The expulsive force of the radial stream would thus drive off this
cometary dust to distances in some inverse ratio of the density of the
atoms; but, a limit would ultimately be reached, when gravitation would
be relatively the strongest--the last force diminishing only as the
squares of the distances, and the first diminishing in the compound
ratio of the squares and the square roots of the distances. At the
extreme verge of the system, this cometary matter would accumulate, and,
by accumulation, would still further gather up the scattered atoms--the
sweepings of the inner space--and, in this condensed form, would again
visit the sun in an extremely elongated ellipse. It does not, however,
follow, that all comets are composed of such unsubstantial materials.
There may be comets moving in parabolas, or even in hyperbolas--bodies
which may have been accumulating for ages in the unknown regions of
space, far removed from the sun and stars, drifting on the mighty
currents of the great ethereal ocean, and thus brought within the sphere
of the sun's attraction; and these bodies may have no analogy to the
periodical comets of our system, which last are those with which we are
more immediately concerned.

The periodical comets known are clearly arranged into two distinct
classes--one having a mean distance between Saturn and Uranus, with a
period of about seventy-five years, and another class, whose mean
distance assigns their position between the smaller planets and Jupiter,
having periods of about six years. These last may be considered the
siftings of the smaller planets, and the first the refuse of the
Saturnian system. In this light we may look for comets having a mean
distance corresponding to the intervals of the planets, rather than to
the distances of the planets themselves. One remarkable fact, however,
to be observed in these bodies is, that all their motions are in the
same direction as the planets, and, with one exception, there is no
periodical comet positively known whose motion is retrograde.

The exception we have mentioned is the celebrated comet of Halley, whose
period is also about seventy-five years. In reasoning on the resistance
of the ether, we must consider that the case can have very little
analogy with the theory of projectiles in air; nor can we estimate the
inertia of an infinitely divisible fluid, from its resisting influence
on atomic matter, by a comparison of the resistance of an atomic flu