"Do!" The man's voice fairly boomed. "I'm going to tear that parachute off your back and pitch you overboard, you infernal outlaw! And I'm going to claim that you resisted arrest!" At that instant he noted Mona for the first time. He started as he recognized her. "The surgeon!" Then his rage came on him again. "You hold your tongue, young woman, or I shall have it--pulled out! Do you understand?" he demanded, thrusting his face up toward hers. And then Fort was upon him. All he cared for now was to get his fingers in Norbith's throat. And next moment Mona was desperately steering his machine clear of the other as it swayed and thrashed about under the struggling of the two men. The advantage was with the deputy. Powerful man that he was, he was more than a match for even Fort's great strength, while the athlete's agility did him no good in the restricted space of the cockpit. The parachute hindered him, too. Down on the ground, on a clear spot, it would have been different. As it was, Fort was quickly thrust to his knees, and, despite all that he could do, he could not fight off the deputy's grip. In a moment it had shifted to the athlete's throat. "You would, would you!" roared the deputy. "By--you'll be dead even before you reach the ground!" Fort struggled wildly. In a moment he was strangling; Mona could see his protruding eyes and lolling tongue. She could not help. She was not athlete enough to leap to his aid. But all of a sudden, just as Fort had once come to her own rescue, her tongue came to his. "Boy! Boy! Tear open his shirt! Tear open his shirt!" Fort heard. For a second he hesitated, dull wonder in his starting eyes; then he reached up, and with a spasmodic jerk of his hands, ripped Norbith's shirt wide open. The man's bare chest was exposed. "Don't you see?" shrieked Mona hysterically. "Look, boy! Look!" And Fort saw. Saw the two silver tubes leading from the brown scar in the breast of this man--the man whose heart had been replaced by a silver instrument. Saw the tubes, leading to a belt around the man's middle, where the pumping mechanism was concealed. And as Fort saw, he understood. With a final burst of strength he raised his quivering fingers and clutched one of the little pipes. A jerk, an exclamation from Norbith; and then, even as Fort's head fell back insensate, his hand snapped the little tube in two. "Good God!" swore the deputy. "You--you've--" He gasped and spluttered; he let go of Fort. The athlete dropped like a log into the bottom of the craft. But Eklan Norbith stood upright, his hands thrashing wildly, his mouth twitching horribly. One end of the broken tube hissed with escaping air; the other end spouted blood. The deputy swayed; his head dropped to his shoulders. And then the air rushed into his lungs for the last time; he gave a single piercing shriek, tottered, and fell backward out of the machine.