A furious volley rattled out from the revolvers, but the range was too great and the breathless policemen had the mortification of seeing the machine disappear rapidly to the south.

Immediately the fastest runner of the party started at a trot for the Professor's house to send out a warning. But it was not necessary. The aerodromes all over the kingdom had been warned by wireless from the Air Ministry, and already a host of machines were scouting in every direction.

The stranger, flying due south, had reached Bradford before he was signalled. Instantly there was a rush of aeroplanes from all parts of the Midlands to cut him off. But he slipped through the cordon, flying very high and at a tremendous speed. Outside Birmingham a fast scout picked him up and reported by wireless, and from the huge aerodrome at Cheltenham over twenty fighting planes leaped into the air to stop the career of the marauder.

There was now no chance, at least, of his getting away un.o.bserved. He was under constant observation, alike from the air and the ground, and every moment wireless messages were pouring into the Air Ministry reporting his progress. But to catch him proved impossible. Only two of the pursuing machines were fast enough to keep up with the stranger, and even they could not overtake him. So the headlong flight went on, drawing ever nearer to the southern coast. If the stranger could get out to sea all chance of stopping him would vanish.

But, unknown to the furious British airmen, help was close at hand.

Warned by Jules' wireless messages of the direction the strange machine was taking, Yvette had steered a course to intercept him somewhere in the neighbourhood of Bournemouth, and the Mohawk, with its wireless chattering incessantly, was now swinging lazily at half speed in a big circle between Salisbury and the Hampshire watering-place.

"Over Salisbury now," called Yvette to d.i.c.k, her voice ringing out clearly above the m.u.f.fled hum of the propeller, the only sound which came from the helicopter, with its beautifully silenced engines.

A few minutes later d.i.c.k pointed to the north. "Here he comes," he shouted.

Far away were three tiny specks in the sky.

Through his gla.s.ses d.i.c.k could make them out clearly enough. The leader was a machine of a type he had never seen before; a mile behind it were a couple of planes which he at once recognised as the Bristol fighters which had been so familiar to him in France.

The pace of the three machines was terrific. It was clear the English airmen were going all out in a desperate effort to catch the stranger before he reached the water, and they were expending every ounce of energy. But a moment or two later it was quite clear they were falling behind. Presently a puff of smoke from the leader signalled "petrol exhausted," and he dropped in a long slant to the ground.

The second machine, however, held on grimly, though slowly losing ground. Evidently his predicament was the same as that of his colleague, and a moment later he, too, dipped earthward and was out of the fight.

Only the Mohawk stood between the stranger and safety!

But it was a Mohawk very different from the comparatively crude machine of a year before, wonderful though that was. d.i.c.k and Jules had worked out a revolutionary improvement in the lifting screws, with the result that a small supplementary engine, using comparatively little power, was now sufficient to keep the machine suspended in the air. As a result the full power of the big twin driving engines was now available for propulsion, and the speed of the Mohawk, when pushed to the limit, was something of which d.i.c.k had hardly dreamed in his earlier days. So far as he knew the Mohawk was easily the fastest craft in existence.

But what of the stranger? Had the men of the mystery craft a still greater secret up their sleeve? That they had something big d.i.c.k could plainly see by the way the fastest craft of the British Air Service, the best in the world, had dropped astern of the stranger. Was the Mohawk fast enough to beat the pirate? They would soon know.

As the big machine came on, Yvette set the elevating propellers of the Mohawk to work, and the helicopter shot upward. The stranger saw the manoeuvre and at once followed suit. But here he was at a disadvantage.

Yvette's object, of course, was to get above him. He would then be at their mercy, for he could not fire vertically, while the gun of the Mohawk was specially constructed so as to be able to fire downwards through a trap which opened in the flooring. If they could get what in the air corresponded to the "weather gauge" at sea, they would have the marauder at their mercy if the Mohawk had speed enough to hold him.

Could they do it?

Plainly the fugitive saw his danger. As Yvette shot upward he must have realised that in speed of climbing he was no match for his antagonist.

He decided to trust to his heels.

Yvette, climbing rapidly, had got a couple of thousand feet above the stranger and was heading to meet him. They were now twelve thousand feet in the air.

