It was the Premier's turn to be surprised.

"Why--who is he?" he asked. "Professor Rudford had never even heard his name and laughed when I suggested that he could have had anything to do with it."

"He won't laugh when he gets some real idea of Gronvold's ability," said Scott bitterly. "The man is one of the mysteries of the world of crime," he went on. "Exactly who he is we don't know--I mean we know little about his life. But we believe he is Norwegian born, though he has strong Russian characteristics. We know he studied at Leipzig.

Tutors who knew him well speak with the utmost admiration of his amazing brain power as a student and the daring of his conceptions. But for some reason he never did well in examinations and attracted no attention whatever outside a very limited circle. Personally, I believe that for some strange reason he deliberately elected not to call attention to himself, for there is not the slightest doubt that he could with ease have captured every honour the University had to bestow. After leaving Leipzig he disappeared for some years. I don't know how he spent them.

But I do know that he is a chemist of amazing ability. He has, moreover, been mixed up with a number of puzzling international crimes, though we have never been able to bring any of them home to him. Do you remember the big bank robbery at Liverpool three years ago?"

The Premier nodded.

"You mean," he said, "when the bank vaults were blown open with dynamite and half a million in gold stolen?"

"That's the case," said Scott. "Only it wasn't dynamite, there was no explosion. The thick steel and stone walls of the vaulted safe had been melted through as if they had been b.u.t.ter. The story of an explosion was deliberately given out to deceive the thieves. But the fact is that some process was used of which we have no knowledge whatever."

And he paused, then went on:

"Now I am pretty sure Gronvold was in that. I was called in before anything had been touched. And in one corner I picked up a sc.r.a.p of paper bearing some queer formulae of which I could make nothing. It had evidently been dropped by accident. And it bore Gronvold's name.

Moreover, as I ascertained by a visit to Leipzig, where I saw some of the old University registers, it was in his handwriting. But where he is, how he got into England, how the burglary was effected and how he got away with such an enormous weight of gold we never could make out.

If he is really in this new discovery we are face to face with a terrible problem. The man is absolutely without scruple, and for three years he has had the use of half a million of money for his experiments.

He may have done anything in that time."

"But how did you know of him?" asked the Premier.

"It's a queer story," replied the other. "Simmons, one of our men in Christiansand came across, quite by accident, a drunken Norwegian sailor who told a strange story of the blowing up of a mountain by a tiny cartridge placed at the bottom of an old mine shaft. He actually mentioned Gronvold's name, and claimed to have been one of his a.s.sistants. When he became sober he was evidently terribly alarmed at having talked, and denied the whole story. The same day he disappeared, and Simmons has been unable to trace him."

He went on after a pause:

"Now the blowing up of a mountain is a fact. A hill nearly a thousand feet high in a wild lonely district north-east of Tonstad has absolutely disappeared--levelled out. To have done the work by ordinary means would have meant years of labour and would have cost a fortune. There can be no doubt that some entirely new force has been employed.

Officially the occurrence is attributed to a landslide; actually it is and can be nothing of the kind. Now this, coupled with what the Norwegian sailor said, suggests that we ought to look into the matter.

Whether the Norwegian Government knows anything about it I do not know, and the matter would be of such importance from the international point of view that we cannot make direct inquiries."

"Will you take it in hand?" asked the Premier. "Whom will you get to help you? I am afraid the ordinary men would be of very little use."

"I think I will run over to Paris and see Regnier," replied Scott. "He has a fellow named Manton who will certainly be useful. He was in our flying corps and was invalided out owing to wounds. He has done some wonderful work and has an entirely new type of aeroplane which he invented and which, by the way, our people would have nothing to do with. Regnier swears by him. He works always with a French girl named Yvette Pasquet, who did some splendid intelligence work during the war, and her brother Jules. They will have nothing to do with anyone else when they are on a case, and they have had some amazing results."

Crossing to Paris by the afternoon air express Scott the same evening was warmly greeted by Regnier. He rapidly explained his visit. Regnier looked grave.

"I have heard of the man," he said, "but have never seen him, I don't think in a case like this you can do better than Manton. He is very well up in all these scientific things; they seem to be a perfect craze with him."

An hour later, Regnier, Scott, d.i.c.k Manton, Yvette, and Jules were closely discussing the problem in Manton's rooms.

"We have got to find that sailor," was d.i.c.k's verdict, "and luck is going to have a good deal to do with it. I suppose Simmons is on the look out for him?"

"Yes," replied Scott, "I wired him at once."

"Do you think Gronvold and the sailor have quarrelled?" put in Yvette.

"I think not," was Scott's reply. "If they had there seems no reason for the man's alarm. I think he calculated on going back to him. That was Simmons' view, too."

d.i.c.k, who had been carefully studying a map, looked up.

"Just look here," he said, "you could hide an army in this place."

The map was in contour and gave a vivid impression of the wild and desolate country, a broken ma.s.s of hills and lakes, stretching north and east from Tonstad.

