COMETS. 93 



has a fainter light than Encke's comet, and, like the 

 latter, its motion is direct, whilst Halley's comet moves in 

 a course opposite to that pursued by the planets. Biela's 

 comet presents the first certain example of the orbit of a 

 comet intersecting that of the Earth. This position, with 

 reference to our planet, may, therefore, be productive of dan- 

 ger, if we can associate an idea of danger with, so extraor- 

 dinary a natural phenomenon, whose history presents no 

 parallel, and the results of which we are consequently unable 

 correctly to estimate. Small masses endowed with enor- 

 mous velocity may certainly exercise a considerable power ; 

 but Laplace has shown that the mass of the comet of 1770 is 

 probably not equal to -goVo of that of the Earth, estimating 

 further with apparent correctness, ihe^mean mass of comets as 

 much below To 1 000 that of the Earth, or about -j^Vo tnat of the 

 Moon.-" 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 passage took place, on the 29th 

 of October, 1832, it required a full month before the Earth 

 would reach the point of intersection of the two orbits. 

 These two comets of short periods of revolution, also inter- 

 sect each other, and it has been justly observed,! that amid 

 the many perturbations experienced by such small bodies 

 from the larger planets, there is a possibility supposing a 

 meeting of these comets to occur in October that the inha- 

 bitants of the Earth may witness the extraordinary spectacle 

 of an encounter between two cosmical bodies, and possibly 

 of their reciprocal penetration and amalgamation, or of their 

 destruction by means of exhausting emanations. Events of this 

 nature, resulting either from deflection occasioned by disturb- 

 ing masses, or primevally intersecting orbits, must have been 

 of frequent occurrence in the course of millions of years in 



* Laplace, Expos, du Syst. du Monde, pp. 216, 237. 



t Littrow, Besclireilende Astron., 1835, s. 274. On the inner comet, 

 recently discovered by M. Faye, at the Observatory of Paris, and whose 

 eccentricity is 0'551, its distance at its perihelion T690, and its distance 

 at its aphelion 5'832, see Schumacher, Astron. Nadir., 1844, No. 495. 

 .Regarding the supposed identity of the comet of 1766 with the third 

 comet of 1819, see Astr. Nachr., 1833, No. 239 ; and on the identity 

 of the comet of 1743 and the fourth comet of 1819, see No. 237 of the 

 last-mentioned work. 



