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tion advanced by him respecting the suspension of clouds. He had, 

 however, to regret that the most essential parts of his communication 

 had not been reported. With the permission of the Society, he would 

 recapitulate his communication, in order that the readers of the "Pro- 

 ceedings," and members who were not present when he spoke on the 

 subject in question, might not have an erroneous impression. He 

 would also add some suggestions which had recently occurred to him. 



It is well known that, although there were occasionally two differ- 

 ent sets of clouds pertaining severally to different currents of air, one 

 above the other, — usually, in fair weather, there was but one set. In 

 either case all the clouds belonging to one current are seen to be 

 situated somewhere between two levels. Above the space, included 

 between these levels, none are seen to rise; nor are any observed to 

 sink below its lower boundary. It was conceived that the causes of 

 this persistence of the clouds between two horizontal planes, of which 

 the lower one is usually more than a mile in height, had never been 

 satisfactorily assigned. 



Agreeably to the prevalent impression that clouds are enduring 

 masses of condensed aqueous vapour, their specific gravity ought to 

 be much greater than that of the subjacent cloudless air, over which 

 they swim; since the little watery bubbles of which they are formed, 

 consist, not only of the air with which they are inflated, but also of a 

 liquid 840 times as heavy. But he had of late years observed that 

 clouds are not as durable as generally supposed. On the contrary, 

 like the steam condensed in escaping from boiling water, they are in- 

 cessantly forming by the condensation of aqueous vapour, and disap- 

 pearing in consequence of its being vaporized again. A cloud may 

 appear to cling to the brow of a mountain, sometimes for more than 

 an hour; when, on closer examination, it may be discovered that, as 

 one portion appears, another vanishes, and that the apparent dura- 

 bility is due to the equality of the causes of condensation and re-vapo- 

 rization. Dr. Hare had enjoyed a fine opportunity of verifying this 

 view of the subject, when involved within a cloud on the summit of 

 the Rhigi, last summer. It was then quite evident, that what might, 

 at a distance, be mistaken for an enduring mass of condensed mois- 

 ture, such as is called a cloud, was really due to a current of air, sa- 

 turated with aqueous vapour, which was rushing up the mountain 

 side. As this current reached a level, at which the temperature was 

 below its dew point, the contained vapour was converted by conden- 

 sation into a cloud ; but as it attained a higher level, where the dew 

 point was sufficiently low to compensate for the cold, the moisture 

 was made to resume the aeriform state. 



