288 CELESTIAL DYNAMICS. 



like the clouds in our air, or in wide-spread planes and flame- 

 like columns, or in rays like the aurora borealis. 



" Near large spots, or extensive groups of them, large 

 spaces are observed to be covered with peculiarly marked 

 lines much brighter than the other parts of the surface ; these 

 lines are curved, or deviate in branches, and are called faculae. 

 Spots are often seen between these lines, or to originate there. 

 These are in all probability the crests of immense waves in 

 the luminous regions of the solar atmosphere, and bear wit- 

 ness to violent action in their immediate neighbourhood." 



The changes on the solar surface evidently point to the 

 action of some external disturbing force ; for every moving 

 power resident in the sun itself ought to exhaust itself by its 

 own action. These changes, therefore, are no unimportant 

 confirmation of the theory explained in these pages. 



At the same time it must be observed that our knowledge 

 of physical heliography is, from the nature of the subject, 

 very limited ; even the meteorological processes and other phe- 

 nomena of our own planet are still in many respects enigmat- 

 ical. For this reason no special information could be given 

 about the manner in which the solar surface is affected by 

 cosmical masses. However, I may be allowed to mention 

 some probable conjectures which offer themselves. 



The extraordinarily high temperature which exists on the 

 sun almost precludes the possibility of its surface being solid ; 

 it doubtless consists of an uninterrupted ocean of fiery fluid 

 matter. This gaseous envelope becomes more rarefied in 

 those parts most distant from the sun's centre. 



As most substances are able to assume the gaseous state 

 of aggregation at high temperatures, the- height of the sun's 

 atmosphere cannot be inconsiderable. There are, however, 

 sound reasons for believing that the relative height of the so- 

 lar atmosphere is not very great. 



Since the gravity is 28 times greater on the sun's surface 

 than it is on our earth, a column of air on the former must 



