e, or at least so little at the bottom
of the valleys that it is imperceptible. No clouds, no fog, no rain nor
snow. The sky is an eternally black space, vaultless, jeweled with stars
by day as by night.

Let us suppose that we arrive among these savage steppes at daybreak:
the lunar day is fifteen times longer than our own, because the Sun
takes a month to illuminate the entire circuit of the Moon; there are no
less than 354 hours from the rising to the setting of the Sun. If we
arrive before the sunrise, there is no aurora to herald it, for in the
absence of atmosphere there can be no sort of twilight. Of a sudden on
the dark horizon come flashes of the solar light, striking the summits
of the mountains, while the plains and valleys are still in darkness.
The light spreads slowly, for while on the Earth in central latitudes
the Sun takes only two minutes and a quarter to rise, on the Moon it
takes nearly an hour, and in consequence the light it sends out is very
weak for some minutes, and increases excessively slowly. It is a kind of
aurora, but lasts a very short time, for when at the end of half an
hour, the solar disk has half risen, the light appears as intense to the
eye as when it is entirely above the horizon; the radiant orb is seen
with its protuberances and its burning atmosphere. It rises slowly, like
a luminous god, in the depths of the black sky, a profound and formless
sky in which the stars shine all day, since they are not hidden by any
atmospheric veil such as conceals them from us during the daylight.

[Illustration: FIG. 72.--The Lunar Apennines.]

The absence of sensible atmosphere must produce an effect on the
temperature of the Moon analogous to that perceived on the high
mountains of our globe, where the rarefaction of the air does not permit
the solar heat to concentrate itself upon the surface of the soil, as it
does below the atmosphere, which acts as a forcing-house: the Sun's heat
is not kept in by anything, and incessantly radiates out toward space.
In all probability the cold is extremely and constantly rigorous, not
only during the nights, which are fifteen times longer than our own, but
even during the long days of sunshine.

We give two different drawings to represent these curious aspects of
lunar topography. The first (Fig. 72) is taken in the neighborhood of
the Apennines, and shows a long chain of mountains beneath which are
three deep rings, Archimedes, Aristillus, and Autolycu