Frank’s hand dropped quickly to his revolver.

“Stand over there,” he commanded in a harsh voice.

The man obeyed, and Frank approached and looked at him carefully.

“He is about the same build, sir,” he said, turning to Lord Hastings.

Lord Hastings confronted the sailor.

“Your name?” he demanded.

“Thompson, sir,” was the reply, and the man let fall his arms, which he had kept above his head when he entered.

Immediately Frank’s revolver flashed forth.

“Hands up there, Thompson,” he said quietly.

Plainly frightened, Thompson obeyed.

“How long have you been in the service?” demanded Lord Hastings.

“Ten years, sir.”

“Ever known by any other name?”

“No, sir.”

“Are you of German descent?”

“There is not a drop of German blood in me, so far as I know, sir.”

“Do you speak German?”

“A little, sir.”

Lord Hastings looked at the man closely, and demanded suddenly:

“What is the matter with your hand?”

“I was helping Smith in the engine room, sir, and a heavy block fell on it, sir.”

“Let me see it.”

Slowly the sailor unwrapped the bandage and exposed his hand. It was very red across the top, and Frank, glancing at it, believed that the mark could have well been caused by the blow of a revolver butt.

“I should say he is the man, sir,” he said quietly to Lord Hastings.

“It would seem so,” was his commander’s reply, “still we must be positive.” He turned again to the sailor. “Is it not true,” he asked, “that only a few moments ago you attacked Mr. Chadwick, and that your wounded hand is the result of a blow from his revolver?”

The sailor looked at his commander in surprise, that seemed genuine enough.

“No, sir,” he replied quietly.

“Then you deny you are a German spy?” asked Lord Hastings.

The sailor started back, and his face turned red.

“A spy, sir!” he cried. “Me a spy? Why if there is one country under the sun for which I would not turn a hand, it is Germany.”

“Circumstances are much against you, in spite of your protestations of innocence,” said Lord Hastings gravely.

“But Smith, the engineer, can vouch for me, sir, and so can Black, who was in the engine room when I injured my hand. Call them, sir.”

“Well, I’ll call them,” said Lord Hastings, “but I doubt if it will do any good.”

At a command from Lord Hastings the engineer and a sailor named Black were sent into the room.

Lord Hastings turned to the engineer.

“Were you in the engine room when Thompson injured his hand?” he asked.

Smith shifted uneasily from one foot to the other.

“Yes, sir,” he replied at length.

“He says that a block fell on his hand. Is that true?”

“Well, sir, I—I——” stammered Smith.

“Come, sir, this is a serious matter,” said Lord Hastings sternly.

“No, sir, it isn’t true,” said Smith. “He——”

“That is enough,” said Lord Hastings. He turned to Black. “Thompson says you were there. Is it true that a block fell on his hand?”

“No, sir,” said Black. “He——”

“That’s enough,” said Lord Hastings again. “It looks to me as if you were guilty.”

“If you please, sir,” said Thompson quietly, “will you allow me to ask Smith and Black one question?”

Lord Hastings nodded his head in assent.

“Smith,” said Thompson, “how did I injure my hand?”

“Why,” said Smith, “you miscalculated, and instead of putting it against Black’s head, you put it against the door.”

Thompson turned to Black.

“Is that true?” he asked.

“Yes,” replied Black.

Thompson turned to Lord Hastings.

“That should be enough, sir,” he said quietly.

“What is the meaning of this?” demanded Lord Hastings, looking from one to the other of the three men.

“It’s plain enough,” said Jack stepping forward. “Thompson and Black were settling an argument of some sort by the use of the fists. And Smith, I take it, was the referee. Am I right?”

“Right you are, sir,” replied Smith. “But we didn’t——”

“Never mind, never mind,” said Jack with a wave of his hand. “All of you line up there alongside Brice.” He turned to Frank and Lord Hastings. “We are on the wrong track,” he said.

“So it seems,” replied Lord Hastings, “but appearances were against him. We’ll have to look further.”

One after another the members of the crew were examined, but in spite of the best efforts put forward by Lord Hastings, Jack and Frank, not one of them could be tripped up.

“But it must have been one of them,” said Frank, after all had been examined and given satisfactory accounts of themselves. “It must be one of them. I certainly didn’t dream I was attacked. But who was it?”

“Can there be a stowaway aboard?” asked Jack.

“I never thought of that,” exclaimed Frank. “Shall I have the ship searched, sir?”

“Immediately,” ordered Lord Hastings. “We’ll leave Johnson here to guard these men, while we make a thorough search.”

From stem to stem they searched the little submarine. Not a single movable obstacle but what they moved. It was as systematic a search as it was possible to conduct, but there was no sign of a stowaway.

“Well, there is no one here,” said Frank, when the search had been concluded. “Therefore, it must have been a member of the crew.”

They went back to Lord Hastings’ quarters, where Johnson still stood on guard. Lord Hastings looked them over carefully, then spoke.

“Men,” he said, and his voice was very grave. “Some place on this vessel there is a traitor. We have searched high and low for some sign of a stranger, but we could find no one. Therefore, the spy must be among you. Will he step forward and save his companions from the finger of suspicion?”

Not a man stirred.

“Then——” began Lord Hastings, but he was interrupted by a sudden motion of the vessel, which seemed to be flying up through the water.

Up to this time it had been perfectly stationary.

“What’s the meaning of that?” cried Lord Hastings when he had recovered his balance.

“I should say someone was forcing the water out of the tanks, sir,” replied Jack calmly.

“But every man is in this room,” replied the commander.

“You forget the stowaway, sir,” said Frank.

“But there is no stowaway.”

“There must be a stowaway! How else do you account for this? The vessel could not do it by itself.”

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.