sketch of a volcanic vent on
the moon in the days when the craters were active. The eruption is here
shown in the fulness of its energy, when the internal forces are hurling
forth ashes or stones which fall at a considerable distance from the
vent. The materials thus accumulated constitute the rampart surrounding
the crater.

The second picture (Fig. 29) depicts the crater in a later stage of its
history. The prodigious explosive power has now been exhausted, and has
perhaps been intermitted for some time. Again, the volcano bursts into
activity, but this time with only a small part of its original energy. A
comparatively feeble eruption now issues from the same vent, deposits
materials close around the orifice, and raises a mountain in the centre.
Finally, when the activity has subsided, and the volcano is silent and
still, we find the evidence of the early energy testified to by the
rampart which surrounds the ancient crater, and by the mountain which
adorns the interior. The flat floor which is found in some of the
craters may not improbably have arisen from an outflow of lava which has
afterwards consolidated. Subsequent outbreaks have also occurred in many
cases.

One of the principal difficulties attending this method of accounting
for the structure of a crater arises from the great size which some of
these objects attain. There are ancient volcanoes on the moon forty or
fifty miles in diameter; indeed, there is one well-formed ring, with a
mountain rising in the centre, the diameter of which is no less than
seventy-eight miles (Petavius). It seems difficult to conceive how a
blowing cone at the centre could convey the materials to such a distance
as the thirty-nine miles between the centre of Petavius and the rampart.
The explanation is, however, facilitated when it is borne in mind that
the force of gravitation is much less on the moon than on the earth.

[Illustration: PLATE VIII.

A NORMAL LUNAR CRATER.]

[Illustration: Fig. 28.--Volcano in Activity.]

[Illustration: Fig. 29.--Subsequent Feeble Activity.]

Have we not already seen that our satellite is so much smaller than the
earth that eighty moons rolled into one would not weigh as much as the
earth? On the earth an ounce weighs an ounce and a pound weighs a pound;
but a weight of six ounces here would only weigh one ounce on the moon,
and a weight of six pounds here would only weigh one pound on the moon.
A labourer who can carry one sack of corn