THE NiTURAL HISTORY OF CLOUDS. ^l 



((^loud is not even so 2;ood a conductor as lias been supposed, 

 and that the fluid, in certain cases, may be very gradually 

 transmitted through it. Positive electricity being that pro- 

 per to the atmosphere in fair weather, we should naturally 

 expect to find it in this cload. 



It mi2:ht be worth while to examine the air above, with a 

 view to discover whether there exists in the latter a negative 

 couuter-chart^e. It will appear, from a consideration of the 

 principles before stated, why this cloud is almost peculiar to 

 the autumn. The j^radual decline of the sun, at this season, 

 keeps the atmo-phere constantly surchiirged with vapoiirj 

 which is ultimately disposed of in rain; and hence follow 

 gales of wind. The stratus, therefore, though an immedi- Indications, 

 ate indication and accompaniment of fair weather, affords 

 an unfavourable prognostic in the early part of summer; as 

 it shows that a tendency has already begun to extensive pre- 

 cipitation, at a time when the usual predominant feature is 

 increasing dryness. ' 



Of the Nature of the Cumuius. 



The heating effect of the sun's rays on the atmosphere is Nature of the 

 greatest near the surface of the Earth, and diminishes gradu- cumulus, 

 ally in ascending. The diminution proceeds in fair weather 

 at the rate of about one degree for each hundred yards, as 



, appears by observations with the thermometer on stations of 

 known ditTerence in altitude. 



This inequality appears to give rise to the cumulus, on 

 the same principles as those of the stratus, but the effects are 

 more complicated. Vapour is generated, as before, at the 



. surface of the Earth, but it is thrown into an atmosphere 

 heated by the sun. Here it maintains its elastic state, and, 



. in proportion to the supply from below, the whole quantity 

 existing in the atmosphere is compelled to rise. In doing 

 this, it changes its climate, and arrives among air of a lower 

 temperature, where a portion is continually decomposed, 

 filling the middle region with heze. Of this, small aggre- 

 gates beo^in to be formed, the increase of which is at first 

 determined by no particular taw. But the aggregate is not 

 in equilibrium with the air. It tend* to subside, and in the 

 E 2 xke^ 



