New York lay below them. The long, straight lines of lights shining up through the semidarkness of the moonlit night made a strange appearance to the two in the swift machine. Davis had mounted to a great height, some ten thousand feet, and the pin points of light outlined more than a dozen cities and towns. The Hudson was a faintly silvery ribbon flowing down placidly from a far-distant source. Because of the ice cake in the Narrows its level had risen two or three feet, but now it flowed smoothly over that great obstacle, melting and carrying it away toward the sea.

The fighting plane roared around in huge circles, seeming strangely alone in the vast expanse of air. One searchlight from below moved restlessly about the sky. A second joined it, then a third. One by one a dozen or more of long, pencil-like beams of light shot up into the sky and moved here and there in seeming confusion, but actually according to a carefully prearranged plan. A hooded red light showed below the biplane in which Teddy and Davis were awaiting some sign of the black flyer. That had been agreed upon, and none of the searchlight beams flashed upon the circling machine. From time to time Davis shut off the motors, and the two of them lifted the ear flaps of their helmets to listen eagerly for the musical humming that would herald Varrhus' approach.

Far to the east they could see where the faintly luminous waters of the ocean came up to and stopped at the darker masses of the land. The harbor below them glittered in the moonlight. The only peculiarity in the scene was the absence of the little harbor craft that ply about busily by day and night upon their multifarious errands. They were all securely docked. The wharves, too, were dark and silent. All the maritime industry of New York was at a standstill.

A wide spiral to twelve thousand feet. The motors were hushed during a two-thousand-feet glide, while the two men in the machine listened intently. For two hours this maneuver had been repeated and re-repeated. No sound save the rush of the wind through the guy wires and past the struts had broken the chilly stillness of the heights. The sky was a blue dome of a myriad winking lights. A pale silver moon shone down.

The nose of the machine pointed down and the motors ceased to roar. Faintly but unmistakably above the whistling and rushing of the wind about the surfaces of the biplane a deep, musical humming could be heard. Abruptly the motors burst into life again. The exhausts began to bellow out their reassuring thunder. The machine began to climb again, circling to every point of the compass, while Teddy and Davis scanned the sky keenly for a sign of the black flyer with its cargo of menace to New York.

"I'm going to fifteen thousand."

Davis' voice sounded with metallic clearness in Teddy's ear. The telephones between the two helmets were working perfectly.

"That was Varrhus, all right?" said Teddy quietly. "Did you signal to the people beneath?"

Davis pushed a button, and a green light glowed beside the red one in the hood below the machine. In a moment the receipt of this signal by those below was evidenced. The searchlights took up their task with renewed vigor, searching the sky frantically for a sign of the black flying machine. The hood below the biplane allowed the signal to be seen by those on the ground, but made the light invisible to any one in the air. The biplane swung in wide circles, Teddy and Davis with every nerve taut and every sense alert, aflame with eagerness to sight their quarry. They saw it, outlined for an instant by the white beam of one of the circling lights.

It was dropping like a stone from the clouds. The searchlight rays glistened from polished black sides and were reflected from shimmering propeller blades above it.

"Helicopter," said Davis crisply. "Now!"

The black flyer was a thousand feet below them and still falling. The nose of the biplane dipped sharply and it dived straight for the still falling machine. Teddy gripped the machine gun and sighted along the barrel. Down, down, the biplane darted, all the power of its eight hundred horse power aiding in the speed of its fall. The glistening black machine checked in its drop and hung motionless in mid-air. The pilot was evidently unconscious of the machine swooping down upon him.

Five hundred feet down, six hundred——Teddy pulled hard on the trigger, and his machine gun spurted fire. A stream of explosive projectiles sped toward the menacing black shape. Teddy saw them strike the shining sides of the machine and explode with little bursts of flame. The biplane was rushing with incredible speed toward the other flyer. Teddy played his machine gun upon it as he might have played a hose, and apparently with as little effect. The tiny explosive shells struck and flashed futilely. The black flyer seemed to be unharmed. After a second's hesitation, it dropped again abruptly. The biplane shot toward the spot the other machine had occupied. The distance was too short to turn or swerve, quickly as it responded to the controls.

