Physical Constitution of the Sun. 349 



sect a sun-spot is explained. Whether we look through the 

 broken cloudy covering right to the scoria, or to a mass of cloud 

 or mist resting immediately above it, which must consequently 

 be cooler and lower than the penumbra-cloud, may at first, as 

 unimportant for the elucidation of the phenomena, be left unde- 

 cided*. 



" From the nature of these whirlwinds it results, further, that 

 towards the centre of a spot there is a descending current in the 

 atmosphere — on the contrary, at the outer borders of the pen- 

 umbra an ascending one. Tt hence follows that the inner border 

 of the penumbra must lie deeper than the outer ; and therefore the 

 entire structure of clouds which appears to us as a penumbra must 

 descend conically, or in the shape of a funnel } in the direction of the 

 centre of the nucleus. 



" The observed widening of the penumbra on the side next 

 the margin of the sun is thus explained, in the known manner, 

 as a phenomenon of the perspective. Just so the peculiar radial 

 shading of the penumbra finds its explanation in the direction of 

 the atmospheric currents. 



"Are the above-mentioned whirlstorms powerful enough to 

 carry up into higher regions the stratum to which is assigned 

 the name of the chromosphere, these parts will appear to us as 

 protuberances. Taking into account the direction of the whirl- 

 ing motion, it follows that the ascending currents must be at the 

 outer border of the penumbra ; and this circumstance explains 

 both the connexion of the protuberances with the sun-spots and 

 also the processes of stronger light observed in the vicinity of the 

 penumbra," 



As a supplement to these explanations, I take leave at present 

 to add only the following. 



According to the views here developed, the space occupied on 

 the sun's disk by the penumbra represents the region of circu- 

 lation of those currents which are produced by the difference of 

 temperature between the cooler mass of scoria and the surround- 

 ing glowing-liquid surface, in the superincumbent atmosphere. 

 The lower currents, directed centrifugally from the centre of the 

 spot, must, as cooled masses, lower the temperature of the por- 

 tions of the sun's surface bathed by them. Taking into account 

 the oppositely flowing upper current, in the penumbra we there- 

 fore look through relatively hotter upon relatively cooler strata 

 of gas ; so that through the breaks in the upper condensation- 



* It is self-evident that we must not imagine our view of the mass of 

 scoria of the nucleus of a spot to be so undisturbed and direct as that we 

 have of the lunar surface. With the vapour-laden atmosphere of the sun 

 and the still very high temperature of cooled scoria, it may be regarded as 

 indubitable that our view does not fall direct upon the solid mass of the 

 nucleus, but on the vapours condensed above it. 



