They circled swiftly, searching for the black flyer. The searchlights were following him now, and they saw that he was rising straight up. He had not yet dropped his cold bomb. Davis put his machine at the ascent at as steep an angle as he dared. They climbed almost as rapidly as the helicopter. The black machine made its first aggressive move now. Davis was climbing in a jerky spiral, rising at an amazing speed. Teddy was busily fitting a new belt of cartridges into his machine gun. The pilot of the other machine darted to one side and a huge cloud of mist sprang into being just below him, darting downward like some pale-gray snake, unfolding itself in the sky. Davis zoomed sharply. Another second and he would have run into the whitish cloud. The biplane recovered and swerved to one side. Twelve thousand feet. Thirteen thousand feet. Fourteen thousand feet. Three miles in the air! Then the black flyer began to drop. The biplane dived after him, Teddy's machine-gun spitting fire and explosive bullets in a furious, well-directed blast. Once, twice, bursts of the little flashes that showed his bullets were striking served to reassure Teddy, but the biplane could not gain on the falling helicopter. Down, down----There were half a dozen quick bursts of flame in the air. Anti-aircraft guns were firing. The black flyer dropped unharmed. Barely a thousand feet above the waters of the bay, the propeller at the bow seemed to be put into motion, for the straight descent changed into a graceful curve. The curve flattened out, and the black machine ceased to fall. It sped madly for the Narrows, with a bedlam of bursting shells all about it and the vengeful, spitting two-seater darting after it like an avenging Nemesis. Again and again spurts of flame against the body of the glistening helicopter showed that Teddy's fire was well directed, but the machine shot onward in a furious rush for the Narrows. Above the Narrows, without pausing, a black object that turned to white in the searchlight rays fell from the misshapen globe below the center of the black flyer's body. The thing that fell seemed to leave a mist of fog behind it as it dropped. Then, its mission accomplished, the dark machine fled toward the west. Teddy and Davis, in the biplane, sped after it at the topmost speed of which their aëroplane was capable. Teddy was nearly insane with baffled rage and disappointment. He knew that he had failed. Another cold bomb had been dropped in the Narrows, and any attempt to destroy it would only result in the death of those who made the attempt. "Faster, faster!" he pleaded to Davis. "If it gets far ahead of us we'll lose it in the darkness."