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502 LECTURE XLII. 



parts below. Besides, the diameter of the sun is 1 1 1 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 imaained 

 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 at- 

 mosphere, imperfectly transparent; conceiving that,without such an atmosphere' 

 the marginal parts, which are seen most obliquely, must appear consider- 

 ably the brightest ; but this opinion is wholly erroneous, and the inferences 

 which have been drawn from it," respecting the sun's atmosphere, are con- 

 sequently 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 Mer- 

 cury. It is said to have been first distinctly described in Childrey's Bri- 

 tannia Baconica, a work published in l66l, and it was afterwards more par- 

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

 almost constantly visible; and in these climates, it may often be distin- 

 guished in the beginning of March, after the termination of twilight, ex- 

 hibiting the appearance of a narrow triangle, somewhat rounded off, of a 

 whiteness resembling the milky way, ascending from the sun as a base, likjC 

 the projection or section of a very flat spheroid, and extending to a distance 

 of more than 50° from the sun. The whole orbit of Venus never subtend* 

 80 great an angle from the earth as 96°, consequently this substance must 

 occasionally involve both Mercury and Venus; and if- it were not extremely 

 rare, it would produce some disturbance in their motions ; while in fact it 

 does not appear to impede the progress even of the tails of the comets, which 

 are probably themselves of very inconsiderable density. It cannot be a 

 continuous fluid atmosphere, revolving with the same velocity as the sun; 

 for the gravitation of such an atmosphere would cause it to assume a form 

 more nearly spherical; and the only probable manner in Avhich it' can be 

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