It was no pleasant thought to contemplate the presence of a bold, even desperate, agent of an enemy government, on board an American transport carrying approximately two thousand souls.

That he was capable of going any lengths, if necessary, already had been proved; and the evidence of his evil genius might come in horrible form at any instant.

Nevertheless, neither the excitement nor the potential danger of the situation was sufficient to prevent Jerry and Slim from taking a full eight hours of much-needed sleep, while Lieutenant Mackinson, Joe and three other officers whom the captain had taken into his confidence in the matter, followed out every possible clue in pursuit of a solution of the baffling mystery.

The record of every enlisted man and officer on the vessel had been most carefully probed, without building up enough suspicion to warrant the singling out of any individual as the probable offender.

Likewise an investigation of the members of the crew had failed to develop anything tangible, even directly suspicious. It was a case of watch everybody, take every precaution, and be prepared for anything. Only nine men on the vessel, however, including the spy himself, knew anything about it, and the rest were in utter ignorance of the treachery that might be directed against them at any time.

Refreshed by their sleep, Jerry and Slim arose about four o'clock that afternoon. Joe, who had rested easily throughout the later excitement of the preceding night, was still in the midst of the investigation and was not then to be found. Jerry had some letters to write, so Slim went to the upper deck alone.

Seeing no one that he knew, and his mind weighted anyway with the menacing mystery of the strange happenings of the night before, he sat down on a coil of rope, just in the lee of the forward smokestack, to think the whole matter over for the twentieth time.

He was thus absorbed when something, at first vague and indefinite, then clearer and clearer until it was unmistakable, began to impress itself upon his mind. Like the awakening call that comes to a man in a sound sleep—seemingly as a far-off whisper that gradually gathers volume and strength until finally the sleeper awakes with a start to find someone standing directly over him, loudly and insistently calling his name—so Slim came to a realization of the strange series of sounds that were being repeated within a few feet of him.

Could it possibly be only the crackling of the steam-pipe that ran along the smokestack to the whistle—a crackling merely from the pressure within? For a moment Slim thought an over-wrought imagination was playing tricks upon him. But he rose hastily and crossed the short intervening distance.

Clearly and distinctly it came to him then. Someone in another part of the vessel was rapping desperately upon that pipe! And in the long and short dashes of the international code that someone was repeating a single word—"Help! Help! Help!"

In another instant, using the heavy end of his jackknife as a crude transmitter, Slim was tapping off the reply:

"Who are you—and where?"

"Lieutenant Mackinson," the message began to come back. "Locked in closet off engine room. Can't make self heard. Can you help?"

"This is Slim," the youth rapped back upon the pipe. "Caught your message on deck. Am coming with help at once."

And he dashed down the deck toward the captain's quarters, almost bowling over the captain's aide as he hurtled into the sanctum of the ship's commander unannounced.

"Well?" the captain demanded sternly. "Why all the haste?"

"Lieutenant Mackinson," Slim blurted out; "he's locked in a closet down near the engine room."

"Locked in a closet!" the captain repeated incredulously. "How do you know?"

"He gave a telegraphic call for help on the steam-pipe which runs through there and connects with the whistle," the lad explained. "I was on deck and heard it. I talked with him over the pipe."

"There is no time to lose, then. Come with me." And the captain himself hurriedly led the way down through the lower depths of the ship, where it became hotter and more oppressive with every step they took.

They had taken a route by which they escaped the attention of anyone else on the ship.

"It should be right about here somewhere," the captain announced, as they approached a particularly dark passage. For a few steps they felt their way along, and then stopped to listen.

There was nothing but the dull and constant hum of the engines and the almost insufferable heat.

"The other side," said the captain in a lowered voice, as they failed to find any trace of the imprisoned lieutenant where they were.

They were crossing a short gallery when Slim abruptly signaled a halt.

"I thought I heard something," he said. "It sounded like another call."

