et forth as follows: Such a ring doubtless had a
diameter of some million miles; we readily perceive that the particles
of matter in the outer part of the belt would have a swifter movement
around the sun than those on the inside. When by some disturbance, as
possibly by the passage of a great meteoric body of a considerable
gravitative power, this ring was broken in two, the particles
composing it on either side would, because of their mutual attraction,
tend to draw away from the breach, widening that gap until the matter
of the broken ring was aggregated into a sphere of the star dust or
vapour. When the nebulous matter originally in the ring became
aggregated into a spherical form, it would, on account of the
different rates at which the particles were moving when they came
together, be the surer to fall in toward the centre, not in straight
lines, but in curves--in other words, the mass would necessarily take
on a movement of rotation essentially like that which we have
described in setting forth the nebular hypothesis.

In the stages of concentration the planetary nebulae might well repeat
those through which the greater solar mass proceeded. If the volume of
the material were great, subordinate rings would be formed, which when
they broke and concentrated would constitute secondary planets or
satellites, such as our moon. For some reason as yet unknown the outer
planets--in fact, all those in the solar system except the two inner,
Venus and Mercury and the asteroids--formed such attendants. All these
satellite-forming rings have broken and concentrated except the inner
of Saturn, which remains as an intellectual treasure of the solar
system to show the history of its development.

To the student who is not seeking the fulness of knowledge which
astronomy has to offer, but desires only to acquaint himself with the
more critical and important of the heavenly phenomena which help to
explain the earth, these features of planetary movement should prove
especially interesting for the reason that they shape the history of
the spheres. As we shall hereafter see, the machinery of the earth's
surface, all the life which it bears, its winds and rains--everything,
indeed, save the actions which go on in the depths of the sphere--is
determined by the heat and light which come from the sun. The
conditions under which this vivifying tide is received have their
origin in the planetary motion. If our earth's path around the cent