in the same plane, so
that the chances of collision are at present _nil_. There is only one
case, indeed, in which it seems to be eventually possible. M. Lespiault
has pointed out that the curves traversed by "Fides" and "Maia" approach
so closely that a time may arrive when the bodies in question will
either coalesce or unite to form a binary system.[1023]

The maze threaded by the 500 asteroids contrasts singularly with the
harmoniously ordered and rhythmically separated orbits of the larger
planets. Yet the seeming confusion is not without a plan.
The established rules of our system are far from being totally
disregarded by its minor members. The orbit of Pallas, with its
inclination of 34 deg. 42', touches the limit of departure from the
ecliptic level; the average obliquity of the asteroidal paths is
somewhat less than that of the sun's equator;[1024] their mean
eccentricity is below that of the curve traced out by Mercury, and all
without exception are pursued in the planetary direction--from west to
east.

The zone in which these small bodies travel is about three times as wide
as the interval separating the earth from the sun. It extends perilously
near to Jupiter, and dovetails into the sphere of Mars.

Their distribution is very unequal. They are most densely congregated
about the place where a single planet ought, by Bode's Law, to revolve;
it may indeed be said that only stragglers from the main body are found
more than fifty million miles within or without a mean distance from the
sun 2.8 times that of the earth. Significant gaps, too, occur where some
force prohibitive of their presence would seem to be at work. The
probable nature of that force was suggested by the late Professor
Kirkwood, first in 1866, when the number of known asteroids was only
eighty-eight, and again with more confidence in 1876, from the study of
a list then run up to 172.[1025] It appears that these bare spaces are
found just where a revolving body would have a period connected by a
simple relation with that of Jupiter. It would perform two or three
circuits to his one, five to his two, nine to his five, and so on.
Kirkwood's inference was that the gaps in question were cleared of
asteroids by the attractive influence of Jupiter. For disturbances
recurring time after time--owing to commensurability of periods--nearly
at the same part of the orbit, would have accumulated until the shape of
that orbit was notably changed. The body