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