ctive crater on our earth, is seven miles in circuit. Larger
extinct craters occur in Japan; but all our terrestrial volcanic
mountains are dwarfed by those observed on the surface of the moon,
which, owing to its smaller size, has cooled more rapidly than our
earth. It is, of course, the explosive force from below which keeps
the crater clear, as a cup-shaped hollow, truncating the cone; and all
stones falling into it would be only thrown out again. It may at the
close of an eruption cool down so completely that a lake can form within
it, such as Lake Averno, near Naples; or it may long remain a seething
sea of lava, such as Kilauea; or the lava may find one or more outlets
from it, either by welling over its rim, which it will then generally
break down, as in many of the small extinct volcanoes ("puys") of
Auvergne, or more usually by bursting through the sides of the cone.

LAVA VARIES VERY MUCH IN LIQUIDITY

It is not generally until the volcano has exhausted its first explosive
force that lava begins to issue. Several streams may issue in different
directions. Their dimensions are sometimes enormous. Lava varies very
much in liquidity and in the rate at which it flows. This much depends,
however, upon the slope it has to traverse. A lava stream at Vesuvius
ran three miles in four minutes, but took three hours to flow the next
three miles, while a stream from Mauna Loa ran eighteen miles in two
hours. Glowing at first as a white-hot liquid, the lava soon cools at
the surface to red and then to black; cinder-like scoriaceous masses
form on its surface and in front of the slowly-advancing mass; clouds of
steam and other vapor rise from it, and little cones are thrown up
from its surface; but many years may elapse before the mass is cooled
through. Thus, while the surface is glassy, the interior becomes
crystalline.

As to what are the causes of the great convulsions of nature known as
the volcano and the earthquake we know very little. Various theories
have been advanced, but nothing by any means sure has been discovered,
and considerable difference of opinion exists. In truth we know so
little concerning the conditions existing in the earth's interior
that any views concerning the forces at work there must necessarily be
largely conjectural.

Sir Robert S. Ball says, in this connection: "Let us take, for instance,
that primary question in terrestrial physics, as to whether the interior
of the earth is liquid