The Moon | Page 5

of England) 50,000 square miles, while the
Mare Humboldtianum, according to Schmidt, includes only about
42,000 square miles, an area which is approached by some formations
not classed with the Maria. This distinction, speaking generally,
prevails among the Maria,--those of larger size, such as the Oceanus
Procellarum, the Mare Nubium, and the Mare Foecunditatis, are less
definitely enclosed, and, like terrestrial oceans, communicate with one
another; while their borders, or, if the term may be allowed, their
coast-line, is often comparatively low and ill-defined, exhibiting many

inlets and irregularities in outline. Others, again, of considerable area,
as, for example, the Mare Serenitatis and the Mare Imbrium, are
bounded more or less completely by curved borders, consisting of
towering mountain ranges, descending with a very steep escarpment to
their surface: thus in form and other characteristics they resemble
immense wall-surrounded plains. Among the best examples of enclosed
Maria is the Mare Crisium, which is considered by Neison to be the
deepest of all, and the Mare Humboldtianum.
Though these great plains are described as level, this term must only be
taken in a comparative sense. No one who observes them when their
surface is thrown into relief by the oblique rays of the rising or setting
sun can fail to remark many low bubble-shaped swellings with gently
rounded outlines, shallow trough-like hollows, and, in the majority of
them, long sinuous ridges, either running concentrically with their
borders or traversing them from side to side. Though none of these
features are of any great altitude or depth, some of the ridges are as
much as 700 feet in height, and probably in many instances the other
elevations often rise to 150 feet or more above the low-lying parts of
the plains on which they stand. Hence we may say that the Maria are
only level in the sense that many districts in the English Midland
counties are level, and not that their surface is absolutely flat. The same
may be said as to their apparent smoothness, which, as is evident when
they are viewed close to the terminator, is an expression needing
qualification, for under these conditions they often appear to be covered
with wrinkles, flexures, and little asperities, which, to be visible at all,
must be of considerable size. In fact, were it possible to examine them
from a distance of a few miles, instead of from a standpoint which,
under the most favourable circumstances, cannot be reckoned at less
than 300, and this through an interposed aerial medium always more or
less perturbed, they would probably be described as rugged and uneven,
as some modern lava sheets.
RIDGES.--Among the Maria which exhibit the most remarkable
arrangement of ridges is the Mare Humorum, in the south-eastern
quadrant. Here, if it be observed under a rising sun, a number of these
objects will be seen extending from the region north of the

ring-mountain Vitello in long undulating lines, roughly concentric with
the western border of the "sea," and gradually diminishing in altitude as
they spread out, with many ramifications, to a distance of 200 miles or
more towards the north. At this stage of illumination they are strikingly
beautiful in a good telescope, reminding one of the ripple-marks left by
the tide on a soft sandy beach. Like most other objects of their class,
they are very evanescent, gradually disappearing as the sun rises higher
in the lunar firmament, and ultimately leaving nothing to indicate their
presence beyond here and there a ghostly streak or vein of a somewhat
lighter hue than that of the neighbouring surface. The Mare Nectaris,
again, in the south-western quadrant, presents some fine examples of
concentric ridges, which are seen to the best advantage when the
morning sun is rising on Rosse, a prominent crater north of Fracastorius.
This "sea" is evidently concave in cross-section, the central portion
being considerably lower than the margin, and these ridges appear to
mark the successive stages of the change of level from the coast-line to
the centre. They suggest the "caving in" of the surface, similar to that
observed on a frozen pond or river, where the "cat's ice" at the edge,
through the sinking of the water beneath, is rent and tilted to a greater
or less degree. The Mare Serenitatis and the Mare Imbrium, in the
northern hemisphere, are also remarkable for the number of these
peculiar features. They are very plentifully distributed round the margin
and in other parts of the former, which includes besides one of the
longest and loftiest on the moon's visible surface--the great serpentine
ridge, first drawn and described nearly a hundred years ago by the
famous selenographer, Schroter of Lilienthal. Originating at a little
crater under the north- east wall of great ring-plain Posidonius, it
follows a winding course across the Mare toward the