Constitution of the Sun and Stars. 307 



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 re- 

 stricted sense of the term, are everywhere of a gauze-like transpa- 

 rency, to admit of the copious radiation towards the sky which is re- 

 quisite ; and this enables spectators 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 higherjthan that of the clouds; 

 and accordingly the mist and rain constitute a background 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 va- 

 pours in which only excessively thin cloud can form, since under these 

 unfavourable circumstances 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 become 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 gradation of light which is observed upon the 

 sun's disk. In the middle of the disk we look vertically through 

 the honeycombed structure which has been described, and see through 

 it the brighter background almost without any intervening obstruc- 

 tion. But as we turn our eyes towards the margin of the disk, we 

 look more and more obliquely across the columns, which progres- 

 sively intercept increasing quantities of the brighter light 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 therefore become abnormally thin. If this process is 

 not carried so far as to put a stop to the incessant rain beneath 

 the clouds, their increased transparency will give rise to a facula 

 when the phenomenon takes place on a large scale, and to the 

 coarsely mottled appearance of the photosphere when it presents 

 itself in smaller patches. Hence we see why a facula retains its 

 brightness up to the margin of the sun's disk, a phenomenon which 

 is inconsistent with the usually received hypothesis that the grada- 

 tion of light on the sun's disk is due to the absorption of the outer 

 atmosphere. If the rain also cease we have the penumbra of a spot ; 

 if the cloud itself is dissolved away, we have its umbra. 



The dark body which is disclosed in the umbra? and penumbrse of 

 spots must be either an untarnished ocean of some highly reflecting 



