, uniil a delicate transparent veil is formed. 

 2. Parallel threads are collected into distinct wroups, lyiag 

 a,t viiriouH anules with the horizon. 3. Flexuous and di- 

 verging fibres are extended from the original stem, formin^g 

 the resemblance of crests of feathers, locks of hair, 8cc. 

 4. The first formed threads, become, as it were, the sup- 

 ports from whicJ^ ^others bUq uely ascend O; desiiend intp 

 the atmospbere. Lastly, A dea.se uucleus is sometim.es 

 formed, and short "fibres shoot out fwm it in all directions, 

 cry lofty. '^^^ great itslevation of the cirrus, has been ascer ained by 

 geometrical observations. " The small white streaks of 

 condensed vapour, which appear on the f re of the sky, I 

 ifiave found';" says Dalton, " by several careful observations, 

 to be from three to five miles above the Earth's surface." 



Viewed from the suviimits of the highest mountains, they 

 appear as distant as from the plains. A more easy and not 

 less convincing proof of iheir elevation may be deduced 

 from their continuing to be ti ged by the sui/s rays.in the 

 evening twilight with the mor-e vivid co juts of the prism, 

 while the denser clouds, having already paseed tlu*o«gh the 

 same gradation, are in the deepest shade. 



The 



