There was still a perceptible motion of the vessel. "We're rising," cried Jack in alarm. "Someone has tampered with the tanks. We are likely to hit a mine." Lord Hastings turned to the sailors. "Men," he said quickly, "until this matter has been finally settled, I must let you all go. But, each of you keep an eye on your companion, and at the first sign of treachery, shoot him. Search the ship." He followed Frank, who had dashed toward the compartments in which were located the water tanks, by means of which the vessel rose and submerged. As he dashed forward there came to his ears the sound of a shot. The commander of the D-16 redoubled his stride and the men, also having heard the shot, piled after him. As he passed the door to the engine room, the figure of a large man, rushing forth at that moment, struck him squarely and knocked him down. Lord Hastings was up in a moment and had his opponent beneath him. The latter was a powerful customer, however, and had it not been for the crew, who rushed to their commander's aid at that moment, it would have fared badly with him. But the crew, angry that they had been under suspicion, now that they believed they had found the man who was responsible for their predicament, leaped upon the man and soon had him bound securely. At that moment Frank and Jack came upon them. "Well, I see you have him," said Frank quietly. "Yes," replied Lord Hastings, "but what was the shot I heard?" "Oh that," said Frank. "He took a shot at me when I came upon him as he was fooling about the tanks." "Did he hit you." "No; just knocked my gun out of my hands. But you see, there was a stowaway on board." "But where on earth was he hiding?" "I found the hiding place," said Frank quietly. "It is in the engine room, right where he could do the most damage should occasion require. He had built himself a little stage beneath the floor, where he could lie, only coming out when it was safe." "But why hasn't he sunk us long ago? That's what I would like to know." "I think I can answer that," said Jack. "I should say that his work was to find out the specifications of the D-16--how she attains her great speed, and how she can remain indefinitely under water. In some way word of her building must have reached the enemy. Am I right?" he asked of the prisoner. The latter shrugged his shoulders. "Think as you please," he said. "Whatever my work, I have failed." "Yes, you have failed," said Lord Hastings. "And you know the penalty?" "Yes," said the prisoner quietly, "it is death." "Yes," repeated Lord Hastings slowly, "it is death!" Then to the men: "Guard him as you value your lives." Motioning to Jack and Frank to follow him, Lord Hastings led the way to his cabin. CHAPTER VII. DEATH OF THE SPY. "What will you do with the spy, Lord Hastings?" asked Frank. "Shoot him," was the brief reply. Frank was silent for some moments.