Such are the views of the origin of the physical features of our satellite which their form and inferred constitution appear to suggest. They are not offered with any intention of dogmatising on a subject which is admittedly obscure, and regarding which we have by no means all the necessary data for coming to a clear conclusion. All that can be affirmed is, that there is a great deal to be said in support of them, and that they are to some extent in harmony with phenomena within range of observation on the surface of our earth. The far greater effects of lunar vulcanicity, as compared with those of our globe, may be accounted for to some extent by the consideration that the force of gravity on the surface of the moon is only one-sixth of that on the surface of the earth. Hence the eruptive forces of the interior of our satellite have had less resistance to overcome than in the case of our planet; and the erupted materials have been shot forth to greater distances, and piled up in greater magnitude, than with us. We have also to recollect that the abrading action 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 Zöllner, 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 Zöllner 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.