Suddenly, with a tremendous nose dive, the foreign aeroplane slipped below them. The manoeuvre was so smartly carried out that Yvette was completely taken by surprise, and before she could recover herself the chance of bringing the stranger to battle had gone. He had pa.s.sed five thousand feet below them, and the issue now depended upon speed and endurance.

With a cry of disappointment, Yvette swung the Mohawk round in pursuit.

Their quarry, by his daring manoeuvre, had gained a couple of miles before she could turn, and was fast disappearing towards the sea.

d.i.c.k shook his head. He had seen the speed of which the fugitive was capable, and he had the gravest doubts whether the Mohawk could equal it.

Waiting for the strange aeroplane, Yvette had set the Mohawk to a comparatively slow pace. She had misjudged the distance and her error had enabled the raider to get a more than useful--possibly a decisive-- lead.

But even as she swung round she had pressed the accelerator and the Mohawk quivered as the big twin engines began to work up to their maximum. Watching keenly, d.i.c.k saw the apparent rush away of the foreigner slacken and finally stop. They were at least holding their own. He signalled Yvette for more speed. She shook her head.

d.i.c.k was in despair. The pace at which they were going was not enough.

He thought it was their best. But he had not calculated on Yvette's resourcefulness.

The French girl had swiftly made up her mind. She knew they had plenty of petrol for several hours' flight. They were holding their own already in the matter of speed, and the Mohawk, though d.i.c.k did not know it, had still some knots in reserve. Yvette would not jeopardise the engines by instantly pushing them to the limit.

But they were "warming up" under her skilful handling. They were two miles behind as they pa.s.sed over Bournemouth and started the long flight to the French coast which the stranger was seeking.

Half an hour slipped by and d.i.c.k suddenly realised that the Mohawk was gaining, slowly, it was true, but unmistakably. He looked inquiringly at Yvette, who nodded and smiled.

"All right, d.i.c.k," she shouted. "We can get them any time we want."

d.i.c.k realised her plan. His own thought, as a fighting man, would have been to close at once and have it out. But Yvette had the radium in mind. If they smashed the stranger over the sea the priceless radium would inevitably be lost.

With the Mohawk gradually gaining, the chase drew near to the French coast. Cherbourg loomed ahead of them, drew near, and disappeared beneath them. They were over France.

Instantly Yvette began to coax the Mohawk to do its best. Splendidly the engines responded, the plane shot forward at a pace which surprised d.i.c.k, and a few minutes later they were directly above the fugitive.

The battle was all but won. In vain their quarry sought, by diving and twisting, to shake them off. His position was hopeless.

Seeing a good landing-place ahead d.i.c.k fired a couple of shots as a signal. They could see the terrified face of the pa.s.senger in the plane below gazing upward at the strange shape of the Mohawk above them.

Then the signal of surrender came, and the fugitive dipped earthward. A couple of minutes later it came to land, and the two occupants stood holding up their hands while the Mohawk came gently to earth fifty yards away, dropping vertically from the sky in a fashion which caused the pilot of the foreign machine the wildest astonishment.

The radium was saved! But it enacted a fearful revenge. The unfortunate pa.s.senger, who they found out later was a well-known Spanish anarchist, had imprudently placed the two tubes in his pocket, apparently ignorant of their terrible power. Even in the short time he had them in his possession he was so terribly burned that he died a couple of days later in spite of the efforts made to save him, while the pilot, who had, of course, been near enough to the tubes to get some of the effects, was also so seriously injured that for weeks his life hung in the balance. It was found impossible to remove the tubes to England until Professor Fortescue, overjoyed at the good news, came bringing the leaden safe into which the precious tubes were placed.

The sequel came a week later. Not even the British War Office could ignore the fact that the Mohawk, single-handed, had achieved a feat at which the British Air Force had signally failed. A highly placed official sought d.i.c.k out. The result was that the plans of the Mohawk were sold jointly to England and France at the price of one hundred thousand pounds.

And Regnier lost his "star" combination. d.i.c.k had no longer before his eyes the fear that had haunted him for so long that in marrying Yvette he would be condemning her to a life of comparative poverty. And so the companionship born amid the stress and tumult of war came at last to perfect fruition in the marriage between the two lovers which took place in Paris just three months after their last air adventure.

The End.