ger craters to a great height and thus becoming illuminated by the
sun's light.]

But the greater part of the moon's hemisphere is dotted over by almost
innumerable circular crater-like hollows; sometimes conspicuously
surmounting lofty conical mountains, at other times only sinking below
the general outer surface of the lunar sphere. On approaching the
margin, these circular hollows appear oval in shape owing to their
position on the sphere; and the general aspect of those that are visible
leads to the conclusion that there are large numbers of smaller craters
too small to be seen by the most powerful telescopes. These cones and
craters are the most characteristic objects on the whole of the visible
surface, and when highly magnified present very rugged outlines,
suggestive of slag, or lava, which has consolidated on cooling, as in
the case of most solidified lava-streams on our earth.[8] One of the
most remarkable of these crateriform mountains is that named
_Copernicus_, situated in a line with the southern prolongation of the
Apennines. Of this mountain Sir R. Ball says: "It is particularly well
known through Sir John Herschel's drawing, so beautifully reproduced in
the many editions of the _Outlines of Astronomy_. The region to the west
is dotted over with innumerable minute craterlets. It has a central,
many-peaked mountain about 2,400 feet in height. There is good reason to
believe that the terracing shown in its interior is mainly due to the
repeated alternate rise, partial congealation and retreat of a vast sea
of lava. At full moon it is surrounded by radiating streaks."[9] The
view regarding the structure of Copernicus here expressed is of
importance, as it is probably applicable to all the craters of our
satellite.

"When the moon is five or six days old," says Sir Robert Ball, "a
beautiful group of three craters will be readily found on the boundary
line between night and day. These are _Catharina_, _Cyrillus_, and
_Theophilus_. Catharina is the most southerly of the group, and is more
than 16,000 feet deep and connected to 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 when the volcanic forces commenced
the formation of Theophilus, the rampart of which encroaches
considerably on its older neighbour. Theophilus stands as a well-defined
round crater, ab