The Moon | Page 7

is most likely to prevail.
Most observers will agree with Schmidt, that observations and
drawings of objects on the sombre depressed plains of the moon are
easier and pleasanter to make than on the dazzling highlands, and that
the lunar "sea" is to the working selenographer like an oasis in the
desert to the traveller--a relief in this case, however, not to an
exhausted body, but to a weary eye.
RING-MOUNTAINS, CRATERS, &C.--It is these objects, in their
almost endless variety and bewildering number, which, more than any
others, give to our satellite that marvellous appearance in the telescope

which since the days of Galileo has never failed to evoke the
astonishment of the beholder. However familiar we may be with the
lunar surface, we can never gaze on these extraordinary formations,
whether massed together apparently in inextricable confusion, or
standing in isolated grandeur, like Copernicus, on the grey surface of
the plains, without experiencing, in a scarcely diminished degree, the
same sensation of wonder and admiration with which they were beheld
for the first time. Although the attempt to bring all these bizarre forms
under a rigid scheme of classification has not been wholly successful,
their structural peculiarities, the hypsometrical relation between their
interior and the surrounding district, their size, and the character of
their circumvallation, the dimensions of their cavernous opening as
compared with that of the more or less truncated conical mass of matter
surrounding it, all afford a basis for grouping them under distinctive
titles, that are not only convenient to the selenographer, but which
undoubtedly represent, as a rule, actual diversities in their origin and
physical character.
These distinguishing titles, as adopted by Schroter, Lohrmann, and
Madler, and accepted by subsequent observers, are WALLED-PLAINS,
MOUNTAIN RINGS, RING-PLAINS, CRATERS, CRATER-CONES,
CRATERLETS, CRATER-PITS, DEPRESSIONS.
WALLED-PLAINS.--These formations, approximating more or less to
the circular form, though frequently deviating considerably from it, are
among the largest enclosures on the moon. They vary from upwards of
150 to 60 miles or under in diameter, and are often encircled by a
complex rampart of considerable breadth, rising in some instances to a
height of 12,000 feet or more above the enclosed plain. This rampart is
rarely continuous, but is generally interrupted by gaps, crossed by
transverse valleys and passes, and broken by more recent craters and
depressions. As a rule, the area within the circumvallation (usually
termed "the floor") is only slightly, if at all, lower than the region
outside: it is very generally of a dusky hue, similar to that of the grey
plains or Maria, and, like them, is usually variegated by the presence of
hills, ridges, and craters, and is sometimes traversed by delicate furrows,
termed clefts or rills.

Ptolemaeus, in the third quadrant, and not far removed from the centre
of the disc, may be taken as a typical example of the class. Here we
have a vast plain, 115 miles from side to side, encircled by a massive
but much broken wall, which at one peak towers more than 9000 feet
above a level floor, which includes details of a very remarkable
character. The adjoining Alphonsus is another, but somewhat smaller,
object of the same type, as are also Albategnius, and Arzachel; and
Plato, in a high northern latitude, with its noble many-peaked rampart
and its variable steel-grey interior. Grimaldi, near the eastern limb
(perhaps the darkest area on the moon), Schickard, nearly as big, on the
south- eastern limb, and Bailly, larger than either (still farther south in
the same quadrant), although they approach some of the smaller "seas"
in size, are placed in the same category. The conspicuous central
mountain, so frequently associated with other types of ringed
enclosures, is by no means invariably found within the walled-plains;
though, as in the case of Petavius, Langrenus, Gassendi, and several
other noteworthy examples, it is very prominently displayed. The
progress of sunrise on all these objects affords a magnificent spectacle.
Very often when the rays impinge on their apparently level floor at an
angle of from 1 deg. to 2 deg., it is seen to be coarse, rough grained,
and covered with minute elevations, although an hour or so afterwards
it appears as smooth as glass.
Although it is a distinguishing characteristic that there is no great
difference in level between the outside and the inside of a walled-plain,
there are some very interesting exceptions to this rule, which are
termed by Schmidt "Transitional forms." Among these he places some
of the most colossal formations, such as Clavius, Maurolycus, Stofler,
Janssen, and Longomontanus. The first, which may be taken as
representative of the class (well known to observers as one of the
grandest of lunar objects), has a deeply sunken floor, fringed with
mountains rising some 12,000 feet above it, though they scarcely stand
a fourth of