“What depth, Mr. Stark?”

Captain McGill, standing by the side of the young middy, asked the question. They were still running under water, but the air, which had just been changed, was as fresh as if they were on the surface. A heavy vibration was noticeable though as the powerful engines forced the cigar-shaped craft through the tremendous pressure of the lower waters.

“Hundred fathoms, sir,” was the rejoinder. The naval officer glanced at his watch. Then his eyes fell on the distance recorder.

“We’ve run forty miles at that depth,” he said, “but keep her submerged. This was to be a thorough test.”

“She’s having it, sir,” ventured the midshipman; “we must be out of the Sound and under the Atlantic by this time.”

“Well, we left Block Island some miles to our stern quite a little while ago,” was the reply. “It’s a queer thing to think that there may be some big liner’s keel right above us at this moment.”

“It is, indeed, sir,” agreed Mr. Stark. Just then, Mr. Lockyer and Lieutenant Parry, with other members of the testing party who had been below examining the engines, entered the conning-tower. They reported everything as working to the pitch of satisfaction.

“Well, Mr. Lockyer, I congratulate you, sir,” said Captain McGill ponderously. “I think that your craft will prove a magnificent success. There is only one thing now to test her at, and that is to ascertain how she stands the vibrations set up by torpedo firing.”

“If we could run across a derelict——,” began Midshipman Stark.

“Good gracious, young man, I hope we do no such thing,” laughingly exclaimed Captain McGill; “at this depth, and at ten miles an hour, we would never reach the surface to tell the tale. However, that does not prevent me from admitting that I’m exceedingly sleepy. Gentlemen, it is almost eight bells. Suppose we turn in for a nap. We can be called if anything occurs.”

“This traveling under water seems to affect one’s wakefulness,” yawned one of the board. “I think your suggestion is an excellent one, captain.”

Soon afterward, leaving orders to be summoned at once if anything out of the way occurred, the officers composing the board retired to their staterooms. Quarters were close on the Lockyer, but room had been found for all. The two apparent castaways had gone to their stateroom some time before.

“Ding-ding! Ding-ding! Ding-ding! Ding-ding!”

The ship’s bell clock sounded out eight bells. Ned and Herc, on watch in the conning-tower, exchanged significant glances. Midshipman Stark was at the wheel, but knowing nothing of the plans on foot, the chiming of the hour meant nothing to him, but that the night was slipping by extremely slowly.

As the last strokes of the bell died away, a hail of “all’s well” came from the engine room.

It was echoed from the conning-tower, where the boys stood with beating hearts. The hour that was to witness their ruse had struck. Presently Lieutenant Parry’s foot sounded on the rounds of the steel ladder.

“Strong and Taylor, come below,” he ordered, in a sharp voice. His tones were low, however.

Both boys instantly obeyed. Their hearts beat a little faster than usual as they descended into the cabin. They were about to attempt a somewhat risky bit of business. Both the supposed plotters were desperate-looking men, and the conversation the lads had overheard did not lead them to suppose that the additions to the Lockyer’s company were any less bad than they looked.

“Now, I’ll go to the door,” said the officer, as the two young blue-jackets faced him, “and give the alarm. Then you leave the rest to me, but the instant the cabin is empty, you dive in there and examine the box.”

The boys nodded.

“Aye, aye, sir!” they said, as if they had received a routine order of some sort.

The officer crossed the cabin floor in a couple of strides. Going to the door of the two Italians, he turned the handle. It was unlocked. Fortune favored him then. He could arouse them without awakening anyone else. But as he opened the door a strange thing happened. One of the men sprang suddenly upright and, for an instant, seemed to be about to spring at the officer’s throat. The next instant he subsided with a low laugh.

“Pardon, sir; I——.”

“Yes, yes, I know,” burst out Lieutenant Parry, in an excited voice, which was excellently assumed; “never mind that now. Get up! Quick! The submarine is in peril!”

“In peril! Santa Maria!”

The fellow sprang from his bunk. It could then be seen that he was fully dressed. His companion, also, must have been feigning sleep, for he, too, was up in an instant. He, likewise, was fully dressed.

“Oh, sir, we are in danger?” he gasped.

“The greatest. Come quick!”

Lieutenant Parry seized both the men in an iron grasp and rushed them out of the cabin. He was afraid if they lingered they might stop to pick up the box.

“This way! This way! Quick! Pray heaven we are not too late!” he cried, as he hustled them through the engine-room door, closing it behind them with a loud clang.

“Now!” exclaimed Ned, “we’ve not a minute to lose.”

Followed by Herc, he darted forward like a hound that has just picked up a hot scent.

Another instant and they were in the cabin lately occupied by the two Italians. Ned thrust an arm under the lower bunk. As he had expected, the box was there—a stout, black receptacle, bound at the corners with brass.

It had a lock on, but drawing his marlin-spike knife, Ned had it burst open in an instant. As he broke the lock there was a loud snap and a queer sound like the ticking of a loud clock was heard.

Tick-tock! Tick-tock!

Ned threw back the lid, and as the contents of the box lay before him, he gave a gasp. At first sight the interior of the thing looked not unlike the works of a clock. It was this machinery that was ticking. In one corner was a tiny hammer, raised above what seemed to be a percussion cap. Below this cap was a thick, gelatinous-looking stuff. As he saw this latter, Ned gave a cry, and thrusting his hand into the box, tore the machinery out of it and hurled it clean across the cabin.

