102 



CELESTIAL PHENOMENA. 



of so much importance in theoretical and physical astro- 

 nomy, and whose last appearance in 1835 was less 

 brilliant than might have been expected from preceding 

 ones) ; the comet of Gibers (of the 6th of March, 1815); 

 and the one discovered by Pons in 1812, the elliptic 

 orbit of which was determined by Encke. The two latter 

 comets were invisible to the naked eye. We now know, 

 with certainty, of nine returns of Bailey's comet, for Lau- 

 gier's ( 56 ) calculations have recently demonstrated the identity 

 of its orbit with that of the comet of 1378, mentioned in 

 the Chinese tables of comets, for the knowledge of which we 

 are indebted to Edouard Biot. During the interval between 

 its first and last recorded appearances, in 1378 and 1835, 

 its periods of revolution have fluctuated between 74'91 

 years and 77'58 years, the mean being 76-1. 



Contrasted with the cosmical bodies of which we have 

 been speaking are a group of comets requiring many thou- 

 sand years to perform their revolutions, of which the 

 periods can only be determined with great difficulty and 

 uncertainty. Argelander assigns a period of 3065 to the 

 fine comet of 1811, and Encke a period of upwards of 

 8800 years to the awfully grand one of 1680. Accord- 

 ing to such views, these bodies recede respectively to dis- 

 tances from the Sun twenty-one and forty-four times greater 

 than that of Uranus, or to 83600 and 70400 millions 

 of miles. At these enormous distances, the attractive force 

 of the Sun still subsists ; but whilst the motion of the comet 

 of 1680 at its perihelion is 212 miles in a second, being 

 thirteen times greater than that of the Earth, its velocity at 

 its aphelion is scarcely ten feet in a second, being only 

 three times greater than that of our most sluggish European 



