is more than 10 deg. above the horizon, are seen to
be traversed by wide and deep valleys. The S. _glacis_ is especially
broad, extending to a distance of 10 or 12 miles before it runs down to
the level of the plain. The shape of the circumvallation, when it is
fully illuminated, approximates very closely to that of an equilateral
triangle with curved sides. There are two bright little craters on the
outer slope, just below the summit ridge on the S.E., and another,
larger, on the N. wall, in which it makes a prominent gap. The interior
is considerably brighter than the surface of the surrounding Mare, and, a
little S. of the centre, includes two crater-like objects with broken
rims. These assume different aspects under different conditions of
illumination, and it is only when the floor is lighted by a comparatively
low morning sun, that their true character is apparent. On the N.W.
quarter of the interior are two smaller distinct craters, and a square
arrangement of ridges. On the N.E. there are some hillocks and minor
elevations. The Plinius rills form an especially interesting system, and
under favourable conditions may be seen in their entirety with a good 4
inch refractor, about the time when the morning terminator passes through
Julius Caesar. They consist of three long fissures, originating amid the
Haemus highlands, on the S. side of the Mare Serenitatis, and diverging
towards the W. The most southerly commences S.S.E. of the Acherusian
promontory (a great headland, 5000 feet high, at the W. termination of
the Haemus range), and, following a somewhat undulating course, runs up
to the N. side of Dawes. Under a low evening sun, I have remarked many
inequalities in the width of that portion of it immediately N. of
Plinius, which appear to indicate that it is here made up of rows of
inosculating craters. The cleft north of this originates very near it,
passes a little S. of the promontory, and runs to the E. edge of the
plateau surrounding Dawes. The third and most northerly cleft begins at a
point immediately N. of the promontory, cuts through the S. end of the
well-known Serpentine ridge on the Mare Serenitatis, and, after following
a course slightly concave to the N., dies out on the N. side of the
plateau. This cleft forms the line of demarcation between the dark tone
of the Mare Serenitatis and the light hue of the Mare Tranquilitatis,
traceable under nearly every condition of illumination, and prominent in
all