“An infernal machine!” he gasped.

“What!” almost screamed Herc.

“Don’t you see,” shouted Ned excitedly; “this yellow stuff is nitro-glycerine. Enough to blow this boat to pieces. That clockwork, when set going, would, in due time, bring the hammer down on the percussion cap, touching off the diabolical affair, and——”

Before he could utter another word something sprang on him, encircling his neck, ape-like, with long arms. Ned saw a bright thing flash above him. Instinctively, he knew that it was a knife. Swiftly he threw up one arm and caught the descending blade in the nick of time. At the same instant, a scream of baffled rage rang out, as strong arms seized Guiseppi, who had sprung upon him, and dragged him off the Dreadnought Boy’s back.

In the doorway of the engine room Ignacio struggled, foaming and blaspheming, in Engineer Bowler’s grip, but the husky ex-foreman held him fast.

“Don’t squirm too lively, you bloomin’ dago,” he muttered, “or I might get nervous and tap you on the head with a wrench.”

Held tightly by Lieutenant Parry and two members of the engine-room crew, Guiseppi, who had made the murderous attempt on Ned, writhed and flung himself about with equal vehemence.

“I had hardly gotten them into the engine room,” explained the young officer, giving the recalcitrant Guiseppi’s arm a twist, “before they discovered it was all a trick. I suppose they knew, in a flash, the object of it, for before I could stop him, this ruffian here had darted through the door and sprang upon your back.”

“You were just in time, sir,” said Ned; “I could hear that knife whistle as he plunged it down. He fairly had me in chancery, too.”

“I fancy you were just in time, too, Ned,” said the young officer warmly.

He shuddered as he spoke. But now stateroom doors began to fly open, and heads were poked out. Presently, the entire naval board was hearing the story, while Midshipman Stark, at the wheel, strained his ears to hear what he could of it. For he had heard the disturbance, but, of course, could not leave his post. It was his duty to stick at the wheel, even if he had known that the submarine was about to be blown up.

“It seems to me,” said Captain McGill, when he had heard Lieutenant Parry’s story, “that these two lads are entitled to a great deal of credit for the part they played in this affair. They not only acted bravely, but with discretion, which is better than mere courage. You, too, Parry, did a clever thing. I think, gentlemen, that all three are to be congratulated for securing a pair of precious scoundrels.”

The two Italians were then, at Captain McGill’s orders, triced up to stanchions. Bound securely, they glowered at their captors furiously, but for some time refused to speak. At last, Ignacio, in response to Captain McGill’s questioning, confessed the whole plot.

They had been hired by Ferriss and Camberly—whose shipyard they had formerly worked in—to carry out the daring plan to wreck the Lockyer. Knowing that they could not get on board by any ordinary means, they had chartered the catboat and purposely capsized her, so that they would have to be taken on board. Their plan had then been to wait till she was in port and then set the machine among her engines, wrecking them hopelessly. Both men denied that they had intended to take any lives. But, in view of the amount of nitro-glycerine contained in their machine, it was practically certain that anyone who had the misfortune to be on board at the time it exploded, would have been, if not killed, seriously injured.

As soon as this confession had been extorted from the men it was set down in writing, and they were compelled to sign it. The submarine was then headed for the surface, and the nitro-glycerine gingerly carried on deck and dumped overboard.

“I don’t care to be shipmates with the stuff in that form many minutes longer than I have to be,” said Captain McGill, amid a general laugh, in which, perhaps, there was a little of the trace of the nervous strain which they had undergone.

At Mr. Lockyer’s request, the two Italians were questioned as to the whereabouts of Ferriss and Camberly, but they professed ignorance of where their employers were to be found. They were to have recovered their money by mail, they declared, but as a considerable sum was found on them, it was always supposed that they received some of their pay for their rascally attempt in advance.

“Well, gentlemen,” announced Captain McGill, a short time later, “the tests, both on the surface, semi-submerged, and submerged, have been perfectly satisfactory. Let us now head about and give these rascals over to justice. If their purpose was to ruin the Lockyer submarine and prevent her sale to the Government, they have failed. I shall report her at Washington as an unqualified success.”

“Thank you, sir!” said the inventor simply, and would have added more, but at this instant there came a sudden sharp hail from Tom Marlin, who had succeeded Midshipman Stark at the wheel.

“Something dead ahead, sir. I——”

Before he could complete the sentence there came a terrific shock. The submarine quivered from stem to stern under the stress of the blow. The party had to clutch at handrails and projections to avoid being thrown flat.

“We’ve struck something!” shouted Mr. Lockyer. A terrible fear burned in his eyes as a wild confusion of shouts and cries arose from below.

The submarine slewed round drunkenly, and a rasping sound rang from her steel plates. The inventor, cool-headed despite his alarm for his craft, sprang to the engine controls. Rapidly, he spun the telegraph indicator.

“Back! Full speed astern!”

Again came that bumping, rasping sensation. It was as if the little vessel had struck a reef or a submerged rock, although the chart showed none in that part of the ocean.