n of water has been
absent from the moon; so that, while accumulations of matter had been
proceeding throughout a prolonged period over its surface, there was no
counteracting agency of denudation at work to modify or lessen the
effects of the ruptive forces.

[1] Correctly speaking, each attracts the other towards its centre of
gravity with a force proportionate to its mass, and inversely as the
square of the distance; but the earth being by much the larger body, its
attraction is far greater than that of the moon.

[2] The variation in the distance is only under rare circumstances
40,000 miles, but ordinarily about 13,000 miles.

[3] _Story of the Heavens_, 2nd edition, p. 525, _et seq._

[4] A series of researches made by Zoellner, of Leipzig, led him to
assign to the light-reflecting capacity of the full-moon a result
intermediate between that obtained by Bouguer, which gave a brightness
equal to 1/300000 part of that of the sun, and of Wollaston, which gave
1/801070 part. We may accept 1/618000 of Zoellner as sufficiently close;
so that it would require 600,000 full moons to give the same amount of
light as that of the sun.

[5] Schroter, however, came to the conclusion that the moon has an
atmosphere.

[6] A chart of the moon's surface, with the names of the principal
physical features, will be found in Ball's _Story of the Heavens_, 2nd
edit., p. 60. It must be remembered that the moon as seen through a
telescope appears in reversed position.

[7] _Ibid._, p. 66.

[8] As represented by Nasmyth's models in plaster.

[9] Ball, _loc. cit._, p. 67.

[10] Ball, _loc. cit._, p. 69.

[11] _Astronomy_, p. 78.

[12] _Outlines of Astronomy_, p. 285.

[13] At rare intervals a few crescent-shaped ridges are discernible on
the lunar sphere, but it is very doubtful if they are to be regarded as
breached craters.

[14] The number of "spots" on the moon was considered to be 244 until
Schroter increased it to 6,000, and accurately described many of them.
Schroter seems to have been the earliest observer who identified the
circular hollows on the moon's surface as volcanic craters.

[15] Drawings of these very curious forms are given by Judd,
_Volcanoes_, p. 127.

CHAPTER III.

ARE WE LIVING IN AN EPOCH OF SPECIAL VOLCANIC ACTIVITY?

The question which we are about to discuss in the concluding chapter of
this volume is one to which we ought to be able to offer a definite
answer. This can only be ar