And that action heartened Dick wonderfully. As Von Kettler's face appeared again, he loosed his turret gun in a sweeping blast, and heard Von Kettler's gun roar futilely. Again they crossed each other's path, and again and again, two faces, only able to gauge roughly the position of their planes. Neither man had succeeded in injuring the other. Once old Lake turned his black ray upon Von Kettler, and for, a moment the plane stood out luminously in the blackness, but Dick leaned forward and yelled to the old man to desist. And once Dick looked back and saw Fredegonde crouched in her cockpit with eyes wide with terror. And yet he read in her eyes the same determination she had expressed in the laboratory. She was through with her half-brother. * * * * * All this while the wind had been increasing, making it difficult to maneuver the heavy plane; but now, of a sudden there came a dead lull, and then, with a whining sound, the wind rushed in again. But this was a wind still more unlike any that Dick had ever known. A mighty gale that revolved circularly, but downward too, like a vortex, catching the plane and sweeping it into an ever tightening circle. A man-made gale, upon whose wings the poison gas would spread northward again, carrying unlimited destruction with it. Dick fought in vain to free himself. He was revolving as in a whirlpool, and it required the utmost presence of mind and watchfulness to hold the plane steady. Round and round he spun--and then, suddenly, out of the void materialized Von Kettler's face. Von Kettler, helpless too, was spinning round upon the opposite side of the vortex. Thus each airship was upon the tail of the other, and it was a matter of chance which would get the other within the ringsights of the turret gun. Von Kettler was so near that his shouts of fury came fitfully to Dick's ears as the wind carried them. Dick, working the controls, knew that not for an instant could he direct his attention from them in order to fire his gun, and the moment Von Kettler attempted to do so, he was doomed. Round and round, struggling, battling in vain--and once more the concussion of the rear gun shook the plane. And a shout from the President reached Dick's ears. Dick turned his head for an instant, long enough to see Von Kettler spinning down through the vortex. And he was going down afire. President Hargreaves, "no military man," had got him, the second time he had ever aligned a gun-barrel upon a target. "Bravo, sir, bravo!" Dick shouted. And desperately he flung the stick forward and nosed down. * * * * * No gale, man-made or heaven-made, could carry on its wings three-quarters of a ton of armored, turreted airship. Swirling like a leaf, the plane broke through the clutch of the blast. Instantly it grew calm. Outside that vortex, hardly a breath of air was stirring. It was as if the whole fury of the air was concentrated within that circle.