356- On the Modification of Clouds, &c. 



exercised at great distances ; and when the cloud has arrived 

 at a considerable size, its protuberances are seen to form, and 

 successively sink down into the mass, in a manner which 

 forces one to suppose a shower of invisible drops rushing 

 upon it from all parts. 



In unsettled weather the rapid formation of large cumuli 

 • iias been observed to clear the sky of a considerable hazy 

 " whiteness, which on the other hand has been found to ensue 



■ upon their dispersion*. 



On these considerations wc are obliged to admit as a co- 



■ orperating cause of the increase of this cloud, that sort of at- 

 traction which large insulated conducting masses exercise, 



■ when charged, on the smaller ones which lie within their 

 ' influence. Instead of ai spherical aggregate, however, we have 



only a sort of hemisphere, becavrse that part of the cloud 

 which presents itself towards the earth can receive no ad- 

 dition from beneath, there being no condensed water. On 



" the contrary, the mass must be continually suffering a di- 

 minution there by the tendency of the cloud to subside, and 

 of the vapour plane to rise during the increase of the diur- 

 nal temperature. It is this evaporation that cuts ofl' all the 

 cumuli visible at one time in the same plane, and it is rea- 

 sonable to conclude that much of the vapour thus produced 



' is again condensed without quitting the cloud, as its course 

 would naturally be mostly upward. Thus the drops of 



■ which a cumulus consists may become larger the longer it 

 is suspended, arrd the electricity stronger from the compa- 

 rative diminution of surface. 



Such is probably the manner in which this curious struc- 

 ture is raised, while the base is continually escaping from 



■ beneath it. That we mav not however be accused of build- 

 ing such a castle in the air by attempting further conjec- 

 ture, we may leave the present modification, after recapitu- 



• lating some of its circumstances which appear to be ae- 

 covmtcd for. 



The cunmlus appears only in the dav time, because the 

 direct action of the sun's rays upon the earth can alone put 

 the atmosphere into that state of incqualitv of temperature 

 which has been described. It evaporates in the evening 



* That clouds are not always evapor.-iiccl when they disappear, but 

 somctinres djipe.rscd so as 10 liecome iiivibiblt as diitiiict aggiegvues^ is 

 a fact prttty v ell ascertained by observHtions. This luippcus soineii'.v.cs 

 by the approach of other ciouds ; at others, the evaporaiioii of part of a 

 cumulus is followed by the dispersion of the remainder. The criterion 

 used was the speedy production of ti luuti^ii^ncy in the one case, and of 

 hazy turbiJness in the other. 



from 



