D. Kirkwood on the origin of the Comets of 1812, ete. 257 
accordingly we find that the aphelia of a large proportion of 
the periodic comets are near the orbits of the major planets, 
“T admit,” says M. Hoek, “that the orbits of comets are by 
nature parabolas or hyperbolas, and that in the cases when el- 
liptical orbits are met with, these are occasioned by planetary 
attractions, or derive their character from the uncertainty of 
our observations. To allow the contrary would be to admit 
Some comets as permanent members of our planetary system, 
to which they ought to have belonged since its origin, and so 
to assert the simultaneous birth of that system and of these 
comets. As for me I attribute to these a primitive wandering 
character. Traveling through space they move from one star 
to another in order to leave it again, provided they do not meet 
any obstacle that may force them to remain in its vicinity. 
Such an obstacle was Jupiter, in the neighborhood of our sun, 
for the comets of Lexell and Brorsen, and probably for the 
greater part of periodical comets ; the other part of whic 
may be indebted for their elliptical orbits to the attractions of 
Saturn and the remaining planets. 
“Generally then, comets come to us from some star or 
other. The attraction of our sun modifies their orbit, as had 
been done already by each star through whose sphere of attrac- 
tion they had passed. We can put the question if they come 
as single bodies or united in systems. This is the point I have 
undertaken to investigate. Since some time already I had felt 
the truth of the following thesis :— 
-“There are systems of comets in space that are broken up 
by the attraction of our sun, and whose members attain, as 
wolated bodies, the vicinity of the Earth during a course of 
to other comets which need not here be specified. Now, the 
Comets of 1812 and 1846 IV. have their aphelions very near 
the orbit of N eptune, and hence the original parabolas in which 
ey moved were probably transformed into ellipses by the per- 
eptune near the same time, and at some distance from eac 
other, their different relative positions with regard to the dis- 
turbing body may account for the slight differences in the ele- 
ments of their orbits. ees 
At what epoch did they enter the solar system ?—The mean 
tween the longitudes of the aphelia of the two comets 1s 
_* Monthly Notices, vol. xxv, p. 243. 
