"It sounds loud enough," answered Jack. "Shoot some more," which the old hunter did. They wandered on still farther, firing at intervals all that day, but there came no answering report or calls to direct them to the projectile. They climbed once more to the tops of towering peaks, but there they found their range of vision limited by peaks still higher, while there were great valleys, in one of which, whether near or far they could not tell, they knew, the _Annihilator_ was hidden. They had almost lost track of time now, and they did not know how far they had wandered. They had sought out lonely caves to sleep in when they were so weary they could go no farther, and they had sat about on bleak rocks shivering, and had eaten their scanty meals--shivering because in spite of their fur garments they were cold, as they did not eat enough to keep their blood properly circulating. They could not when they did not have the food to eat! Andy used up all but a few of his cartridges in firing signals, but to no purpose. Their water was all but gone, and of their food only enough remained for a day longer, though their life-torches still gave forth plenty of vapor. "Well, what's to be done?" asked Jack, as they sat about, looking helplessly at one another. "Might as well give up," suggested Mark bitterly. "Give up? Not a bit of it!" cried Andy, as cheerfully as he could. "Let's keep on. We'll find the projectile sooner or later." So they kept on. It was while making their way between two great mountain peaks that towered above their heads on either side, thousands of feet up, making a sort of natural gateway, that Jack, who was in the lead, cried out in astonishment at the sight that met his gaze when he had passed the pinnacles. "Look!" he shouted, pointing forward. What he indicated was a great crater--larger and deeper than any they had yet met with. It seemed a mile across, and, if gloom and darkness were any indications, it was a hundred miles deep. But it was not the size of the great hole in the ground, not its fearful gloom, that attracted their attention. What did was a great natural or artificial bridge of stone that was thrown across the middle of it from edge to edge. A bridge of stone that spanned the abyss; a roadway, fifty feet wide, which reached into some unknown land, connecting it with the desolate country in which our friends had been wandering. "A bridge of stone across the cavern," said Jack, "but see. Here is a house of stone. This was the guard-house, I'll wager--the guardhouse at the entrance to some city, and that bridge is the means by which the inhabitants entered and left. Maybe we are at the edge of the inhabited part of the moon!" His words thrilled them. They pressed forward to the beginning of the bridge across the crater. They looked into the stone hut. Clearly it had been made by hands, for it was composed of blocks of stone, neatly fitted together. Jack's theory seemed confirmed.