aring these altitudes with
those of mountains on our earth, we must for the latter add the depth of
the sea to the height of the land. Reckoned in this way, our highest
mountains are still higher than any we know of in the moon.

We must now discuss the important question as to the origin of these
remarkable features on the surface of the moon. We shall admit at the
outset that our evidence on this subject is only indirect. To establish
by unimpeachable evidence the volcanic origin of the remarkable lunar
craters, it would seem almost necessary that volcanic outbursts should
have been witnessed on the moon, and that such outbursts should have
been seen to result in the formation of the well-known ring, with or
without the mountain rising from the centre. To say that nothing of the
kind has ever been witnessed would be rather too emphatic a statement.
On certain occasions careful observers have reported the occurrence of
minute local changes on the moon. As we have already remarked, a crater
named Linne, of dimensions respectable, no doubt, to a lunar inhabitant,
but forming a very inconsiderable telescopic object, was thought to have
undergone some change. On another occasion a minute crater was thought
to have arisen near the well-known object named Hyginus. The mere
enumeration of such instances gives real emphasis to the statement that
there is at the present time no appreciable source of disturbance of the
moon's surface. Even were these trifling cases of suspected change
really established--and this is perhaps rather farther than many
astronomers would be willing to go--they are still insignificant when
compared with the mighty phenomena that gave rise to the host of great
craters which cover so large a portion of the moon's surface.

We are led inevitably to the conclusion that our satellite must have
once possessed much greater activity than it now displays. We can also
give a reasonable, or, at all events, a plausible, explanation of the
cessation of that activity in recent times. Let us glance at two other
bodies of our system, the earth and the sun, and compare them with the
moon. Of the three bodies, the sun is enormously the largest, while the
moon is much less than the earth. We have also seen that though the sun
must have a very high temperature, there can be no doubt that it is
gradually parting with its heat. The surface of the earth, formed as it
is of solid rocks and clay, or covered in great part by t