110 COSMOS. 



times that of Uranus, while a Centaiiri is 11,000, and 61 

 Cygni 31,000 times that of Uranus, according to Bessel's de- 

 terminations. 



Having considered the greatest distances of comets from 

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

 the greatest proximity hitherto measured. Lexell and Burck- 

 hardt's comet of 1770, so celebrated on account of the disturb- 

 ances 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 

 ]769 and 1779, through the sj^stem of Jupiter's four satellites 

 without producing the slightest notable change in the well- 

 known orbits of these bodies. The great comet of 1680 ap- 

 proached at its perihelion eight or nine times nearer to the 

 surface of the Sun than Lexell's comet did to that of our 

 Earth, being on the 17th 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 

 among 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 possi- 

 ble evils threatened by comets have acquired more weight as 

 their direction has become more defuiite. The certainty that 

 there are within the known planetary orbits comets which re- 

 visit our regions of space at short intervals — that great dis- 

 turbances 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 cosmical vapor, which, acting as 

 a resisting and impeding medium, tends to contract all orbits 

 — the individual difference of comets, which would seem to 

 indicate considerable decreasing gradations in the quantity of 

 the mass of the nucleus, are all considerations more than equiv- 

 alent, both as to number and variety, to the vague fears en- 

 tertained in early ages of the general conflagration of the world 

 hy Jiammg stvords, and stars with fieri/ streaming hair. As 

 the cons'Dlatory considerations which may be derived from the 

 calculus of probabilities address themselves to reason and to 



