on the earth's surface, is sufficient evidence
of the absence of air; and, as no cloud has ever been noticed to veil
even for a moment any part of our satellite's face, we are pretty safe
in concluding that there is no water; or at least, if there be any, that
it is inappreciable in quantity.[5] Hence we infer that there is no
animal or vegetable life on the moon's surface; neither are there
oceans, lakes or rivers, snowfields or glaciers, river-valleys or
canyons, islands, stratified rocks, nor volcanoes of the kind most
prevalent on our own globe.

[Illustration: Fig. 38.--Photograph of the moon's surface (in part)
showing the illuminated "spots," and ridges, and the deep hollows. The
position of "Tycho" is shown near the upper edge, and some of the
volcanic craters are very clearly seen near the margin.]

Now on looking at a photographic picture of the moon's surface (Fig.
38), we observe that there are enormous dark spaces, irregular in
outline, but more or less approaching the circular form, surrounded by
steep and precipitous declivities, but with sides sloping outwards.
These were supposed at one time to be seas; and they retain the name,
though it is universally admitted that they contain no water. Some of
these hollows are four English miles in depth. The largest of these,
situated near the north pole of the moon, is called _Mare Imbrium_; next
to it is _Mare Serenitatis_; next, _Mare Tranquilitatis_, with several
others.[6] Mare Imbrium is of great depth, and from its floor rise
several conical mountains with circular craters, the largest of which,
_Archimedes_, is fifty miles in diameter; its vast smooth interior being
divided into seven distinct zones running east and west. There is no
central mountain or other obvious internal sign of former volcanic
activity, but its irregular wall rises into abrupt towers, and is marked
outside by decided terraces.[7]

The Mare Imbrium is bounded along the east by a range of mountains
called the _Apennines_, and towards the north by another range called
the _Alps_; while a third range, that of the _Caucasus_, strikes
northward from the junction of the two former ranges. Several circular
or oval craters are situated on, and near to, the crest of these ridges.

[Illustration: Fig. 39.--A magnified portion of the moon's surface,
showing the forms of the great craters with their outer ramparts. The
white spot with shadow is a cone rising from the centre of one of the
lar