96 COSMOS. 



the central body, it now remains for us to notice instances 

 the greatest proximity hitherto measured. Lexell and 

 Burckhardt's comet of 1770, so celebrated on account of the 

 disturbances it experienced from Jupiter, has approached 

 the Earth within a smaller distance than any other comet. 

 On the 28th of June, 1770, its distance from the Earth was 

 only six times that of the Moon. The same comet passed 

 twice, viz. in 1769 and 1779, through the system of Jupiter's 

 four satellites without producing the slightest notable change 

 in the well-known orbits of these bodies. The great comet 

 of 1680 approached at its perihelion eight or nine times 

 nearer to the surface of the Sun than Lex ell's comet did to 

 that of our Earth ; being on the 1 7th of December, a sixth 

 part of the Sun's diameter, or seven-tenths of the distance 

 of the Moon, from that luminary. Perihelia occurring beyond 

 the orbit of Mars can seldom be observed by the inhabitants 

 of the Earth, owing to the faintness of the light of distant 

 comets ; and amongst those already calculated, the comet of 

 1729 is the only one which has its perihelion between the 

 orbits of Pallas and Jupiter ; it was even observed beyond 

 the latter. 



Since scientific knowledge, although frequently blended 

 with vague and superficial views, has been more extensively 

 diffused through wider circles of social life, apprehensions of 

 the possible evils threatened by comets have acquired more 

 weight, as their direction has become more definite. The 

 certainty that there are within the known planetary orbits, 

 comets which revisit our regions of space at short intervals 

 that great disturbances have been produced by Jupiter and 

 Saturn in their orbits, by which such as were apparently 

 harmless have been converted into dangerous bodies the 

 intersection of the Earth's orbit by Biela's comet the cos- 

 mical vapour, which acting as a resisting and impeding 

 medium, tends to contract all orbits the individual difference 

 of comets, which would seem to indicate considerable decreas- 

 ing gradations in the quantity of the mass of the nucleus are 

 all considerations more than equivalent both as to number 

 and variety, to the vague fears entertained in early ages, of 

 the general conflagration of the world by flaming swords, and 

 stars with Jiery streaming hair. As the consolatory considera- 

 tions which may be derived from the calculus of probabilities, 