"Flares," gasped Davis, but before he spoke Teddy was pressing the small button that would set them off.

A burst of tiny lights shot out before the biplane, many-colored balls of fire driven forward from a tube below the fusilage. They illuminated the air for a short distance, entering the space from which the black flyer had just dropped. Teddy and Davis saw a small cloud of what seemed to be mist or fog hanging in the air. The tiny fire balls darted into it the fraction of a second before the biplane itself had to traverse the same space. As the first of the lights struck the fringe of the whitish cloud it flared up. The fire ball had touched a droplet of liquified gas and set it flaming. It burned fiercely and with incredible rapidity, setting fire to the remainder of the cloud. Teddy ducked his head as the aËroplane shot madly through a huge globe of blazing gas in mid-air.

"Great God!" gasped Davis. "Now where's Varrhus?"

The heavy masks the two aviators had worn had protected them from the flaming hydrogen, and their goggles had saved their eyes. Now Davis was only eager to make a second attempt upon the black machine. He swerved and circled. The searchlights below were waving frantically through the air. The flare aloft had been seen, and they concentrated upon the space below the spot. In a second the black flyer was once more outlined by half a dozen beams. Davis banked sharply and darted toward it again.

The pilot of the strange machine seemed to be quite confident that he had disposed of his antagonist, and was apparently busy with something inside the cabin. He was probably preparing to release his cold bomb, but was again interrupted. The biplane approached. Teddy saw his explosive bullets strike and flash. He knew they struck, but they seemed incapable of doing harm. The black flyer was clearly defined by the searchlights, and Teddy could see it distinctly. It was a long, needlelike body with a glass-inclosed cabin near the center. Above it four whirring disks of comparatively huge size showed the position of the vertical propellers that enabled it to rise and fall and to hang suspended motionless in the air. A fifth propeller spun slowly at the bow. That was evidently not running at full speed. Below the needlelike body hung a misshapen globe, like the bulging ovipositor of some strange insect.

Flash! Flash! The impact of the explosive bullets was marked by spiteful cracks as they burst. Teddy was aiming for the cabin of the machine.

"Got him!" he exclaimed.

The glass of the cabin windows had splintered into fragments. The aËroplane shot toward the motionless black flyer.

"Shall I ram?" asked Davis in a perfectly even voice. He was quite prepared to sacrifice both his and Teddy's lives to make absolutely certain of the destruction of the menacing helicopter with its more than dangerous occupant.

Teddy, with lips compressed, nodded. He had forgotten that in the darkness Davis could not see his movement. As the biplane sped forward the black machine dropped again. Again the whitish cloud was left behind it, clearly defined in the searchlight rays. Teddy had barely time to press the flare button before they reached the cloud. The mist of atomized liquid hydrogen seemed to burst into flame all about them. The aËroplane roared through hell-fire for a moment. Flame was before Teddy's aviator's goggles. He was in a veritable inferno. Then the aËroplane shot free again.

"Ram him!" panted Teddy. "Smash him! Do anything, only we've got to get him!"

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."

Davis pressed his lips together and used every artifice he knew of to increase the speed of his machine, but the glistening black body ahead of them drew steadily farther away. At last it could barely be seen. Then, as if in derision, a light appeared in the cabin of the black flyer. It winked oddly. Dot-dash, dot-dash——

"He's signaling," said Davis.

Dot-dash, dot-dash——

"W-a-t-c-h," spelled Davis, "t-h-e M-i-s-s-i-s-s-i-p-p-i.—V-a-r-r-h-u-s."

"Watch the Mississippi, Varrhus," repeated Teddy. "He's getting away! He's getting away!"

The light ahead of them winked and disappeared. The sky was empty except for the biplane roaring after a vanished enemy.

"He's gotten away," half sobbed Davis. "Damn him! He killed Curtiss, and he's gotten away!"

Teddy stared into the empty night with something of Davis' disappointment and despair.