COMETS. 107 



sribed to the altered form of the small nebulous stai in the 

 n(5inity of the Sun, and to the action of the unequal density 

 of the strata of cosmical ether.^ These facts, and the inves- 

 tigations to which they have led, belong to the most interest- 

 ng results of modern astronomy, Encke's comet has been 

 ilie means of leading astronomers to a more exact investiga- 

 lion of Jupiter's mass (a most important point with reference 

 to the calculation of perturbations) ; and, more recently, the 

 X)urse of this comet has obtained for us the first determina- 

 tion, although only an approximative one, of a smaller mass for 

 Mercury. 



The discovery of Encke's comet, which had a period of only 

 3id years, was speedily followed, in 1826, by that of another, 

 Biela's comet, whose period of revolution is 6;Sth years, and 

 which is likewise planetary, having its aphelion beyond the 

 orbit of Jupiter, but within that of Saturn. It has a fainter 

 light than Encke's comet, and, like the latter, its motion is 

 direct, while Halley's comet moves in a course opposite to that 

 pursued by the planets. Biela's comet presents the first cer- 

 tain example of the orbit of a comet intersecting that of the 

 Earth. This position, with reference to our planet, may there- 

 fore be productive of danger, if we can associate an idea of 

 danger with so extraordinary a natural phenomenon, whose 

 history presents no parallel, and the results of which we are 

 consequently unable correctly to estimate. Small masses en- 

 dowed with enormous velocity may certainly exercise a con- 

 siderable power ; but Laplace has shown that the mass of the 

 comet of 1770 is probably not equal to joVo^h of that of the 

 Earth, estimating further with apparent correctness the mean 

 mass of comets as much below yooVooth that of the Earth, 

 or about yaVo^^ ^^^^ of the Moon.f We must not confound 

 the passage of Biela's comet through the Earth's orbit with 

 its proximity to, or collision with, our globe. When this pas- 

 sage took place, on the 29th of October, 1832, it required a 

 full month before the Earth would reach the point of inter- 

 section of the two orbits. These two comets of short periods 

 of revolution also intersect each other, and it has been justly 

 observed, $ that amid the many perturbations experienced by 



* Encke, in the Astronomische Nachrlchien, 1843, No. 489, s. 130-132. 



t Laplace, Expos, du Syst. du Monde, p. 216, 237. 



X Littrow, Beschreibende Astron., 1835, s. 274. On the inner comet 

 recently discoven-ed by M. Faye, at the Observatory of Paris, and whose 

 eccentricity is 0-.5j1, its distance at its perihelion 1*690, and its distance 

 at its aphelion ,5-832, see Schumacher, Astron. Nachr., 1844, No. 495. 

 Regarding the siipposcd identity of the comet of 1766 with the third 



