ystem, it is
at least a body which before its approach to the sun had the same motion
through the stellar spaces that the sun has. As this unity of motion
must have been maintained from the beginning, we may regard comets as
belonging to the solar system in the sense of not being visitors from
distant regions of space.

The acceptance of this seemingly inevitable conclusion leads to another:
that no comet yet known moves in a really hyperbolic orbit, but that the
limit of eccentricity must be regarded as 1, or that of the parabola. It
is true that seeming evidence of hyperbolic eccentricity is sometimes
afforded by observations and regarded by some astronomers as sufficient.
The objections to the reality of the hyperbolic orbit are two: (1) A
comet moving in a decidedly hyperbolic orbit must have come from so
great a distance within a finite time, say a few millions of years, as
to have no relation to the sun, and must after its approach to the sun
return into space, never again to visit our system. In this case the
motion of the sun through space renders it almost infinitely improbable
that the orbit would have been so nearly a parabola as all such orbits
are actually found to be. (2) The apparent deviation from a very
elongated ellipse has never been in any case greater than might have
been the result of errors of observation on bodies of this class.

This being granted, a luminous view of the causes which lead to the
observed orbits of comets is readily gained by imagining these bodies to
be formed of nebulous masses, which originally accompanied the sun in
its journey through space, but at distances, in most cases, vastly
greater than that of the farthest planet. Such a mass, when drawn
towards the sun, would move round it in a nearly parabolic orbit,
similar to the actual orbits of the great majority of comets. The period
might be measured by thousands, tens of thousands, or hundreds of
thousands of years, according to the distances of the comet in the
beginning; but instead of bodies extraneous to the system, we should
have bodies properly belonging to the system and making revolutions
around the sun.

Were it not for the effect of planetary attraction long periods like
these would be the general rule, though not necessarily universal. But
at every return to perihelion the motion of a comet will be to some
extent either accelerated or retarded by the action of Jupiter or any
other planet in the neighbourhood of