t, under favourable
conditions, may be traced across the floor to the broken S.E. border. It
is probably a true "fault." On the N.E., the inner slope of the wall is
very broad, and affords a fine example of a vast landslip.

The spots and faint light markings on the floor are of a particularly
interesting character. During the years 1869 to 1871 they were
systematically observed and discussed under the auspices of the Lunar
Committee of the British Association. Among the forty or more spots
recorded, six were found to be crater-cones. The remainder--or at least
most of them--are extremely delicate objects, which vary in visibility in
a way that is clearly independent of libration or solar altitude; and,
what is also very suggestive, they are always found closely associated
with the light markings,--standing either upon the surface of these
features or close to their edges. Recent observations of these spots with
a 13 inch telescope by Professor W.H. Pickering, under the exceptionally
good conditions which prevail at Arequipa, Peru, have revived interest in
the subject, for they tend to show that visible changes have taken place
in the aspect of the principal crater-cones and of some of the other
spots since they were so carefully and zealously scrutinised nearly a
quarter of a century ago. The gradual darkening of the floor of Plato as
the sun's altitude increases from 20 deg. till after full moon may be
regarded as an established fact, though no feasible hypothesis has been
advanced to account for it.

On the N.E. of Plato is a large bright crater, A; and, extending in a
line from this towards the E., is a number of smaller rings, the whole
group being well worth examination. On the N. there is a winding cleft,
and some short crossed clefts in the rugged surface just beyond the foot
of the wall, which I have seen with a 4 inch achromatic. The region on
the W., imperfectly shown in the maps, includes much unrecorded detail.
On the Mare Imbrium S. of Plato is a large area enclosed by low ridges,
to which Schroter gave the name "Newton." It suggests the idea that it
represents the ruin of a once imposing enclosure, of which the
conspicuous mountain Pico formed a part.

TIMAEUS.--A very bright ring-plain, 22 miles in diameter, with walls
about 4500 feet in height, on the coast-line of the Mare Frigoris, and
associated with the E. side of the great enclosed plain W.C. Bond.
Schmidt shows a double hill, nearly central, and