which ours would be unnoticeable, like a single star among the
multitude glittering in the terrestial sky on a clear night. Some
might be in the form of a wreath, like our own; some might be
globular, like the great star-clusters in Hercules and Centaurus; some
might be glittering circles, or disks, or rings within rings. If we
could enter them we should probably find a vast variety of
composition, including elements unknown to terrestrial chemistry; for
while the visible universe appears to contain few if any substances
not existing on the earth or in the sun, we have no warrant to assume
that others may not exist in infinite space.

And how as to gravitation? We do not know that gravitation acts beyond
the visible universe, but it is reasonable to suppose that it does. At
any rate, if we let go its sustaining hand we are lost, and can only
wander hopelessly in our speculations, like children astray. If the
empire of gravitation is infinite, then the various outer systems must
have some, though measuring by our standards an imperceptible,
attractive influence upon each other, for gravitation never lets go
its hold, however great the space over which it is required to act.
Just as the stars about us are all in motion, so the starry systems
beyond our sight may be in motion, and our system as a whole may be
moving in concert with them. If this be so, then after interminable
ages the aspect of the entire system of systems must change, its
various members assuming new positions with respect to one another. In
the course of time we may even suppose that our universe will approach
relatively close to one of the others; and then, if men are yet living
on the earth, they may glimpse through the openings which reveal
nothing to us now, the lights of another nearing star system, like the
signals of a strange squadron, bringing them the assurance (which can
be but an inference at present) that the ocean of space has other