ON THE SOLAR SYSTEM. 399 



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

 conceives the sun to be surrounded, are left nearly or entirely bare. Dr. 

 Wilson* and Dr. Herschel are disposed to consider this ocean as consisting 

 jjiher of a flame than of a liquid substance, and Dr. Herschel attributes 

 ''iybr-Fspots to the emission 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 appear- 

 ance 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 appear- 

 aacfc somewhat mottled ; some parts of it, appearing brighter than others, 

 hav;r ^nerally ^ een called faculae ; but Dr. Herschel distinguishes them 

 by the names of ridges and nodules. The spots are usually surrounded 

 by margins less dark than themselves, which Dr. Herschel calls shallows, 

 and which he considers as parts of an inferior stratum consisting of 

 opaque clouds, capable of protecting the immediate surface of the sun 

 from the excessive heat produced by combustion in the superior stratum, 

 and perhaps of rendering it habitable to animated 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 be convinced 

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

 parts below. Besides, the diameter of the sun is 111 times as great as that 

 of the earth ; and at its surface, a heavy body would fall through no less 

 than 450 feet in a single second ; so that if every other circumstance per- 

 mitted human beings to reside on it, their own weight would present an 

 insuperable difficulty, since it would become nearly thirty times as great as 

 upon the surface of the earth, and a man of moderate size would weigh 

 above two tons. Some of the most celebrated astronomers have imagined, 

 from the comparative light of different parts of the sun's disc, or apparent 

 surface, that he is surrounded by a considerably dense and extensive atmo- 

 sphere, imperfectly transparent ; conceiving that, without such an atmo- 

 sphere, the marginal parts, which are seen most obliquely, must appear 

 considerably the brightest ; but this opinion is wholly erroneous, and the 

 inferences which have been drawn from it, respecting the sun's atmosphere, 

 are consequently without foundation. 



We are, however, assured, by direct observation, of the existence of some 

 aerial substance in the neighbourhood of the sun, producing the appearance 

 called the zodiacal light, which is sometimes seen, nearly in the plane 

 of the sun's rotation on its axis, extending beyond the orbit of Mercury. 

 It is said to have been first distinctly described in Childrey's Britannia 

 Baconica, a work published in 1661 ; and it was afterwards more par- 

 ticularly observed by Cassini,t Mairan,* and others. In the torrid zone it 



* Ph. Tr. 1774, p. 1 ; 1783, p. 144. See also ibid. vi. 2216, 2295, and 3020 

 Cassini, Mem. de 1'Acad. x. 581. Herschel, Ph. Tr. 1795, p. 46 ; 1801, pp. 265, 

 354. Mossotti, Cesaris Effemeridi, 1820-1. Nicollet, Connoissance des Temps 



t Hist, et Mem. vii. 119 ; viii. 193. 



Mairan, Traite de 1' Aurore Boreale, Suite des Mem. de 1'Acad. Par. 1731 and 

 1751, 4to, Paris, 1733. 



