l
lunar crater may be described as a circular wall, enclosing a central
plain, or "floor," which is often much depressed below the level of the
surface outside. These so-called "craters," or "ring-mountains," as they
are also termed, are often of gigantic proportions. For instance, the
central plain of one of them, known as Ptolemaeus,[17] is about 115 miles
across, while that of Plato is about 60. The walls of craters often rise
to great heights; which, in proportion to the small size of the moon,
are very much in excess of our highest terrestrial elevations.
Nevertheless, a person posted at the centre of one of the larger craters
might be surprised to find that he could not see the encompassing
crater-walls, which would in every direction be below his horizon. This
would arise not alone from the great breadth of the crater itself, but
also from the fact that the curving of the moon's surface is very sharp
compared with that of our earth.

[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL
"CRATERS," MOUNTAIN RANGES, AND "SEAS"

In this, as in the other plates of the Moon, the _South_ will be found
at the top of the picture; such being the view given by the ordinary
astronomical telescope, in which all objects are seen _inverted_.

(Page 199)]

We have mentioned Ptolemaeus as among the very large craters, or
ring-mountains, on the moon. Its encompassing walls rise to nearly
13,000 feet, and it has the further distinction of being almost in the
centre of the lunar disc. There are, however, several others much wider,
but they are by no means in such a conspicuous position. For instance,
Schickard, close to the south-eastern border, is nearly 130 miles in
diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
almost exactly at the east point, is nearly as large as Schickard.
Another crater, Clavius, situated near the south point, is about 140
miles across; while its neighbour Bailly--named after a famous French
astronomer of the eighteenth century--is 180, and the largest of those
which we can see (see Plate IX., p. 198).

Many of the lunar craters encroach upon one another; in fact there is
not really room for them all upon the visible hemisphere of the moon.
About 30,000 have been mapped; but this is only a small portion, for
according to the American astronomer, Professor W.H. Pickering, there
are more than 200,000 in all.

Notwithstanding the fact that the crater is the type of mo