36 



Mr. G. J. Stoney on the Physical 



[Recess, 



with dry air up to the point of being but just unable to precipitate itself 

 while exposed underneath to the heat of the penumbra. If a current so 

 charged with vapour happen to cross the umbra, it will receive less heat from 

 below, and some of the vapour in it will now be able by radiation to main- 

 tain itself as cloud. This cloud will be peculiarly circumstanced. It is 

 formed from an isolated body of vapour, and once formed will continue in 

 existence, since the hot currents which will rise at intervals through it when 

 convection sets in, will consist of dry air unable to generate the cloud 

 overhead, which would otherwise screen it from the open sky. It will 

 accordingly often find itself under circumstances to become by reason of 

 this prolonged existence progressively cooler ; and as the temperature falls, 

 more of the vapour is able to precipitate itself, until at length the cloud 

 becomes so dense that rain sets in. The rain is probably caught and 

 dissolved in the dry air below, long before it can reach the body of the sun ; 

 but if it last through a space of even a few thousand metres, it will give to 

 the bridge of vapour the brightness of a facula. In other cases the vapour 

 either carried into the umbra from around, or perhaps rising into it 

 from a steaming ocean beneath, appears to form mere pellicles of cloud 

 that mottle its deep shadow. When the storm is of the nature of a 

 whirlwind, a current of dry outer air which has not lapped up mois- 

 ture from the photosphere, usually seems also sucked in, and manifests 

 its presence in the dark spot which Mr. Dawes has called the nucleus of 

 the umbra. 



61. It appears more reasonable to suppose that the phenomena which 

 have hitherto been explained by the transference of ponderable matter over 

 immense distances in incredibly short times, the filling up of gulfs, and the 

 like, are phenomena of the rapid formation or dissolution of cloud, and lose 

 much of their marvellous character. Terrestrial cloud may be seen to form 

 within a very few minutes over the whole of the visible heavens, and often 

 when there is no wind, or apparently advancing against the wind. 



62. If there be a substance in the sun of low vapour-density, but not 

 capable of existing in a state of vapour in the coolness of the height to 

 which it would otherwise rise, and if this refractory substance is volatile at 

 the temperature and pressure which exist lower down, it will behave in a 

 very peculiar manner. In the lower strata of the sun's atmosphere it will 

 exist as a vapour ; and from this situation it will keep continually making- 

 its way upward in its effort to find its natural level. Before it reaches its 

 destination, however, the gas incessantly streaming upward will as inces- 

 santly be precipitated. If the particles of the cloud so formed are heavier 

 than the surrounding atmosphere, they will begin to subside. Not only 

 so, but the chill caused by their radiation in their new solid or liquid state, 

 will make the inverse flame spoken of in § 8 burn downwards, until it 

 sinks to that level at which the upward supply of vapour, owing to its 

 tendency to diffuse itself upwards, or caused by currents of convection, 

 exactly balances the downward motion of the fiery cloud from subsi- 



