much satellite and primary as smaller and greater sisters, made out of one mass, and consequently made of the same material. And since the density of the moon is only three-fifths that of the earth, there can be nothing for it but that she is hollowed out by a great system of caverns. There was no necessity, said Sir Jabez Flap, F.R.S., that most entertaining exponent of the facetious side of the stars, that we should ever have gone to the moon to find out such easy inferences, and points the pun with an allusion to Gruyère; but he certainly might have announced his knowledge of the hollowness of the moon before. And if the moon is hollow, then the apparent absence of air and water is, of course, quite easily explained. The sea lies within at the bottom of the caverns, and the air travels through the great sponge of galleries, in accordance with simple physical laws. The caverns of the moon, on the whole, are very windy places. As the sunlight comes round the moon, the air in the outer galleries on that side is heated, its pressure increases, some flows out on the exterior and mingles with the evaporating air of the craters (where the plants remove its carbonic acid), while the greater portion flows round through the galleries to replace the shrinking air of the cooling side that the sunlight had left. There is, therefore, a constant eastward breeze in the air of the outer galleries, and an up-flow during the lunar day up the shafts, complicated, of course, very greatly by the varying shape of the galleries and the ingenious contrivances of the Selenite mind. . . .
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