219 



greater than ordinary rapidity. At a considerable distance 

 from the thundercloud, where the atmosphere is free from 

 cloud, the air descends, acquiring temperature according to 

 the law of couvective equilibrium in dry air. The air then 

 traverses the ground towards the region where the storm is 

 raging, acquiring moisture as it proceeds, but probably 

 without much diminution of temperature, on account of 

 the heated ground making up for the cold of evaporation. 

 Arrived under the thundercloud, the air rises, losing tempera- 

 ture, but at a diminished rate, owing to the condensation of 

 its vapour to form part of the immense cumulus cloud which 

 overcasts the sky on these occasions. The upward current of 

 air carries the cloud and incipient rain drops upwards, but 

 presently, in consequence of the increased capacity of the mass 

 from the presence of a large quantity of water, the refrigera- 

 tion of the air, in consequence of its dilatation, will be so far 

 diminished as to prevent the condensation of fresh vapour, 

 and ultimately to redissolve the upper portion of the cloud. 

 This phenomenon, which has been noticed by Rankine in the 

 cylinder of the steam engine, will account for the defined 

 outline of the upper edges of cumulus clouds. The upward 

 current no doubt extends occasionally to regions below the 

 freezing temperature. If cloud be carried with it, snow 

 or hail will be formed, which, if sufficiently abundant, 

 will pass through the cloud and fall to the ground before 

 it is melted. Now, the dry cold air in which the snow 

 and hail are formed is a perfect insulator. Ice has also 

 been proved, by Achard, of Berlin, to be a non-conductor 

 and an electric. Even water, in friction against an insulator, 

 is known from the experiments of Armstrong, explained by 

 himself and Faraday, to be able to produce powerful electric 

 effects, and this fact has been suggested by Faraday to 

 explain powerful electric effects in the atmosphere. Sturgeon 

 has noted the remarkable development of electricity by hail 

 showers. Few heaw thunderstorms occur without the fall 



