COMETS. 103 



rivers, and but half that which I found in the Cassiquiare, 

 an arm of the Orinoco. Amongst the countless host of un- 

 calculated or still undiscovered comets, it is highly proba- 

 ble that there are many, the major axes of whose orbits may 

 far exceed even that of the comet of 1680. In order to 

 afford through the medium of figures some idea, I do not 

 say of the extent of the sphere of attraction, but, of the 

 distance in space of a fixed star or other sun from the aphe- 

 lion of the comet of 1680, (the one of the bodies of our 

 solar system which, according to our present knowledge, 

 attains the greatest remoteness,) I would here remind the 

 reader, that, according to the most recent determinations of 

 parallax, even the nearest fixed star is at least 250 times 

 more distant from our Sun than this comet at its aphelion. 

 The comet's distance is only 44 times that of Uranus, whilst 

 that of a Centauri is 11000, and of 61 Cygni, according to 

 Bessel's determination, 31000 times that of Uranus. 



Having thus considered the greatest known distances of 

 comets from the central body, we may proceed to notice 

 instances of the greatest proximity hitherto measured. The 

 smallest distance between a comet and the Earth occurred 

 in the case of Lexell and Burkhardt's comet of 1770, which 

 has acquired so much celebrity from the perturbations it 

 underwent from the mass of Jupiter. On the 28th of June, 

 1770, this comet's distance from the Earth was only six 

 times that of the Moon from the Earth. The same comet 

 passed twice, in 1767 and in 1769, through the system of 

 the four satellites of Jupiter, without causing the slightest 

 sensible derangement in these small bodies, whose move- 

 ments are so well known. The great comet of 1680, when 

 at its perihelion on the 17th of December, was only 



