189 



As in condensing, steam relinquishes as much heat as would make 

 it red-hot, if retained while under sufficient pressure to keep it in the 

 liquid state, it follows that, as the cloud is formed, the temperature of 

 the air with which it is associated is raised so much as to produce a 

 buoyancy which enables it to float or even to ascend ;* but as soon 

 as it reaches a point where the air is so devoid of aqueous vapour as 

 to permit it to be revaporized, a proportionable refrigeration and in- 

 crease of density ensues. Thus the buoyancy produced at one level, 

 is compensated by a commensurate opposite influence at another. Of 

 course, the clouds are always seen to occupy an interval between two 

 horizontal planes, one above the other. As soon as the aqueous va- 

 pour of the air rises above the lower plane it condenses ; before the 

 cloud thus produced can get beyond the upper one it is reconverted 

 into vapour. 



When the causes of condensation are more potent than those of re- 

 vaporization, rain ensues ; when the opposite is the case, there must 

 be a tendency to fair weather. 



Although of opinion that in hurricanes and other violent rain 

 storms, there must be an exchange of position between the lower and 

 upper strata of the air, it was conceived that showers, unaccompanied 

 by gales or squalls, were to be explained as above suggested. 



Dr. Hare had conceded that there might be more than one cause 

 for the buoyancy of clouds. Dr. Thomson, in his treatise respect- 

 ing Heat and Electricity, suggests electricity as a cause. The fact 

 demonstrated by the experiment, the results of which had been com- 

 municated to the Society at their last meeting in April, that moisture 

 does not render air a conductor of electricity, gives support to this 

 view of the subject ; especially since it has been discovered, that in 

 condensing, steam becomes highly electrified. It seems inevitable 

 that the aqueous globules, of which clouds are constituted, must sepa- 

 rate from each other, as pith balls are seen to do when similarly ex- 

 cited; and that the particles of air with which they are associated 

 must be similarly actuated: hence a cause of rarefaction, and of 

 course of buoyancy. Another cause might co-operate. It is known 

 that the radiation of heat, which causes dew and sometimes hoar- 

 frost, is so completely checked by clouds, that the last mentioned phe- 

 nomenon never takes place when the sky is overcast. Moreover, it is 

 known that the solar rays pass through the air without imparting heat, 



* See a verbal communication of Dr. Hare, made July 3d, 1S40, and pub- 

 lished in the " Proceedings" for that time. 



