1868.] 



Constitution of the Sun and Stan 



35 



Local showers are in other cases the cause of brightness in the umbra and 

 penumbra. 



The sudden increase of density of the sun's atmosphere at the pho- 

 tosphere must serve to keep the luminous stratum in a nearly spherical 

 form. The surfaces of the gases above the photosphere may be violently 

 tossed about by the storms of the solar atmosphere, but the surface of the 

 photosphere is never carried further than to the top of a facula or the bot- 

 tom of the umbra of a spot. 



5 7. The winds which affect the photosphere may be distinguished into 

 two classes, those of the sun's outer atmosphere, and those of the regions 

 within the photosphere. Both classes may coexist in different parts of 

 the same storm. The former class sweeping through the open space above 

 the photosphere, and through rarefied air, will often come from far, and as 

 a general rule be the swiftest. Those below, moving in the dense part of 

 the atmosphere, and perhaps within a confined space, can but seldom 

 attain the same high velocity. 



58. Both classes of wind tend to obliterate the cool film in which clouds 

 usually exist, and to replace it by hotter air. But the hotter air substi- 

 tuted by winds from below, will be equally charged with moisture ; while 

 winds from above will tend to dilute with dry air both the cool film and 

 the adjoining strata immediately under it. In both cases new and more 

 transparent clouds will form ; but in the former case the rain will not cease, 

 and we have only facula ; in the latter it may and often does, — in fact, 

 whenever the film of clouds and the subjacent stratum with which it is 

 mixed by convection, have been rendered sufficiently dry. When by pro- 

 longed convection this state of things is passing away, there will be a 

 struggle between dry weather and wet, which we shall see in the patched 

 appearance of the penumbra. 



59. An umbra presents itself when the cloud, too, is removed, and the 

 dusky body of the sun seen through the opening. It does not seem likely 

 that this can take place so long as there is any of the moist stratum at a 

 temperature below its boiling-point and exposed to radiation. If this view 

 be correct, the umbra can only occur either when the depression caused 

 by a rotatory storm, or by winds impinging from above, has obliterated the 

 dense stratum and brought the air into contact with the ocean ; or when, 

 by the influx of hot air from above or the upheaving of the hot strata 

 beneath, it has come to pass that throughout the whole of a vertical column 

 there is no place where the vapour which forms cloud is at a temperature 

 below its boiling-point. If this happen through the rise of subjacent 

 strata, we should have an umbra without penumbra ; and it does not seem 

 impossible that the same appearance may sometimes present itself where a 

 depression is caused by a wind impinging from above which has not exerted 

 much horizontal friction against the surrounding parts of the photosphere. 



60. It must often happen that a hot current sweeping over the surface 

 of the penumbra dissolves away part of the cloud, diluting the vapour 



D 2 