They stood silent a moment, and then, faint and indistinct, apparently from somewhere several feet ahead of them, they both heard repeated that which had made Slim stop. As the letters were tapped off upon the pipe the lad repeated them for the information of the captain.

"S-M-O-T-H-E-R-I-N-G."

"Smothering!" echoed the commander of the ship. "Great Scott! I believe I know now where he is. This way," and he started down the passageway toward a narrow stairs leading to a still lower chamber in the vessel.

Three turns—two to the right and one to the left—and the captain stopped again to listen. Seemingly from within the wall, right at their elbows, there came a feeble knock. The officer whipped out a pocket flashlight. They were directly in front of a heavy wooden door. It was locked.

"Run get a cold chisel or a heavy screwdriver and hammer," the captain ordered, and Slim hastened away, to return two minutes later with all three tools.

"Stand back as far as you can from the door," said the captain, placing his lips close to the keyhole. But there was no response from within.

Realizing now that Lieutenant Mackinson must have lost consciousness, and that moments might mean life or death to him, the captain worked with feverish haste. He drove the heavy chisel into the crack between the door and the jam, and then, standing off to get a wider swing with the hammer, struck it sidewise.

A panel of the door cracked and loosened. Two more attempts and the panel fell in strips to the floor. Thus given something for a grip-hold, the captain, who was a massive man, took hold with both hands, put his right foot against the wall, and, with one tremendous tug, into which he threw the whole weight of his body, brought the entire door from its hinges.

The captain went staggering backward from the force of his effort and the weight of the door.

The unconscious form of Lieutenant Mackinson tumbled out upon the floor. His face was almost blue from suffocation.

The captain sounded three short, sharp blasts upon a whistle which he had taken from his pocket, and two oilers came running to the spot.

"Help us carry this man to fresh air immediately," he ordered. "He has been overcome."

With one of the oilers carrying the lieutenant by the feet, and the other man and Slim at either shoulder, the unconscious young officer was carried up flight after flight of steps until, the captain leading the way, they arrived at the promenade deck.

A seaman was dispatched for the ship's surgeon, who arrived a few minutes later to find the first-aid efforts of the four men just bringing Lieutenant Mackinson back to consciousness.

As the physician forced some aromatic spirits of ammonia between his lips the lieutenant opened his eyes and gazed about vaguely.

"What's the matter?" he asked weakly; but before anyone could answer he had relapsed again, and there was another wait of several minutes.

But this time the lieutenant's mind was clearing.

"Somebody shoved me—in that closet," he gasped, "and then—slammed and—locked—the door."

He recognized the captain and the doctor. As his eyes closed again he added, in an almost inaudible whisper: "I was getting too close on somebody's trail."

The captain looked at the ship's doctor significantly and dismissed the two oilers with instructions to return to their duties.

"Found him locked in a small compartment down near the auxiliary engine room," the commander said briefly. "Hotter than blazes, and no air whatever where he was. He made his whereabouts known by tapping a message on a steam-pipe."

"H'm," said the doctor, whose youthful appearance might not give a stranger a proper measure of his long and varied experience. "Nearly suffocated, too. He couldn't have lasted there much longer. His heart action is pretty weak even yet. Better have him removed to his bed, and kept there for the rest of the day, at least."

At that moment Jerry came hurrying down the deck. He was visibly excited, but, unlike Slim, he did not forget that not only must a soldier never permit his feelings to run away with him, but that he must be equally mindful of respect for superiors.

And so, even as two men carried Lieutenant Mackinson away, he remained standing at salute, waiting for the captain to recognize him with a return of the salute.

"And now what?" asked the captain.

Jerry stepped forward, with difficulty repressing his excitement.

"I stepped out of the wireless room for only a few moments," he said. "When I returned I found this lying upon the table."

He opened his left hand. In it lay a piece of light chain, both ends broken.

"Beside it," he continued, "was this note."

From his pocket he extracted a piece of paper, the edges of which were roughly torn. He handed it to the captain, who read aloud:

"Let this be a warning that no further interference will be of avail."