"Flares!" cried Davis in an agony of apprehension, though his voice was only audible to Teddy by means of the telephone connection between the two helmets.
As he spoke the men on the boat shot up the little fire balls that had protected the aëroplane in its former fight. A dozen balls of light sped up to meet the menacing cloud of liquified gas. They reached it, sped into it, glowing feebly! The white cloud did not ignite, but fell on toward the boat. It reached and enveloped the little vessel, and suddenly the guns were still.
"Damn him!" said Teddy in a voice that shook with rage. "He's not using hydrogen. We can't close in on him now. Our flares are no good."
Davis tilted the nose of his machine upward, and Teddy stared down his sights. He pulled the trigger. The gun kicked backward, but the recoil cylinders did their work. The tracer shell left a little line of smoke behind it. It passed below the black body.
"Too low," said Teddy grimly, and fired again.
Varrhus began to climb. Straight up his machine went, but with the picric acid giving added impetus to the explosions in the cylinders the two-seater climbed as rapidly. Varrhus' ascent swerved. He was directly over the aëroplane. A whitish cloud appeared below his machine and blotted it out for an instant.
"We zoom," said Davis almost gayly, and the fighting plane seemed to be dancing on its tail for an instant. The cloud of gas unfolded itself down to the surface of the water, barely twenty yards before the space in which Davis had checked his course.
Around and around a huge circle. The biplane had caught up with the black flyer, and Davis turned toward it for an instant to give Teddy an opportunity to fire. There was a flash at the stern of the slender black body, and the symmetry of the glistening form was marred by a ragged edge where the tip of the tail had been blown off.
"Almost," said Teddy grimly.
"He'll dive now."
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."
The black speck ahead of them was drawing no nearer, it is true, but it was not dwindling. The silvery wings of the biplane cut through the air with fierce impatience. It flew in the straightest of straight lines after the other craft. Dark-brownish smoke blew backward from the bellowing exhausts, tinged almost to saffron by the presence of the explosive acid. The sunlight kissed the upper surfaces of the wings of the pursuing plane. Below them the ocean rolled and tossed.
Whistling wind and roaring engines. Speed, speed, speed! The biplane rushed with incredible swiftness through the air. The black flyer skimmed lightly on, barely in advance of its white-winged enemy. Twice Teddy essayed a shot, but the biplane trembled so that accuracy was impossible, and he could see by the smoke of his tracer shell that he had gone far wide of the black machine. The space between the black speck and the waves below it seemed to increase.
"Rising," said Davis. "Now we'll get him."
Teddy kept his eyes fixed on Varrhus' slender, needlelike craft. He was barely conscious of the upward tilt of the machine in which he was riding, but he saw that they were keeping pace with Varrhus as he rose in the air.
"Four thousand feet," said Davis crisply. "And two hundred and twenty-nine miles an hour. There's land ahead."
Teddy saw a mountainous coast line becoming visible far away. The black flyer continued to rise.
"Six thousand feet," said Davis again, "and two hundred and thirty-two miles——"
The pilot of the other machine saw that they were gaining. He dropped abruptly.
"Now!" exclaimed Davis fiercely.
He dived downward. The descent, coupled with the immense power of the engines—now delivering vastly more than the eight hundred horse power for which they were designed—made them shoot toward the black flyer with increasing speed. The other machine was barely more than half a mile away and every detail of its construction was visible. Teddy noticed for the first time a slender tube rising between the two center sustaining propellers. He instantly leaped to the conclusion that it was the means by which the jets of liquified gas had been shot out. He fired.
"A hit!" cried Davis.
There had been a flash from the top of the cabin. A jagged rent appeared in the polished roofing, and the slender tube vanished. The black flyer seemed to abandon all hopes of escape. It sped madly for a gap between two of the tall mountains that rose along the coast line. At the unprecedented speed with which both machines had been traveling the coast seemed fairly to rush at them. No villages were visible, but it seemed to be a habitable, if not an inhabited, land. The black flyer swept on across country, Varrhus evidently making every effort to gain even a few yards on his adversaries, and Davis just as fiercely determined that he should not. Once, twice, three times Teddy fired.