Listening intently for a moment, to get the exact location of the other man, Joe flung open the door and made a flying leap in the other's direction. The man was leaning over, and Joe landed squarely upon his back. With a muffled exclamation of surprise the man jerked himself forward and Joe went hurtling over his head, his arms, however, still clasped tightly about the other man's neck. Joe knew in an instant that he was in combat with a man larger and more powerful than himself, but his own youth and suppleness were in his favor. Throwing all his strength into the movement, he twisted about and at the same time jumped, so that he managed to wrap his legs about the other man's waist. With another lithe movement he was again upon his back and reaching for his antagonist's throat, at the same time squeezing with all the strength of his powerful young limbs upon the other's ribs. Back and forth across the narrow confines of the little room they staggered, now one having a temporary advantage, and again the other. Just as Joe was managing to fasten his fingers in at the throat, and the other was hammering terrible elbow blows into his stomach, the bigger man stumbled. As he fell he turned, and his full weight came down upon the lad, almost crushing him. Joe was not done for yet, however. With the strength of desperation he held on to the other fellow's shirt. He felt something hard and metallic under it, and in a new grasp included that in his fist. Again the struggle began. Unable to break Joe's grip, the intruder tried to sink his teeth into the lad's wrist. Failing in this, he gave an evidence of his strength by rising, dragging Joe upward with him. There was an instant of terrible whirling about the room, and then the man landed a smashing blow on Joe's jaw. Still gripping the man's shirt, and the unknown metallic thing beneath it, the lad reeled. The shirt ripped, there was another sharp snap, and the boy fell backward, dazed. He heard the man run swiftly, almost noiselessly toward the stern of the ship; brilliant and many-colored lights flashed before his eyes--and he knew no more. [Illustration: There was an Instant of Terrible Whirling about the Room.] CHAPTER VI THE MYSTERY OF THE IRON CROSS When Joe came back to consciousness it was with his head pounding terribly, and Lieutenant Mackinson bending over him, swathing his face with a cool wet cloth, while Jerry and Slim, whom the lieutenant had wakened, were standing nearby, one holding a basin of water, the other a bottle containing a liniment or lotion. "You've been done up pretty badly," said Lieutenant Mackinson, as Joe went through the painful motion of moving his head from left to right, letting his gaze take in the now lighted wireless room. "Yes," he answered with an effort. "Nothing serious, though, I guess." And then, full recollection coming to him, "Did he get away?" "Who?" asked the lieutenant quickly. "Who was it beat you up so?"