But it is with the circular craters that we are most concerned. Judging from analogy with the lava-craters present on our globe, we must suppose them to be due to the extrusion, and piling up, of lava through central pipes, followed in some cases by the subsidence of the floor of the crater. It seems not improbable that it was in this way the greater number of the circular craters lying around Tycho, and dotting so large a space round the margin of the moon, were constructed. (See [Fig. 38].) In general they appear to consist of an elevated rim, enclosing a depressed plain, out of which a central cone arises. The rim may be supposed to have been piled up by successive discharges of lava from a central orifice; and after the subsidence of the paroxysm the lava still in a molten condition may have sunk down, forming a seething lake within the vast circular rampart, as in the case of the Hawaiian volcanoes. The terraces observable within the craters in some instances have probably been left by subsequent eruptions which have not attained to the level of preceding ones; and where a central crater-cone is seen to rise within the caldron, we may suppose this to have been built up by a later series of eruptions of lava through the original pipe after the consolidation of the interior sea of lava. The mamelons of the Isle of Bourbon,[15] and some of the lava-cones of Hawaii, appear to offer examples on our earth's surface of these peculiar forms.
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.