52 THE NATURAL HISTORY Of CLOUDS. 



Nature of the mean time the increase of temperature is proceeding upward* 

 cumulus. Henee the lower part sooo finds a position in a plane of air 



sufficiently warm to e\'aporate it: and as this eifect is rcjgu* 

 lated, in general, by the elevation alone, we see these ag»« 

 gregates assume each a fiat base, resting as it were on th^ 

 same plane, parallel to the Earth's surface. The remainder 

 of the cloud sports in all the varieties of the spheroid, and 

 more rarely of the cone; according to the course of the 

 showers of minute particles of water, which we may consider 

 (though invisible in their progress) as descending upon it. 

 The vapour generated at the base is, probably, in part con* 

 ddnsed dn the surface of the colder particles of the cloud 

 above. While the supply from the haze exceeds the waste 

 by evaporation, the cloud iucreages : when the latter has be- 

 gun to prevail, it may be traced through various stages of 

 diminution to its final wreck, on sinking wholly into the 

 warmer atmosphere. This happens commonly about sun- 

 set; because the ascending current of vapour, the source 

 of the phenomenon, then slackens or ceases; and the lower 

 air parting with its redundant caloric to the higher, we un- 

 expectedly see the dense clouds evaporate, at the very time 

 when th^ chill of the evening h felt below, and the dew 

 falls. 



But it cloes not appear, that the causes we have hitherto 

 enumerated are fully adequate to the phenomenon. The 

 increase of the cumulus is often more rapid than consists 

 with the notion of simple attraction, exercised between dis- 

 tant particles of water, in a resisting medium. When a 

 cumulus is thus increasing, the small aggregates in its way 

 do not usually join it, but seem to vanish before it. Lastly, 

 the cumulus itself, however dense, never descends in rain. 

 It is difficult to conceive, that so powerful an attraction could 

 exist for many hours, without bringing the particles toge- 

 ther into larger and larger drops, until they were too heavy 

 for longer suspension. If we suppose, however, that^ from 

 the coruniencement of its aggregation, the cumulus becomes 

 a positively electrified mass, these difficulties vanish. This 

 mass may electrify negatively, and attract into itself, from 

 great distances, both the dispef^ieid particles of water and 

 those which have already united in much smaller masses. 



Its 



