“I will explain. The moon is a much lighter body than the earth bulk for bulk, a cubic yard of it containing on an average only six tenths as much matter as an average yard of earth. The reason of this is that a very large part of the moon’s bulk is made up of interstices, caves and openings. Now it is a remarkable fact that the hemisphere of the moon facing the earth is much lighter than the further one, so much so that the center of gravity is 33 miles further from this side of the moon than from the further side. This fact has been suspected by some of your astronomers. The consequence of it is that the sea has all gone to the further hemisphere, and the near hemisphere is in the highest place, about 33 miles above the level of the sea. It is much as if a concave cap, the material of which is 33 miles thick at the center and tapers to zero all round the rim, were fitted on to a sphere. This rim is at the edge of the moon, as seen from the earth. Our atmosphere like yours, gets lighter as we ascend and is too thin to support life at a height of five miles, so that the great plateaus of our hither hemisphere are over 20 miles higher than any appreciable atmosphere. So you can see the impossibility of life on the hither surface of the moon if you reflect a moment what the conditions would be on a mundane plateau 33 miles above the sea level. Your highest mountains are only between five and six miles high, and you know the impossibility of either vegetable or animal life at even that altitude. “On the earth such elevations are regions of perpetual snow, and the hither surface of the moon would be such a region if it possessed water and an atmosphere. But while the surface on this side is uninhabitable, there are immense tracts of underground space, that have been converted into habitable territory. This underground country lies so far below the surface that it is practically near the sea level throughout. It is approached at all parts of the rim of the cap just described, and there are many thousands of tunnels entering it all round this rim, especially in the equatorial parts of the moon. A great amount of labor has been expended, not only on these entrances, but on the internal cavities to which they lead; but compared with the work performed for us by nature, our own labor is but an insignificant item—hardly so much as the labor of your race in fitting up the earth for your residence. The entrances are all volcanic craters, and the vast cavities to which they lead, were excavated long ages ago by volcanic action. The material blown out of the volcanoes, mostly fell upon the hither side of the moon increasing the bulk of the cap; most of the volcanoes being on this side. But even the material thrown from the lateral regions was drawn this way by the attraction of the earth and after describing a longer or shorter curve, fell on the hither side of the moon.