“I beg your pardon, sir,” he said deferentially, “but you tell us what you think of it; what your theory is as to where we are and why we hang here in mid-air, and why the ship rolled over every time we passed this point?” “I can only account for it,” I replied, “upon a single and rather preposterous hypothesis, which is that the Moon is a hollow sphere, with a solid crust some two hundred and fifty miles in thickness. Gravity is preventing us from rising above the point where we now are, while centrifugal force keeps us from falling.” The others nodded. They too had been forced to accept the same apparently ridiculous theory, since there was none other that could explain our predicament. Norton had walked across the room to read the barometer which he had rather neglected while the ship had been performing her eccentric antics far below the surface of the Moon. I saw his brows knit as he glanced at it, and then I saw him studying it carefully, as though to assure himself that he had made no mistake in the reading. Then he turned toward us. “There must be something wrong with this instrument, sir,” he said. “It is registering pressure equivalent to that at the Earth’s surface.” I walked over and looked at the instrument. It certainly was registering the pressure that Norton had read, nor did there seem to be anything wrong with the instrument. “There is a way to find out,” I said. “We can shut down the insulating generator and open an air-cock momentarily. It won’t take five seconds to determine whether the barometer is correct or not.” It was, of course, in some respects a risky proceeding, but with West at the generator, Jay at the air-cock and Norton at the pump I knew that we would be reasonably safe, even if there proved to be no atmosphere without. The only danger lay in the chance that we were hanging in a poisonous gas of the same density as the earthly atmosphere, but as there was no particular incentive to live in the situation in which we were, we each felt that no matter what chance we might take it would make little difference in the eventual outcome of our expedition. I tell you that it was a very tense moment as the three men took their posts to await my word of command. If we had indeed discovered a true atmosphere beneath the surface of the Moon, what more might we not discover? If it were an atmosphere, we could propel the ship in it, and we could, if nothing more, go out on deck to breathe fresh air. It was arranged that at my word of command West was to shut off the generator, Jay to open the air-cock, and Norton to start the pump. If fresh air failed to enter through the tube Jay was to give the signal, whereupon Norton would reverse the pump, West start the generator, and immediately Jay would close the air-cock again.