disk or a
body of circular shape, with much the greater diameter in the plane of
its whirling. As the process of concentration went on, this disk is
supposed to have divided into ringlike masses, some approach to which
we can discern in the existing nebulae, which here and there among the
farther fixed stars appear to be undergoing such stages of development
toward solar systems. It is reasonably supposed that after these rings
had been developed they would break to pieces, the matter in them
gathering into a sphere, which in time was to become a planet. The
outermost of these rings led to the formation of the planet farthest
from the sun, and was probably the first to separate from the parent
mass. Then in succession rings were formed inwardly, each leading in
turn to the creation of another planet, the sun itself being the
remnant, by far the greater part of the whole mass of matter, which
did not separate in the manner described, but concentrated on its
centre. Each of these planetary aggregations of vapour tended to
develop, as it whirled upon its centre, rings of its own, which in
turn formed, by breaking and concentrating, the satellites or moons
which attend the earth, as they do all the planets which lie farther
away from the sun than our sphere.

[Illustration: Fig. 1.--Saturn, Jan. 26, 1889 (Antoniadi).]

As if to prove that the planets and moons of the solar system were
formed somewhat in the manner in which we have described it, one of
these spheres, Saturn, retains a ring, or rather a band which appears
to be divided obscurely into several rings which lie between its group
of satellites and the main sphere. How this ring has been preserved
when all the others have disappeared, and what is the exact
constitution of the mass, is not yet well ascertained. It seems clear,
however, that it can not be composed of solid matter. It is either in
the form of dust or of small spheres, which are free to move on each
other; otherwise, as computation shows, the strains due to the
attraction which Saturn itself and its moons exercise upon it would
serve to break it in pieces. Although this ring theory of the
formation of the planets and satellites is not completely proved, the
occurrence of such a structure as that which girdles Saturn affords
presumptive evidence that it is true. Taken in connection with what we
know of the nebulae, the proof of Laplace's nebular hypothesis may
fairly be regarded as complete.

It sho