COMETS. 109 



seventy-six years, the first in point of importance with respect 

 to theoretical and physical astronomy is Halley's comet, whose 

 last appearance, in 1835, was much less brilliant than was to 

 be expected from preceding ones ; next we would notice Ol- 

 bers's comet, discovered on the 6th of March, 1815 ; and, 

 lastly, the comet discovered by Pons in the year 1812, and 

 whose elliptic orbit has been determined by Encke. The two 

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

 with certainty of nine returns of Halley's large comet, it hav- 

 ing recently been proved by Laugier's calculations,* that in 

 the Chinese table of comets, first made known to us by Ed- 

 ward 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 cosmical 

 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 respect- 

 ively 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 while the velocity of the comet 

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

 thirteen times greater than that of the Earth, it scarcely 

 moves ten feet in the second when at its aphelion. This ve- 

 locity 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, among the innumerable 

 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 fur- 

 thest 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 



* Laugier, in the Comptes Rendus des Stances de f Academie, 18431 

 t xvi., p. 1006. 



