One mystery had been solved, but it had given rise to a more sinister problem. Michael Splendor was the one who pointed this out, as Don Winslow and his friends sat that evening at the officers’ mess.

How, he asked, could the thief have known Don was making a search of the enlistment records?

There were various answers offered on the spur of the moment. Mercedes Colby suggested that Don and the captain had been overheard talking about it on their way to the cabin; but that point was quickly settled. Neither man had mentioned it before reaching Riggs’ cabin.

Red’s answer was that the thief had happened to see Don through the open door, as he sat at the captain’s table. This sounded reasonable, until Michael Splendor told them he had tried looking through the door. From outside the cabin, he stated, neither table nor record file could be seen.

“The conclusion is, me friends,” he said with a troubled frown, “that the man who struck down Commander Winslow knew in advance what he was going to do, and why. He had time to plan the job, and wait for his chance to catch his victim alone. He knew the very moment we decided to search those records for a clue!”

In the shocked silence which followed Splendor’s words, Riggs pushed back his chair. The captain’s face had the look of a man just charged with murder.

“In other words, you accuse me, Mr. Splendor!” he said hoarsely, rising to his feet. “By George, sir! If you were not a cripple, I would....”

“Please, Captain Riggs!” the voice of Michael Splendor rang sharp as a trumpet call. “I am accusing no one of us—least of all yourself. Now, look me in the eye and smile, me friend; for in faith I would sooner accuse meself than anyone in this cabin!”

Slowly the color came back into Captain Riggs’ cheeks. Sinking back into his chair at the head of the table, he did his best to smile, though it was a hard attempt.

“I believe you, Mr. Splendor!” he said huskily. “But the way you put the evidence gave me an ugly start, sir. It seemed to point to me alone as the attacker, or at least as the one person whose whereabouts were unaccounted for at the time of the attack on Winslow.”

“Indeed it did, Captain,” admitted the man in the wheel chair, apologetically, “and sorry I am for not choosing me words more carefully! I was just tryin’ to show all of ye how closely we are being spied upon. I meself could have sworn that no one was in earshot of our party this afternoon. Yet our every speech was heard and noted by the enemy. Our future plans must be told in whispers behind locked doors, I fancy.”

Throughout the rest of the meal there was a lively argument about dictaphones and other means of eavesdropping which the spy might have used. It all proved highly amusing to the Scorpion spy, who was listening in on every word, by the aid of a clever electrical “ear.”

Small and easily concealed as a man’s wrist watch, the device was a powerful amplifier of ordinary sounds. These were transmitted over threadlike wires to an earphone, palmed in the spy’s hand.

Turned toward a ship’s ventilator or porthole, or toward a party conversing on deck, the mechanical “ear” could pick up even whispered speech without the slightest difficulty.

But while this comparatively harmless eavesdropping was in progress, a far more sinister drama was being enacted below decks. It was the old, old game of death, which has been played since life began upon the earth, and the first killer stalked his unsuspecting prey.

Deep in the bowels of the ship, amid the click and whir of oiled machinery, Chief Petty Officer Ahern began his evening watch. He was a man of about thirty years, with a well-muscled body, and keen, blue Irish eyes. In six years he had risen from Fireman Second Class to Chief Machinist’s Mate.

At the moment, Ahern was the only man in the engine room of the Gatoon. Lieutenant Allen, the Engineer Officer, was in his own stateroom, cleaning up for the evening meal. The other Machinist’s Mates were off watch. The nearest members of the “black gang” were sweating in the boiler room, forward.

Ahern was whistling an old Irish ditty as he moved about, checking the smoothly running machinery. Thought of danger was the farthest thing from his mind as he paused to glance up at the stars shining through the fiddley hatch above his head.

All at once his body stiffened as if in agony. His hands clawed at his throat. Mouth open and eyes popping from their sockets, he reeled backward in a grotesque dance of death.

As he fell, struggling, to the iron deck, a man in ordinary seaman’s uniform dodged past him toward the main steam line. There was a quick sharp hammering; a hiss of escaping steam. Then the seaman reappeared, his features covered by a white handkerchief.

Briefly he stooped over Ahern’s limp body, fumbling at the swollen, purple neck. The next moment, swift as a startled rat, he slipped out of sight behind a bulkhead.

Five minutes later, Lieutenant Allen came on deck for a breath of air before going to mess. Glancing toward the fiddley hatch, he noticed a wisp of steamy vapor rising from it. In alarm he sprang forward to look below. The heavy reek of hot engine oil met his nostrils as he bent over the hatch. The hum and clink of smoothly moving metal rose with it to reassure him. Only the steamy mist between decks, and a slowing of the engine’s rhythmic beat spelled danger to the officer.

Turning to the engine room ladder, Lieutenant Allen made the lower deck in record time. Through a mist of steam, he made out the body of the Chief Machinist’s Mate. Pausing beside it only long enough to read the signs of death, he pressed on as far as he dared toward the broken steam line.

Up on the Gatoon’s bridge, Lieutenant Darnley caught the engine room’s urgent signal. Picking up the speaking tube, he barked a short acknowledgement.

“Allen speaking,” came the terse reply. “Inform Captain Riggs of attempt to sabotage the ship’s engines. Chief Machinist’s Mate Ahern is dead at his post. Must stop engines and pull boiler fires at once.”

The meal was just over in the officers’ mess. With a low exclamation Don Winslow jumped up and stepped to the nearest porthole.

“If I’m not mistaken, Captain,” he said, turning to face the others, “this ship is losing way. The engines, ... hear that, sir? The vibration has stopped completely!”

Captain Riggs sprang to his feet, scowling.

“You’re right, Commander!” he cried. “We’ll be losing steerage way in a few moments. I’ll ring the bridge and find out....”

A heavy knocking on the cabin door interrupted. Opening it, the captain faced a breathless yeoman, whose message of disaster fairly tumbled from his lips.

“Trouble with the main steam line, sir!” the enlisted man reported. “And the Chief Machinist’s Mate has met with an accident, too. Lieutenant Allen requests your presence at once in the engine room!”