Davis was prepared for the maneuver, and almost as soon as the helicopter began to drop the biplane darted down after it, Teddy firing viciously. The streaks of smoke that his shells left behind them told him where he missed. Varrhus shifted the course of his fall, and again a cloud drifted in the air just before the pursuing plane. Davis flung the "joy-stick" forward, and the fighter fell into an absolutely vertical dive. A second more and it had turned upon its back and was flying upside down, away from the threatening mist. Davis twisted in mid-air and righted his machine. Varrhus was darting away, barely two hundred feet above the surface of the water. Again the two-seater dived upon him. Teddy's shells were zipping dangerously near the black machine. It began to zigzag, to twist and turn like a snake. It doubled back and shot directly under the biplane, but too far below for the deadly mist to be used. Davis banked at a suicidal angle and went after it again. They passed directly above the silent small boat, drifting aimlessly on the waves. Little icicles were forming on the bulwarks, showing that the cold of the liquified gas was still intense. For one instant Teddy had a perfect sight, and pulled the trigger with the peculiar confidence of a marksman who knows he is making a perfect shot. There was a flash upon the upper portion of the black hull. A dark object shot off at a tangent from one of the whirring disks. The helicopter sank rapidly. Teddy gave a shout. "Landed!" The black machine recovered again. One of the disks was badly injured and now slowed and stopped, showing that the blade of one of the four sustaining propellers had been broken, but the remaining three increased their speed. Varrhus seemed to abandon the idea of fighting. He began to shoot away toward the northeast. He was more than a mile away, and Teddy had stopped firing. Varrhus had had no difficulty in distancing the same machine a week before, and anticipated no trouble in losing it, even with his own flyer partially crippled. He had not reckoned on the picric compound now being used for fuel. The biplane sped madly after the fleeing black aircraft. The motors roared hugely, and the wind was like a solid mass, pushing fiercely against Teddy's exposed head. A small half-moon of glass protected Davis from the wind, but for the gunner no such protection was practicable. The rushing of the wind through the wires and along the sides of the stream-line body amounted to a shriek. Never had such speed been known before. Davis' voice came quietly to Teddy above the sounds outside, muted by the heavy, padded helmet. The telephone receivers were fast against Teddy's ears. "We're making two hundred and twenty-six." "We're not gaining," said Teddy grimly. "Wait until he rises. The motor's adjusted to be most efficient at about seven thousand feet."