outhern face. This conclusion
would, of course, have been an assured one, had the previous observation
of a dark division on the northern face been remembered. With the
knowledge which we now possess, indeed, the darkness of the seeming
stripe would be sufficient evidence that there must be a real division
there between the rings; for we know that no mere darkness of the ring's
substance could account for the apparent darkness of the stripe. It has
been well remarked by Professor Tyndall, that if the moon's whole
surface could be covered with black velvet, she would yet appear white
when seen on the dark background of the sky. And it may be doubted
whether a circular strip of black velvet 2000 miles wide, placed where
we see the dark division between the rings, would appear nearly as dark
as that division. Since we could only admit the possibility of some
substance resembling our darker rocks occupying this position (for we
know of nothing to justify the supposition that a substance as dark as
lampblack or black velvet could be there), we are manifestly precluded
from supposing that the dark space is other than a division between two
distinct rings.

Yet Sir W. Herschel, in examining the rings of Saturn with his powerful
telescopes, for a long time favoured the theory that there is no real
division. He called it the 'broad black mark,' and argued that it can
neither indicate the existence of a zone of hills upon the ring, nor of
a vast cavernous groove, for in either case it would present changes of
appearance (according to the ring's changes of position) such as he was
unable to detect. It was not until the year 1790, eleven years after his
observations had commenced, that, perceiving a corresponding broad black
mark upon the ring's southern face, Herschel expressed a 'suspicion'
that the ring is divided into two concentric portions by a circular gap
nearly 2000 miles in width. He expressed at the same time, very
strongly, his belief that this division was the only one in Saturn's
ring-system.

A special interest attached at that time to the question whether the
ring is divided or not, for Laplace had then recently published the
results of his mathematical inquiry into the movements of such a ring as
Saturn's, and, having _proved_ that a single solid ring of such enormous
width could not continue to move around the planet, had expressed the
_opinion_ that Saturn's ring consists in reality of many concentric
rings, ea