on the Orhlts of Comets. 25* 



move in very eccentric ellipses, in one of the foci of which is 

 placed our sun ; for, as La Place says in his System of the 

 World, analogy leads us to imagine that comets move int 

 orbits, which, instead of being nearly circular, like those of 

 the other planets, are very eccentric, and the sun extremely 

 near that part in which they are visible to us; and to observe 

 the same law as the other planets. 



Hence is it not probable that they revolve about two 

 fixed stars, placed in the two foci of their orbits? This 

 opinion, I think, is strengthened by the amazing eccen- 

 tricity of their orbits, which, as was observed above, ap- 

 proaches very near to a parabola, no comet has yet been seen 

 that would answer to an hyperbola : of this amazing di- 

 stance, the exceeding small part we see before a comet ap- 

 proaches the sun, and when it leaves him, would not differ 

 much from a right line. Again, as the two foci of the 

 ellipse in which it moves are so very distant, is it not pro- 

 bable there are two attracting powers ? that, is one in each 

 focus"; and as the attraction of one body begins at the point 

 where the other ends, let us conceive the comet to be put 

 in motion a little beyond that point, as at A r and by the 

 time it arrived at B, its centrifugal C 



force becomes great enough to jy r~~~~ * >n 



throw it within the attraction Of n^ '+£m ^ 



the focus D, which we will sup- A 



pose at C : it is now acted upon by the attractive power at 

 D, and acquires in moving from C to D a velocity great 

 enough to bring it again to A \ and thus it will revolve 

 about the two fixed stars B D, in a very eccentric ellipse. 

 This will also account for their appearance from every part 

 of the heavens: and it is supposed that more than 450 have 

 been seen in different directions ; for about the same fixed 

 star many may revolve, yet only one about the same two 

 fixed stars. 



Ferguson, in his Astronomy, estimates the nearest fixed 

 star at about 32,000,000,000,000 miles distance from the 

 earth, consequently it is 32,000,082,000,000 miles from 

 the sun; and Adams, in his Astronomical Essays, says that 

 the comet seen by Brydone at Palermo in 1770 moved at 

 the rate of 60,000,000 miles an hour. Now admitting this 

 to be its average rate, and that it performed a revolution 

 once in 129 year9, which is the period assigned to that 

 which appeared in 1 061, we shall have 67,802,400,000,000 

 miles for the length of its orbit ; and it is not improbable 

 that this would be the perimeter of an eccentric ellipse 

 whose foci were the distance above mentioned. . „ 



