PHYSICAL ASTRONOMY. ' 56) 



as if the planet were, in the progress of his revolutions, receding fur- 

 ther from the Sun. This is explained, if we suppose that there is, in 

 the region of Mercury, a resisting medium which moves round the 

 Sun in the same direction as the Planets move. Evidence of a kind 

 of nebulous disk surrounding the Sun, and extending beyond the orbits 

 of Mercury and Venus, appears to be afforded us by the phenomenon 

 called the Zodiacal Light ; and as the Sun itself rotates on its axis, 

 it is most probable that this kind of atmosphere rotates also. 8 On the 

 other hand, M. Le Verrier conceives that the Comets which now re- 

 volve within the ordinary planetary limits have not always done so, 

 but have been caught and detained by the Planets among which they 

 move. In this way the action of Jupiter has brought the Jomets of 

 Faye and Vico into their present limited orbits, as it drew the Comet 

 of Lexell out of its known orbit, when the Comet passed over the 

 Planet in 1779, since which time it has not been seen. 



Among the examples of the boldness with which astronomers as- 

 sume the doctrine of gravitation even beyond the limits of the solar 

 system to be so entirely established, that hypotheses may and must be 

 assumed to explain any apparent irregularity of motion, we may reck- 

 on the mode of accounting for certain supposed irregularities in the 

 proper motion of Sirius, which has been proposed by Bessel, and 

 which M. Peters thinks is proved to be true by his recent researches 

 (Astr. Nach. xxxi. p. 219, and xxxii. p. 1). The hypothesis is, that 

 Sirius has a companion star, dark, and therefore invisible to us; and 

 that the two, revolving round their common centre as the system 

 moves on, the motion of Sirius is seen to be sometimes quicker and 

 sometimes slower. 



The Earths Density. 



" Cavendish's experiment," as it is commonly called the measure 

 of the attractions of manageable masses by the torsion balance, in or- 

 der to determine the density of the Earth has been repeated recently 

 by Professor Reich at Freiberg, and by Mr. Baily in England, with 

 great attention to the means of attaining accuracy. Professor Reich's 

 result for the density of the Earth is 5'44 ; Mr. Baily's is 5-92. Cav- 

 endish's result was 5'48 ; according to recent revisions 9 it is 5-52. 



6 M. Le Verrier, Annales de V Obs. de Paris, vol. i. p. 89. 



9 The calculation has been revised by II. Edward Schmidt. Humboldt's Kosmoe 

 i. p. 425. 



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