"Suppose Gronvold is there," said d.i.c.k, "he could hide anything he wanted to. I don't think he would have travelled far from its base to blow up the hill--that was probably experimental. My idea is that he has established his laboratory somewhere in the hills about there.

There is no population and little or no traffic through the district.

He must send to one of the towns for supplies, and Christiansand is the most likely. I should guess that the sailor had come there for that purpose and may come again."

"He did not leave the town by boat," declared Scott. "Simmons made the most careful inquiries on all the boats in the harbour and no one of his description was seen."

Three tourists a week later were lodged in a comfortable hotel in the Dronningens Gade, one of the princ.i.p.al streets in the busy port of Christiansand. They were Yvette, Jules, and Scott. d.i.c.k had flown the Mohawk direct to the wild district north-east of Tonstad, and with the help of a light tent had pitched a camp in a little wood a couple of miles from the southern edge of the blown-up hill. He had taken pains in the selection of a suitable place and his camp and the Mohawk were so admirably hidden that they were safe from discovery, unless some one actually walked right up to them, a contingency which in that roadless, unpopulated country was extremely unlikely. But though hidden himself he commanded a wide view.

For two days d.i.c.k devoted himself to a thorough examination of the surrounding country, quartering it thoroughly either on foot or in the Mohawk. He could however see nothing in the least suspicious.

Then came a surprise.

His only method of receiving news from the others was to "listen in" on the wireless telegraph set with which the Mohawk was fitted for messages which, directed to an address in England, were handed to the Christiansand radio station for dispatch, but were really intended for him. These messages were handed in at eight o'clock precisely and d.i.c.k usually got them within half an hour.

On the third day of his watch came the message:

"Sailor located. Travelling north with pack mules. We follow.

Osterluis road."

The man, as he was to learn later, had been spotted by Yvette in Christiansand. She had seen him leave a small cafe much frequented by sailors, and had been struck by his likeness to the description given by Simmons. She had followed him for some time while he made a variety of purchases at numerous shops, and had been struck by the fact that a mere sailor should evidently have such a large sum of money at his disposal.

Luckily she had encountered Simmons, who at once recognised the man and had promptly disappeared to avoid arousing his suspicions.

Yvette was able to learn that all the man's purchases were being delivered to a small inn on the outskirts of the town, and a few inquiries showed that he had four mules stationed there.

The matter began now to clear up. They were sure of the man; at least he could not leave without his mules and stores. Jules and Scott took up the watch at the inn, while Yvette shadowed the suspect. It was thought best that Simmons should not appear. It soon became evident that the man had no a.s.sociates in Christiansand. All he did was to visit shops, paying cash for all his purchases and having them sent to the inn where his mules were stabled.

The next day, with his mules heavily loaded, he set out from Christiansand, taking the road to Trygstand and Ostersluis.

Yvette, Jules, and Scott decided to follow him on foot. To have taken horses would have told him he was being followed as soon as he left the road, as they were pretty sure he would, sooner or later. Luckily all three were splendid walkers and felt they would have no trouble in keeping up with the heavily-laden mules. Cramming a few necessities into rucksacks they were soon on the track of their quarry.

Man and mules made steady progress. They were soon through Trygstand and, shortly after, caught sight of the Mohawk high above them and evidently following the road on the watch for them.

With a handkerchief tied to a stick Yvette swiftly signalled to d.i.c.k the brief facts, and the Mohawk pa.s.sed on towards Christiansand. When the sailor and the mules were hidden in a dip in the road d.i.c.k landed, and all four held a brief consultation as to their future plans.

As a result Scott put on his best speed and soon pa.s.sed the sailor who had stopped for a rest. The man was now between two parties on the ground and under observation from d.i.c.k from the air. He certainly could not escape.

A few miles beyond Trygstand he suddenly left the high road, and turned westward and north across the open country. Evidently he was not bound for Ostersluis. But where could he be going? For miles there was not even a house in the deserted track of country into which he had plunged.

But it was evident he knew his bearings thoroughly. Hour after hour he jogged along, and soon the pursuers realised that they had been wise not to bring horses. No horse could have crossed the country over which the sure-footed mules went swiftly without a stagger.

At nightfall the man camped. Apparently he paid no attention to the pa.s.sing of the aeroplane, for he barely glanced at it. Building a small fire under the shelter of a rock, the three pursuers spent a comfortless night. d.i.c.k had flown to his camp, intending to pick the party up again at dawn.

Early next morning the man was afoot and continued his journey. He was now in the wild country well to the west of Ostersluis, and travelling due north. Yvette, Jules, and Scott were a mile behind, following with the utmost care not to reveal their presence and so rouse the man's suspicions.

They had gone but a few miles when the man paused on the flat top of a high hill, which on the side away from them sloped steeply into a deep gorge at the foot of which ran a small stream. They watched him narrowly.