"And it is the only one we have," said Mr. Roumann. "We need it very much, too, for the projectile can't very well be moved without it." "How can we get it?" asked Jack. "I'm sorry. It was my fault." "It was as much mine as yours," asserted Mark. "I guess it's up to us to go back after it. It isn't far. We can easily walk it." There seemed to be nothing else to do, and, after some discussion, it was decided to have the two boys walk back after the missing tool, which was a very valuable one. "Take fresh life-torches with you," advised Mr. Henderson, "and you had better carry some food with you. It may be farther back than you think, and you may get hungry." "I guess it will be a good thing to take some lunch along," admitted Jack. "And some water, too. We can't get a drink here unless we come to a spring, and we haven't seen any since we arrived." "I'll go with you, if you don't mind," said Andy. "I may see something to shoot." The three of them, each one carrying a freshly charged vapor-torch, a basket of food and a bottle of water, started off, well wrapped in their fur coats. Andy had a compass to enable them to make their way back to where the tool was left, for, amid the towering peaks and the valley-like depressions, very little of the level surface of the moon could be seen at a time. They walked on for several hours, every now and then hoping that they had reached the place where the projectile had been halted, and where they expected to find the tool. But so many places looked alike that they were deceived a number of times. At length, however, they reached the spot and found the instrument where Jack had carelessly dropped it. They picked it up and turned to go back, when Andy Sudds saw a large crater off to one side. "Boys, I'm going to have a look down that," he said. "It may contain a bear or wildcat, and I can get a shot." "Guess there isn't much danger of a bear being on the moon," said Mark, but the old hunter leaned as far over the edge of the crater as he dared. "No, there's nothing here," he announced, with almost a sigh, and he straightened up. As he did so there came a tinkling sound, as if some one had dropped a piece of money. "What's that?" asked Jack. "By heck! It's the compass!" cried Andy. "It slipped from my pocket when I stooped over. Now it's gone!" There was no question of that. They could hear the instrument tinkling far down in the unfathomable depths, striking from side to side of the crater as it went down and down. "We'll never see that again," spoke Mark dubiously. "Can we get back to the projectile without it?" asked Jack. "Oh, I fancy I can pick my trail back," answered the hunter. "It isn't going to be easy, for there are no landmarks to guide me, but I'll do my best. I ought to have known better than to put a compass in that pocket."