y-five
miles in diameter. Thebit, more than thirty miles across, is very deep.
East of Thebit lies the celebrated "lunar railroad," a straight,
isolated wall about five hundred feet high and sixty-five miles long,
dividing at its southern end into a number of curious branches, forming
the buttresses of a low mountain. Purbach is sixty miles broad, and
south of that comes a wonderful region where the ring mountains Hell,
Ball, Lexell, and others, more or less connected with walls, inclose an
area even larger than Ptolemaeus, but which, not being so distinctly
bordered as some of the other inclosed plains, bears no distinctive
name.

[Illustration: LUNAR CHART NO. 4, SOUTHEAST QUARTER.]

The next conspicuous object toward the south ranks with Copernicus among
the grandest of all lunar phenomena--the ring, or crater, Tycho. It is
about fifty-four miles in diameter and some points on its wall rise
17,000 feet above the interior. In the center is a bright mountain peak
5,000 feet high. But wonderful as are the details of its mountain ring,
the chief attraction of Tycho is its manifest relation to the mysterious
bright rays heretofore referred to, which extend far across the surface
of the moon in all directions, and of which it is the center. The
streaks about Copernicus are short and confused, constituting rather a
splash than a regular system of rays; but those emanating from Tycho are
very long, regular, comparatively narrow, and form arcs of great circles
which stretch away for hundreds of miles, allowing no obstacle to
interrupt their course.

Southwest of Tycho lies the vast ringed plain of Maginus, a hundred
miles broad and very wonderful to look upon, with its labyrinth of
formations, when the sun slopes across it, and yet, like Maurolycus,
invisible under a vertical illumination. "The full moon," to use
Maedler's picturesque expression, "knows no Maginus." Still larger and
yet more splendid is Clavius, which exceeds one hundred and forty miles
in diameter and covers 16,000 square miles of ground within its fringing
walls, which carry some of the loftiest peaks on the moon, several
attaining 17,000 feet. The floor is deeply depressed, so that an
inhabitant of this singular inclosure, larger than Massachusetts,
Connecticut, and Rhode Island combined, would dwell in land sunk two
miles below the general level of the world about him.

In the neighborhood of the south pole lies the large walled plain of
Newton, whos