Lieutenant Stein bade the boys good-night and went below. After some further talk, Captain Von Cromp followed him, and the boys were left alone on the submarine, save for the single man on look out. Frank walked up to the latter and engaged him lit conversation. A few moments later he turned Away, saying to the sailor that he and his friend "would take a turn or two about before going below." Walking swiftly up to Jack, Frank said in a low voice: "See if you can't find that small boat they used to pick us up." "What--?" began Jack, but Frank interrupted him. "Never mind the reason," he said. "Help me find it, that's all. We'll have to hurry. Where do you suppose they put it?" A few moments later they came upon the little craft, now above water, placed where the sea could not reach it when the submarine was submerged. Luckily it was out of view of the German on the bridge, and the two lads succeeded in unloosening it and getting it overboard without being seen. Then Frank walked quickly back to the spot where the periscope protruded from below. Opening his coat he took the explosive out and, drawing a handkerchief from his pocket, tied it to the diminutive mine and hung the latter on the tube. "Now for this German," he said to himself. "It wouldn't do for him to see that before I am ready." He approached the man once more and asked several questions. "Well," he said finally, "I guess I shall have to say good-night." The German's reply was choked in his throat. Frank sprang forward, flung one arm around the man's, neck, and with the other clutched him by the throat, to prevent an outcry. Then he freed one arm and struck out heavily. The German fell without a murmur. Frank ran across the deck to where he had left Jack. "Into the boat quick!" he exclaimed. Jack needed no further urging. Frank dropped lightly in after him, and soon they were rowing rapidly away. "Give me that gun," said Frank after they had pulled some distance from the submarine. "What are you going to do with it?" asked Jack. "I'll show you," replied Frank grimly. "Give me the gun!" Without another word Jack passed the weapon to his friend. "Now," said Frank, "lower yourself over the side of the boat and when I say dive, dive!" "See here," said Jack, taking Frank by the arm. "Have you gone crazy? What do you think you are going to do?" "I don't think anything about it," replied Frank, more quietly now. "I know what I am going to do." "Well, what is it then? Out with it." "Do you see that object hanging to the periscope tube on the submarine?" asked Frank. "Yes, I see it. Why?" "Don't you know what it is?" "No; what is it?" "Well, that's the little plaything I found in Lieutenant Stein's cabin. I'm going to bore a little hole through it with this gun you were kind enough to get for me." "Good Heavens!" exclaimed Jack "You'll blow tip the submarine with all on board!" "My idea exactly," replied Frank carefully. "But--" protested Jack.