ers will be favourably placed for observation. They are
named _Catharina_, _Cyrillus_, and _Theophilus_. Catharina, the most
southerly of the group, is more than 16,000 feet deep, and connected
with Cyrillus by a wide valley; but between Cyrillus and Theophilus
there is no such connection. Indeed, Cyrillus looks as if its huge
surrounding ramparts, as high as Mont Blanc, had been completely
finished before the volcanic forces commenced the formation of
Theophilus, the rampart of which encroaches considerably on its older
neighbour. Theophilus stands as a well-defined circular crater about 64
miles in diameter, with an internal depth of 14,000 to 18,000 feet, and
a beautiful central group of mountains, one-third of that height, on its
floor. Although Theophilus is the deepest crater we can see in the moon,
it has suffered little or no deformation from secondary eruptions, while
the floor and wall of Catharina show complete sequences of lesser
craters of various sizes that have broken in upon and partly destroyed
each other. In the spring of the year, when the moon is somewhat before
the first quarter, this instructive group of extinct volcanoes can be
seen to great advantage at a convenient hour in the evening.

[Illustration: PLATE VII.

TRIESNECKER.

(AFTER NASMYTH.)]

24. _Petavius_ is remarkable not only for its great size, but also for
the rare feature of having a double rampart. It is a beautiful object
soon after new moon, or just after full moon, but disappears absolutely
when the sun is more than 45 deg. above its horizon. The crater floor is
remarkably convex, culminating in a central group of hills intersected
by a deep cleft.

25. _Hyginus_ is a small crater near the centre of the moon's disc. One
of the largest of the lunar chasms passes right through it, making an
abrupt turn as it does so.

26. _Triesnecker._--This fine crater has been already described, but is
again alluded to in order to draw attention to the elaborate system of
chasms so conspicuously shown in Plate VII. That these chasms are
depressions is abundantly evident by the shadows inside. Very often
their margins are appreciably raised. They seem to be fractures in the
moon's surface.

Of the various mountains that are occasionally seen as projections on
the actual edge of the moon, those called after Leibnitz (_i_) seem to
be the highest. Schmidt found the highest peak to be upwards of 41,900
feet above a neighbouring valley. In comp