f this plain, about three hundred miles from the southern end of
the _Mare Crisium_, is the mountain ring, or circumvallation, called
Langrenus, about ninety miles across and in places 10,000 feet high.
There is a fine central mountain with a number of peaks. Nearly a
hundred miles farther south, on the same meridian, lies an equally
extensive mountain ring named Vendelinus. The broken and complicated
appearance of its northern walls will command the observer's attention.
Another similar step southward, and still on the same meridian brings us
to a yet finer mountain ring, slightly larger than the others, and still
more complicated in its walls, peaks, and terraces, and in its
surroundings of craters, gorges, and broken ridges. This is Petavius.
West of Petavius, on the very edge of the disk, is a wonderful
formation, a walled plain named Humboldt, which is looked down upon at
one point near its eastern edge by a peak 16,000 feet in height. About a
hundred and forty miles south of Petavius is the fourth great mountain
ring lying on the same meridian. Its name is Furnerius. Look
particularly at the brilliantly shining crater on the northeast slope of
the outer wall of Furnerius.

[Illustration: LUNAR CHART NO. 2, SOUTHWEST QUARTER.]

Suppose that our observations are now interrupted, to be resumed when
the moon, about "seven days old," is in its first quarter. If we had
time, it would be a most interesting thing to watch the advance of the
lunar sunrise every night, for new beauties are displayed almost from
hour to hour; but, for the purposes of our description it is necessary
to curtail the observations. At first quarter one half of the lunar
hemisphere which faces the earth is illuminated by the sun, and the line
of sunrise runs across some of the most wonderful regions of the moon.

We begin, referring once more to Lunar Chart No. 1, in the neighborhood
of the north pole of the moon. Here the line along which day and night
meet is twisted and broken, owing to the roughness of the lunar surface.
About fifteen degrees southwest of the pole lies a remarkable
square-cornered, mountain-bordered plain, about forty miles in length,
called Barrow. Very close to the pole is a ring mountain, about
twenty-five miles in diameter, whose two loftiest peaks, 8,000 to 9,000
feet high, according to Neison, must, from their situation, enjoy
perpetual day.

The long, narrow, dark plain, whose nearest edge is about thirty degrees
so