they are the remains of
gigantic bubbles which were raised in the moon's "skin," when the globe
was still molten, by volcanic gases from below. A few astronomers think
that they are, as is popularly supposed, the craters of extinct
volcanoes. Our craters, on the earth, are generally deep cups, whereas
these ring-formations on the moon are more like very shallow and broad
saucers. Clavius, the largest of them, is 123 miles across the interior,
yet its encircling rampart is not a mile high.

The mountains on the moon (Fig. 16) rise to a great height, and are
extraordinarily gaunt and rugged. They are like fountains of lava,
rising in places to 26,000 and 27,000 feet. The lunar Apennines have
three thousand steep and weird peaks. Our terrestrial mountains are
continually worn down by frost acting on moisture and by ice and water,
but there are none of these agencies operating on the moon. Its
mountains are comparatively "everlasting hills."

The moon is interesting to us precisely because it is a dead world. It
seems to show how the earth, or any cooling metal globe, will evolve in
the remote future. We do not know if there was ever life on the moon,
but in any case it cannot have proceeded far in development. At the most
we can imagine some strange lowly forms of vegetation lingering here and
there in pools of heavy gas, expanding during the blaze of the sun's
long day, and frozen rigid during the long night.

METEORS AND COMETS

We may conclude our survey of the solar system with a word about
"shooting stars," or meteors, and comets. There are few now who do not
know that the streak of fire which suddenly lights the sky overhead at
night means that a piece of stone or iron has entered our atmosphere
from outer space, and has been burned up by friction. It was travelling
at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
above our heads it began to glow, as at that height the air is thick
enough to offer serious friction and raise it to a white heat. By the
time the meteor reached about twenty miles or so from the earth's
surface it was entirely dissipated, as a rule in fiery vapour.

Millions of Meteorites

It is estimated that between ten and a hundred million meteorites enter
our atmosphere and are cremated, every day. Most of them weigh only an
ounce or two, and are invisible. Some of them weigh a ton or more, but
even against these large masses the air acts as a kind of "torpedo-net."