OF ARTS AND SCIENCES. 151 



be thin, or its temperature cannot be relatively high throughout, as 

 demanded by the theory. This evident condition doubtless gives the 

 reason why the hydrogen lines, though the broadest in the solar spec- 

 trum, are not sensibly double. 



The theory of the constitution of the sun above proposed may be 

 briefly recapitulated thus : — 



Convection currents, directed generally from the centre of the sun, 

 start from a lower level, where the temperature is probably above the 

 vaporizing temperature of every substance. As these currents move 

 upward they are cooled, mainly by expansion, until a certain element 

 (probably of the carbon group) is precipitated. This precipitation, 

 restricted from the nature of the action, forms the well-known granules. 

 There is nothing which has come under my observation which would 

 indicate a columnar form in these granules under ordinary circum- 

 stances. 



The precipitated material rapidly cools, on account of its great ra- 

 diating power, and forms a fog or smoke, which settles slowly through 

 the spaces between the granules till revolatilized below. It is this 

 smoke which produces the general absorption at the limbj and the 

 " rice-grain " structure of the photosphere. 



When any disturbance tends to increase a downward convection 

 current, there is a rush of vapors at the outer surface of the photo- 

 sphere toward this point. These horizontal currents, or winds, carry 

 with them the cooled products of precipitation, which, accumulating 

 above, dissolves slowly below in sinking. This body of smoke forms 

 the solar spot. 



The upward convection currents in the region of the spots are bent 

 horizontally by the centripetal winds. Yielding their heat now by 

 the relatively slow process of radiation, the loci of precipitation are 

 much elongated, thus giving the region immediately surrounding a 

 spot the characteristic radial structure of the penumbra. 



This conception of the nature of the penumbra implies a ready in- 

 terpretation of a remarkable phenomenon, amply attested by the most 

 skilful observers, and, as far as my knowledge goes, wholly unex- 

 plained ; namely, the brightening of the inner edge of the penumbra in 

 every well-developed spot.* 



* Relating to this phenomenon we see important observations by Professor 

 Langley, Am. Jour. Sci., Vol. IX. (1875,) p. 194; also Le Soleil, par Le P. 

 A. Secclii, Paris, 1875, Chap. IV. p. 81, and particularly Fig. 46, p. 90, with 

 explanatory text. 



