They listened eagerly while d.i.c.k told his story.

"And now," said d.i.c.k, half ruefully, as he concluded, "I don't know whether we are on the track of something or whether I have been an idiot."

Yvette's eyes were dancing with merriment.

"Well, d.i.c.k," she said, "you are certainly a pretty Englishman not to know one of the most famous places in your own country. Don't you really know Chalkley?"

"No," replied d.i.c.k in bewilderment. "What do you know about it?"

For answer Yvette rummaged among a pile of newspapers and produced a copy of the "Times" dated a week before.

"There?" she said. "Read that."

"That" was a closely printed column which d.i.c.k proceeded to scan with attention. It was an article describing the wonderful deposits of pitchblende, the ore from which radium is extracted, which had been discovered in the Ural region in the neighbourhood of Zlatoust. An English combine had secured the monopoly of the working for fifteen years, and already a supply of radium valued at one hundred and fifty thousand pounds had been brought home by the famous Professor Fortescue for the use of British chemists and medical men.

The discovery and acquisition of the monopoly by British interests, the article pointed out, had put England far ahead in the field of radium research, for she had now a big supply of the precious commodity at her disposal, while other nations were struggling along with the tiny quant.i.ties obtained from other and far less rich deposits. And, as was fully explained, it was not in medicine alone that the radium would be valuable; there was hardly a department of commerce, to say nothing of the arts of warfare, in which radium was not playing a considerable and constantly increasing part. So many new discoveries were being made by the band of experts, of whom Professor Fortescue was the acknowledged head, that it was beginning to be realised that radium in the future was likely to be as valuable as coal and oil had been in the past.

But--and here was the fact of most significance to d.i.c.k--the radium was at Chalkley, Professor Fortescue's home in the wilds of the Durham moors. He had taken it there on his return from Zlatoust for use in some critical experiments he had in hand before it was sent on to the young but growing school of Medicine at Durham University.

They had at least approached the heart of the mystery! It was evident that some band of international desperadoes had designs on the precious radium. In spite of their enormous value, the two tubes containing the salt could easily be carried in a man's pocket, and in Germany there would be a ready market for it among the great chemical firms, whose business consciences were sufficiently elastic to permit them to pay a big price and ask no awkward questions.

d.i.c.k was reading the report carefully, when he suddenly gave a startled exclamation.

"Why, look here," he said, "the radium is only to be kept at Chalkley till the twenty-ninth. That explains the twenty-nine in the advertis.e.m.e.nt. And to-day is the twenty-seventh. If anything is to happen it must be at once or they will be too late. I must ring up Regnier." Regnier was with them in half an hour. He was filled with excitement when he learned the facts which Yvette had discovered.

"That," he said, "puts an entirely new complexion on the affair. There can now be very little doubt about the matter. Clearly `lead' means radium, and I think we can interpret `bull market' as an intimation that it is a big prize. They are evidently well informed, whoever they are.

We must tell London at once."

But before anything could be done a messenger for Regnier arrived post haste from the bureau of the Secret Service in the Quai d'Orsay with strange news.

A big aeroplane, flying at a tremendous speed, had crossed the Franco-Spanish frontier near Bagneres de Luchon having apparently come right across the Pyrenees. It had ignored all the signals of the French frontier guards, whose aeroplanes had, in consequence, gone up in pursuit. Only one of them was fast enough to approach the stranger, and a fight had followed in which the French machine was crippled and forced to descend. Thereupon the strange machine had proceeded, flying in the direction of Bordeaux. Telephone messages had brought warning of its approach, and several attempts had been made to stop it, but without success. It had been reported, chased by French aeroplanes over Bordeaux, Nantes, and St Malo, and at the latter place, just as dusk was falling, it had left the French coast and laid a course apparently for England. No further news of it had been received.

Regnier looked grave.

"Of course," he said, "we have absolutely no reason to couple this machine with the advertis.e.m.e.nt in the `Diario,' but I confess I am uneasy. There is at Chalkley radium worth a fortune, easily carried if anyone can get hold of it, and readily convertible into cash. What better device could be employed than a fast aeroplane which could get to Durham and away before anyone could hope to stop it? In any case, I am going to telephone Scotland Yard at once."

Half an hour later he was in communication with Inspector c.u.mmings, the senior officer on duty at the Yard. To him he explained his suspicions, half afraid, with the Frenchman's dread of ridicule, that the other would laugh at his story as an old woman's tale.

But Inspector c.u.mmings was too experienced to be neglectful or sceptical of anything which could disturb Regnier, whom he well knew to be one of the most astute and level-headed of men. He took the matter seriously enough.

"We have heard nothing yet," he said. "But I will 'phone Durham at once and let you know in the course of an hour."

They waited anxiously for the reply. It came at last.

