himself was found lying unconscious in his laboratory, havingapparently been sandbagged. The raiders had leisurely helped themselvesto food, and, having cut the telephone wires, had departed without anyparticular haste.
But the great leaden safe, weighing several hundredweights, in which theprecious radium had been brought to England, was found to have beenbroken open. _The radium was gone_!
Nothing in the meantime had been heard of the strange aeroplane. But afew hours later an old shepherd walked into one of the localpolice-stations and told a queer story.
His sheep the previous evening, he reported, had been disturbed by thepassing of an aeroplane which, flying very low, had landed on the moorsa few miles away from the Professor's house. It had stayed there allnight and, so far as he knew, was still there. He had been unable toapproach it closely as it was separated from where he had been by a deepgorge and a stream which he could not cross without making a detour ofseveral miles. He had seen two men near the machine who had walked awayand disappeared in the folds of the moor.
A strong party of police, Cummings added, had left at once for the spotwhere the aeroplane had been seen, taking the shepherd with them asguide. The place was remote from any road, and it would be an hour ortwo before they could get there. But the Air Ministry had been warned,and already aeroplanes were going up in the hope of locating the strangemachine.
"I must be in this," said Dick. "Ask him if I can come over. I cannot,of course, go unasked."
"Of course," said Cummings in reply to Regnier's request. "We shall beonly too glad to have Mr Manton. Miss Pasquet can come too, if shelikes. But I'm afraid he won't be able to get here in time. We shalleither have got these fellows or lost them hopelessly in a few hours."
Dick turned to Jules.
"Ring up the British Air Ministry," he said, "and ask them if thestrange machine gets off the ground to send us every movement as it isreported. Keep the telephone on all the time. I am going to try to cutthese chaps off with the Mohawk. You will have to report to me bywireless every movement as it comes through. From what we have heard Ifancy there are very few machines in England fast enough to catch thosefellows if they once get started. Of course you will come, Yvette?"
An hour later, Dick and Yvette, seated in the helicopter, were in fullflight for England. Yvette was at the controls; Dick, in view of thework that might be before them, crouched over the tiny machine gun whichpeered from the bow of the machine.
------------------------------------------------------------------------
Professor Fortescue was in a terrible state of distress. He had beenworking in his laboratory, when a slight noise had caused him to turnround. A man, apparently a foreigner as the Professor judged from thehasty glance he got at him, was standing close behind him. Before theProfessor could speak or move he received a violent blow on the head,and remembered nothing more till he recovered consciousness some timelater under the care of the police.
His chief concern was for the radium, and his distress at its loss waspitiful. It was a disaster from which he seemed unable to recover. Buthe appeared to derive a strange satisfaction from the danger in whichthe thieves would find themselves.
"I don't know how they will get it away," he declared to the policeinspector. "It was dangerous to stay very near the safe for long owingto the terrible power of the radium rays. If the thieves try to carrythe tubes in their pocket they will not get very far. Surely theycannot realise the terrible risk they are running. However, that neednot distress us; all we want is to get the radium back."
In the meantime a strong party of police had arrived from Durham at theProfessor's house, and, under the guidance of the old shepherd, startedacross the moors for the spot at which the strange aeroplane had beenseen. It was slow going over rough and difficult ground which testedthe endurance even of the younger men. The only unconcerned person wasthe old shepherd who trudged stolidly on at a pace with which they foundit difficult to keep up.
They had gone eight or nine miles before the old man spoke.
"Not far now," he said.
A mile farther on he halted.
"It's just over yon hill," he said, pointing to a small eminence a fewhundred yards away. "You will see it as soon as you get at the top."
Breasting the rise, the police cautiously approached the ridge andglanced over. There in the valley, only five or six hundred yards away,was the aeroplane. Two men in air kit stood beside it.
Scattering into a thin line the police rushed down the slope, every manwith a revolver ready in his hand.
But they were just too late. They had only gone a few yards when themen hastily took their places in the machine, there was a loud whirr asthe engine broke into action, and while the policemen were still ahundred yards away, the strange machine rose into the air and was gone.A furious volley rattled out from the revolvers, but the range was toogreat and the breathless policemen had the mortification of seeing themachine disappear rapidly to the south.
Immediately the fastest runner of the party started at a trot for theProfessor's house to send out a warning. But it was not necessary. Theaerodromes all over the kingdom had been warned by wireless from the AirMinistry, and already a host of machines were scouting in everydirection.
The stranger, flying due south, had reached Bradford before he wassignalled. Instantly there was a rush of aeroplanes from all parts ofthe Midlands to cut him off. But he slipped through the cordon, flyingvery high and at a tremendous speed. Outside Birmingham a fast scoutpicked him up and reported by wireless, and from the huge aerodrome atCheltenham over twenty fighting planes leaped into the air to stop thecareer of the marauder.
There was now no chance, at least, of his getting away unobserved. Hewas under constant observation, alike from the air and the ground, andevery moment wireless messages were pouring into the Air Ministryreporting his progress. But to catch him proved impossible. Only twoof 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 getout 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 machinewas taking, Yvette had steered a course to intercept him somewhere inthe neighbourhood of Bournemouth, and the Mohawk, with its wirelesschattering incessantly, was now swinging lazily at half speed in a bigcircle between Salisbury and the Hampshire watering-place.
"Over Salisbury now," called Yvette to Dick, her voice ringing outclearly above the muffled hum of the propeller, the only sound whichcame from the helicopter, with its beautifully silenced engines.
A few minutes later Dick pointed to the north. "Here he comes," heshouted.
Far away were three tiny specks in the sky.
Through his glasses Dick could make them out clearly enough. The leaderwas a machine of a type he had never seen before; a mile behind it werea couple of planes which he at once recognised as the Bristol fighterswhich had been so familiar to him in France.
The pace of the three machines was terrific. It was clear the Englishairmen were going all out in a desperate effort to catch the strangerbefore he reached the water, and they were expending every ounce ofenergy. But a moment or two later it was quite clear they were fallingbehind. Presently a puff of smoke from the leader signalled "petrolexhausted," and he dropped in a long slant to the ground.
The second machine, however, held on grimly, though slowly losingground. Evidently his predicament was the same as that of hiscolleague, and a moment later he, too, dipped earthward and was out ofthe fight.
Only the Mohawk stood between the stranger and safety!
But it was a Mohawk very different from the comparatively crude machineof a year before, wonderful though that was. Dick and Jules had workedout a revolutionary improvement in the lifting screws, with the resultthat a sma
ll supplementary engine, using comparatively little power, wasnow sufficient to keep the machine suspended in the air. As a resultthe full power of the big twin driving engines was now available forpropulsion, and the speed of the Mohawk, when pushed to the limit, wassomething of which Dick had hardly dreamed in his earlier days. So faras he knew the Mohawk was easily the fastest craft in existence.
But what of the stranger? Had the men of the mystery craft a stillgreater secret up their sleeve? That they had something big Dick couldplainly see by the way the fastest craft of the British Air Service, thebest in the world, had dropped astern of the stranger. Was the Mohawkfast enough to beat the pirate? They would soon know.
As the big machine came on, Yvette set the elevating propellers of theMohawk to work, and the helicopter shot upward. The stranger saw themanoeuvre and at once followed suit. But here he was at a disadvantage.Yvette's object, of course, was to