most interesting of all these gigantic
formations is Maurolycus, whose diameter exceeds one hundred and fifty
miles, and which has walls 13,000 or 14,000 feet high. Yet, astonishing
though it may seem, this vast and complicated mass of mountain walls,
craters, and peaks, is virtually unseen at full moon, owing to the
perpendicularity of the sunlight, which prevents the casting of shadows.

We shall next suppose that another period of about seven days has
elapsed, the moon in the meantime reaching its full phase. We refer for
guidance to Lunar Chart No. 3. The peculiarity of the northeastern
quadrant which immediately strikes the eye is the prevalence of the
broad plains called _Maria_, or "seas." The northern and central parts
are occupied by the _Mare Imbrium_, the "Sea of Showers" or of "Rains,"
with its dark bay the _Sinus AEstuum_, while the eastern half is covered
by the vast _Oceanus Procellarum_, the "Ocean of Storms" or of
"Tempests."

Toward the north a conspicuous oval, remarkably dark in hue, immediately
attracts our attention. It is the celebrated ringed plain of Plato,
about sixty miles in diameter and surrounded by a saw-edged rampart,
some of whose pinnacles are 6,000 or 7,000 feet high. Plato is a
favorite subject for study by selenographers because of the changes of
color which its broad, flat floor undergoes as the sun rises upon it,
and also because of the existence of enigmatical spots and streaks whose
visibility changes. South of Plato, in the _Mare Imbrium_, rises a
precipitous, isolated peak called Pico, 8,000 feet in height. Its
resemblance in situation to the conical mountain Pico in the Azores
strikes the observer.

[Illustration: LUNAR CHART NO. 3, NORTHEAST QUARTER.]

Eastward of Plato a line of highlands, separating the _Mare Imbrium_
from the _Mare Frigoris,_ carries the eye to the beautiful semicircular
_Sinus Iridum_, or "Bay of Rainbows." The northwestern extremity of
this remarkable bay is guarded by a steep and lofty promontory called
Cape Laplace, while the southeastern extremity also has its towering
guardian, Cape Heraclides. The latter is interesting for showing,
between nine and ten days after full moon, a singularly perfect profile
of a woman's face looking out across the _Mare Imbrium_. The winding
lines, like submerged ridges, delicately marking the floor of the _Sinus
Iridum_ and that of the _Mare_ beyond, are beautiful telescopic
objects. The "bay" is about one hundred