"c.u.mmings speaking," said the voice on the 'phone. "I have spoken to Durham. They have heard nothing there, but they are unable to obtain any reply from Professor Fortescue. The telephone exchange reports his line out of order.

"But here is a queer thing. A big aeroplane, evidently a foreigner, was reported this morning to have been seen over the Midlands flying north.

There was a lot of mist about, and we have not been able to trace the machine yet. But it was certainly not one of ours."

"Well," said Regnier, "will you keep me posted? I fancy you will have more news before long. In any case, you will have Durham warned?"

"I have warned them myself," replied c.u.mmings, "and they are sending a couple of men out in a motor to make inquiries. You know Chalkley is about twenty-five miles from Durham and quite in the wilds. Professor Fortescue was, a couple of years ago, carrying out some experiments in which it was absolutely necessary he should be away from anything like traffic vibrations, and he chose this place for the purpose because it was remote from any railway or heavy traffic. He has stayed there ever since; he said it suited him to be `out of the world,' as he called it."

Three hours later came still more startling news.

The police officers who had gone from Durham to Chalkley had found that two armed men had made a raid on Professor Fortescue's house. They had gagged the servants, who were found lying bound and helpless, and the Professor himself was found lying unconscious in his laboratory, having apparently been sandbagged. The raiders had leisurely helped themselves to food, and, having cut the telephone wires, had departed without any particular haste.

But the great leaden safe, weighing several hundredweights, in which the precious radium had been brought to England, was found to have been broken open. _The radium was gone_!

Nothing in the meantime had been heard of the strange aeroplane. But a few hours later an old shepherd walked into one of the local police-stations and told a queer story.

His sheep the previous evening, he reported, had been disturbed by the pa.s.sing of an aeroplane which, flying very low, had landed on the moors a few miles away from the Professor's house. It had stayed there all night and, so far as he knew, was still there. He had been unable to approach it closely as it was separated from where he had been by a deep gorge and a stream which he could not cross without making a detour of several miles. He had seen two men near the machine who had walked away and disappeared in the folds of the moor.

A strong party of police, c.u.mmings added, had left at once for the spot where the aeroplane had been seen, taking the shepherd with them as guide. The place was remote from any road, and it would be an hour or two before they could get there. But the Air Ministry had been warned, and already aeroplanes were going up in the hope of locating the strange machine.

"I must be in this," said d.i.c.k. "Ask him if I can come over. I cannot, of course, go unasked."

"Of course," said c.u.mmings in reply to Regnier's request. "We shall be only too glad to have Mr Manton. Miss Pasquet can come too, if she likes. But I'm afraid he won't be able to get here in time. We shall either have got these fellows or lost them hopelessly in a few hours."

d.i.c.k turned to Jules.

"Ring up the British Air Ministry," he said, "and ask them if the strange machine gets off the ground to send us every movement as it is reported. Keep the telephone on all the time. I am going to try to cut these chaps off with the Mohawk. You will have to report to me by wireless every movement as it comes through. From what we have heard I fancy there are very few machines in England fast enough to catch those fellows if they once get started. Of course you will come, Yvette?"

An hour later, d.i.c.k and Yvette, seated in the helicopter, were in full flight for England. Yvette was at the controls; d.i.c.k, in view of the work that might be before them, crouched over the tiny machine gun which peered from the bow of the machine.

Professor Fortescue was in a terrible state of distress. He had been working in his laboratory, when a slight noise had caused him to turn round. A man, apparently a foreigner as the Professor judged from the hasty glance he got at him, was standing close behind him. Before the Professor could speak or move he received a violent blow on the head, and remembered nothing more till he recovered consciousness some time later under the care of the police.

His chief concern was for the radium, and his distress at its loss was pitiful. It was a disaster from which he seemed unable to recover. But he appeared to derive a strange satisfaction from the danger in which the thieves would find themselves.

"I don't know how they will get it away," he declared to the police inspector. "It was dangerous to stay very near the safe for long owing to the terrible power of the radium rays. If the thieves try to carry the tubes in their pocket they will not get very far. Surely they cannot realise the terrible risk they are running. However, that need not distress us; all we want is to get the radium back."

In the meantime a strong party of police had arrived from Durham at the Professor's house, and, under the guidance of the old shepherd, started across the moors for the spot at which the strange aeroplane had been seen. It was slow going over rough and difficult ground which tested the endurance even of the younger men. The only unconcerned person was the old shepherd who trudged stolidly on at a pace with which they found it 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 few hundred yards away. "You will see it as soon as you get at the top."

Breasting the rise, the police cautiously approached the ridge and glanced 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 man with a revolver ready in his hand.

But they were just too late. They had only gone a few yards when the men hastily took their places in the machine, there was a loud whirr as the engine broke into action, and while the policemen were still a hundred yards away, the strange machine rose into the air and was gone.