55 rAt HAfXftiAL SJ«TORY OT CttfUDS. 



Nature of th« ingby subsidence iu too great plenty to be immediately assi- 

 cumulo-stra- ^lilated ; in consequence of which they tend to unite among 

 themselves. A still greater quantity of haze, in the region 

 next above tbe cutnulus, givets rise to tht curious pheuonie- 

 non of the cloud-capped cloud ; when the cumulus is covered 

 at its summit with a cirro-stratus; in the same manner as, 

 iii mountainous tracts, this cloud reposes on an elevated 

 point of land. The (rause is probably alike in each casQ, 

 vthether it be a lower teiopfcrature, or a diminished electri- 

 city, which determines to this particular spot the commence- 

 ment of the aggregation of the cirro-stratus. We may next 

 c'6nsider the cumulo-stratus perfectly formed, and endea- 

 V6tir to assign a cause for its occasional long continuance: 

 which, however, exceeds the day of its formation only on the 

 approach 6f thunder: this cloud, as well as the cumulus, 

 very commonly vanishing abdut sun-set, and reappearing 

 the next day, for some time. The two strata of ihe atmo- 

 sphere, which forn) the superior and inferior boundaries of 

 the cloud, are probahly, during this time, in somewhat dif- 

 ferent states of electricity ; the one a ls»'«ide positing water, 

 the other receiving it; the broad surface of the cumulo* 

 stratus may be regarded as a coating, applied to the upper 

 stratum; and receiving from it s^ continual accession of 

 charged particles of water, the electricity of which is slowly 

 transmitted, through the intermediate j>ovtion, down to the 

 base of the cloud, which is often some hundred feet below ; 

 and where a continual evaporation counteracts the increase 

 above. Here, while ti)e fiiass continues in this moditication, 

 the progress of the electricity downwards is arrested by the 

 dry air: for although the insulated rod is found sometimes 

 to be affected with positive, sometimes with negative signs, 

 tthile the base of such r'louds is over it, this effect is com- 

 (fiftonly influential; and the rod is not charged, as by the 

 ]fjas6age of the nimbus. How the electricity of this cloud 

 Js affected by the constaht evapbration of a portion at the 

 base remains to be ascertained ; and the same may be said 

 1« to th6 diirntilut;. 



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