28 



Mr. Gr. J. Stoney on the Physical [June 20^ 



hydrogen alone, the temperature goes on decreasing till it becomes exces- 

 sively cold. These results are made out with probabilities 2 and 3. 



Within the luminous clouds the temperature very rapidly waxes, and the 

 density, too, appears to receive a nearly sudden increase. All gases with a 

 vapour-density more than about eighty times that of hydrogen are imprisoned 

 within the shell of clouds by the comparative chill which there prevails, 

 cooperating with the intensity of the force of gravity exerted by the sun. 

 Between the film of clouds and the stratum immediately beneath there are 

 violent motions of convection, which both carry up fresh vapour to be con- 

 densed into cloud, and carry down the cloud into a region where it becomes 

 mist and rain. It is convenient to restrict the word cloud to cloud in that 

 situation in which it can form, giving the names mist or rain to the cloud 

 when carried down, either by currents of convection or by subsidence, into 

 a position from which there is not that abundant radiation towards the 

 sky which is essential to its forming. The clouds, in this restricted sense 

 of the term, are everywhere of a gauze-like transparency to admit of the 

 copious radiation towards the sky which is requisite ; and this enables spec- 

 tators upon the earth to see through them the light emitted by the mist and 

 rain beneath. This mist and rain seem everywhere, except in the solar 

 spots, to be dense enough to be opake, and therefore emit the maximum 

 light corresponding to their temperature. This temperature is higher than 

 that of the clouds, and accordingly the mist and rain constitute a back- 

 ground brighter than the luminous clouds. 



Hence the finely-granulated appearance of the surface of the sun, the 

 currents of convection creating a kind of honeycombed structure in the 

 stratum of clouds ; the ascending currents carrying up hot vapours in which 

 only excessively thin cloud can form, since under these unfavourable cir- 

 cumstances its lowest parts cannot tolerate even the slight obstruction to 

 their radiating freely which a cloud of the average density would offer ; and, 

 on the other hand, the descending currents carrying down those portions 

 which by prolonged radiation have cooled down abnormally, and thus be- 

 come both more opake by the condensation of more cloud, and less bright. 

 Those portions which by the most persistent radiation cool down the most, 

 seem to furnish the very dark specks which have been taken notice of by 

 observers. 



Hence also arises the gradationof light which is observed upon the sun's 

 disk. In the middle of the disk we look vertically through the honey- 

 combed structure which has been described, and see through it the brighter 

 background almost without any intervening obstruction. But as we turn 

 our eyes towards the margin of the disk, we look more and more obliquely 

 across the columns, which progressively intercept increasing quantities of 

 the brighter hght from beyond, and substitute for them their own feebler 

 radiations. 



If by disturbances in the atmosphere the hotter stratum on either side 

 is made in certain places to encroach upon the luminous clouds, they are 

 unable to maintain in this situation as low a temperature as elsewhere, and 



