ON THE SOLAR SYSTEM. 501 



revolutions, excepting those of some of the comets, of which the motions 

 are retrograde, and those of some of the satellites of the Georgian planet, 

 which revolve in planes so distant from those of the other planetary motions, 

 that the directions of their revolutions can scarcely be called either direct 

 or retrograde. 



The time and direction of the stm's rotation is ascertained by the change 

 of the situation of the spots, which are usually visible on his disc, and which 

 some astronomers suppose to be elevations, but others, apparently on 

 better foundations, to be excavations or deficiencies in the luminous matter 

 covering the sun's surface. These spots are frequently observed to appear 

 and disappear, and they are in the mean time liable to great variations, 

 but they arc generally found about the same points of the sun's surface. 

 Lalande imagines that they are parts of the solid body of the sun, which, 

 by some agitations of the luminous ocean, with which he conceives the sun 

 to be surrounded, are left nearly or entirely bare. Ur. Wilson and Dr. 

 Herschel are disposed to consider this ocean as consisting rather of a flame 

 than of a liquid substance, and Dr. Herschel attributes the spots to the 

 Anission of an aeriform fluid, not yet in combustion, which displaces the 

 general luminous atmosphere, and which is afterwards to serve as fuel for 

 supporting the process; hence he supposes the appearance of copious 

 spots to be indicative of the approach of warm seasons on the surface of the 

 earth, and he has attempted to maintain this opinion by historical evidence. 

 The exterior luminous atmosphere has an appearance somewhat mottled, 

 some parts of it, appearing brighter than others, have generally been called 

 faculae; but Dr. Herschel distinguishes them by the names of ridges and 

 nodules. The spots are usually surrounded by margins less dark, than them- 

 selves, which Dr. Herschel calls shallows, and which he considers as parts 

 of an inferior stratum consisting of opaque clouds, capable of protecting tlie 

 immediate surface of the sun from the excessive heat produced by combus- 

 tion in the superior stratum, and perhaps of rendering it habitable to ani- 

 mated beings. (Plate XXXI. Fig. 465 . . 469.) 



But if we inquire into the intensity of the heat which must necessarily 

 exist wherever this combustion is performed, we shall soon bd conviucecJ 

 that no clouds, however dense, could impede its rapid transmission to the 



