Book III. 



OF THE ATMOSPHERE. 



248 



551 



latter, or superadding a wide-spread structure to its base (/) ; 7. Cumulo-cirro-stratus, 

 vel Nimbus ; the rain-cloud, a cloud or system of clouds from which rain is falling. 

 It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it 

 laterally and from beneath (g, g) y 8. The Fall Cloud, resting apparently on the surface 

 of the ground {It) . 



2293. The cirrus appears to have the least density, the greatest elevation, the greatest variety of extent 

 and direction, and to appear earliest in serene weather, being indicated by a few threads pencilled on the 

 sky Before storms they appear lower and denser, and usually in the quarter opposite to that from which 

 the storm arises. Steady high winds are also preceded and attended by cirrous streaks, running quite across 

 the sky in the direction they blow in. - , . . 



2294. The cumulus has the densest structure, is formed m the Ipwer atmosphere, and moves along with 

 the current next the earth. A small irregular spot first appears, and is, as it were, the nucleus on which 

 they increase. The lower surface continues irregularly plane, while the upper rises into conical or hemi- 

 spherical heaps ; which may afterwards continue long nearly of the same bulk, or rapidly rise into moun- 

 tains. They will begin, in fair weather, to form some hours after sunrise, arrive at their maximum in 

 the hottest part of the afternoon, then go on diminishing, and totally disperse about sunset. Previous to 

 rain the cumulus increases rapidly, appears lower in the atmosphere, and with its surface full of loose 

 fleeces or protuberances. The formation of large cumuli to leeward in a strong wind, indicates the ap- 

 proach of a calm with rain. When they do not disappear or subside about sunset, but continue to rise, 

 thunder is to be expected in the night. 



2295. The stratus has a mean degree of density, and is the lowest of clouds, its inferior surface commonly 

 resting on the earth in water. This is properly the cloud of night, appearing about sunset. It compre- 

 hends all those creeping mists which in calm weather ascend in spreading sheets (like an inundation of 

 water) from the bottoms of valleys, and the surfaces of lakes and rivers. On the return of the sun, the 



