It was now Kirkwood's chance. The hostile monoplane was still within easy range, although momentarily her superior speed was taking her further and further away from her pursuer. She had broken through the cordon. Ahead was a straight, unimpeded run for home. The Lewis gun began to splutter. Half--three-quarters of the drum of ammunition was expended without tangible result. The Hun observer, too, had got his machine-gun in working order and was pumping out nickel at the rate of five hundred rounds a minute. It was a duel to the death. At that dizzy height no human being could fall and reach the surface of the sea alive. No cover, no sheltering trenches protected the four combatants. In the blue vault of heaven they were compelled to kill or be killed, or even deal out complete and horrifying destruction to each other. "Got him, by Jove!" shouted Kirkwood, as the Hun at the machine-gun threw up his arms and toppled inertly across the barrel of the weapon. For perhaps ten seconds he hung thus, till the monoplane, rocking through an air-pocket, tilted violently. For a brief instant the body trembled in the balance, then slipping sideways the dead Boche toppled over the edge of the fuselage and fell like a stone through space. "Keep it up, you're on it!" yelled Barcroft, never for a moment taking his eyes off the fugitive monoplane. His observer heard the shout but the words were unintelligible in the deafening rush of air Nevertheless he maintained a steady fire at the enemy machine. To give the Hun pilot his due he made no attempt to throw up the sponge. He might have made a nose-dive, trusting to flatten out' and gain the surface of the water. The machine would have sunk like a stone, but there was a faint chance of the pilot being able to unbuckle the strap that held him to the seat and make an attempt to save himself by swimming. The Hun did unfasten the leather strap, but for a different purpose. The monoplane, being of a self-steering type, could be relied upon to continue her flight more or less in a straight line, without a controlling touch on the rudderbar. With a stealthy, cat-like movement the German made his way to the observer's seat, and gripping the firing mechanism of the machine gun prepared to return the dangerous greetings from his pursuer. Less than fifteen miles off--twelve minutes flight--lay the flat outlines of the Belgian coast. Unless Fritz could be brought down rapidly the raider would win through. Suddenly the monoplane tilted and settled down to a dizzy nose-dive. Whether a vital part had been hit or whether the uncontrollable drop was due to faulty construction neither Barcroft nor his companion knew. For the moment the flight-sub imagined that it was a daring ruse on the part of the Hun pilot, until he realised that the latter was in the observer's seat when the catastrophe occurred.