COMETS. 95 



returns of Halley's large comet, it having recently been 

 proved by Laugier's calculations,* that in the Chinese table 

 of comets, first made known to us by Edward Biot, the comet 

 of 1378 is identical with Halley's; its periods of revolution 

 have varied in the interval between 1378 and 1835 from 

 74-91 to 77-58 years, the mean being 76'1. 



A host of other comets may be contrasted with the cos- 

 mical bodies of which we have spoken, requiring several 

 thousand years to perform their orbits, which it is difficult to 

 determine with any degree of certainty. The beautiful comet 

 of 1811 requires, according to Argelander, a period of 3065 

 years for its revolution, and the colossal one of 1680 as much 

 as 8800 years, according to Encke's calculation. These 

 bodies respectively recede, therefore, 21 and 44 times further 

 than Uranus from the Sun, that is to say, 33,600 and 70,400 

 millions of miles. At this enormous distance the attractive 

 force of the Sun is still manifested ; but whilst the velocity of 

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

 that is, thirteeen times greater than that of the Earth, it 

 scarcely moves ten feet in the second when at its aphelion. 

 This velocity is only three times greater than that of water in 

 our most sluggish European rivers, and equal only to half 

 that which I have observed in the Cassiquiare, a branch of 

 the Orinoco. It is highly probable, that amongst the innu- 

 merable host of uncalculated or undiscovered comets, there 

 are many whose major axes greatly exceed that of the comet 

 of 1680. In order to form some idea by numbers, I do not 

 say of the sphere of attraction, but of the distance in space of 

 a fixed star, or other sun, from the aphelion of the comet of 

 1680 (the furthest receding cosmical body with which we are 

 acquainted in our solar system), it must be remembered that, 

 according to the most recent determinations of parallaxes, the 

 nearest fixed star is full 250 times further removed from our 

 sun than the comet in its aphelion. The comet's distance is 

 only 44 times that of Uranus, whilst a Centauri is 11,000, 

 and 61 Cvgni 31,000 times that of Uranus, according to Bes- 

 sel's determinations. 



Having considered the greatest distances of comets from 



* Laugiar, in the Cumpto* rendua des Seances de VAc/ulfwu, 

 1843, t. xvi. p. IOCS. 



