gentlemen--and I've had nothing but newspaper since eight o'clock. By the way, you may as well take the only clue we have--this scrap of pulp. It is French, as you see. And, Teddy, don't get into hot water on my account. The resources of civilisation--as expressed in high explosives--are not exhausted."

Burton stuffed the newspaper into his pocket, and in three minutes was already well on the way to Folkestone. Micklewright watched the flying-boat until it was lost to sight; then, pressing his hand to his aching side, he returned slowly to the house.

The distance from the Luddenham Marshes to Folkestone is about twenty-five miles as the crow flies, and Burton had made the flight once in his flying-boat. Consequently, he was at no loss in setting his course. A brisk south-west wind was blowing, but it very little retarded his speed, so that he felt pretty sure of reaching the harbour by four o'clock. Keeping at an altitude of only a few hundred feet, he was able to pick up the well-known landmarks: Hogben's Hill, the Stour, the series of woods lying between that river and the Elham valley railway line; and just before four he alighted on the sea leeward of the pier, within a few yards of the steamer.

A small boat took him ashore. He avoided the crowd of holiday makers who had already gathered to watch him, and making straight for the pier, accosted a police inspector.

"Have you seen three men ride up on motor cycles, inspector?" he asked.

"No, sir, I can't say I have."

"Three foreigners, one a tall big fellow?"

"Plenty of foreigners have gone on board, sir. Is anything wrong?"

"Yes, they've assaulted and robbed a friend of mine--you may know his name: Dr. Bertram Micklewright, the inventor. They've stolen Government property, and it's of the utmost importance to prevent their crossing the Channel."

"Where did this take place, sir, and at what time?"

"At Luddenham Marshes beyond Faversham, just before three o'clock."

"They'd hardly have got here, would they? They'd have to come through Canterbury, between thirty and forty miles, and with speed limits here and there they'd only just about do it."

"I'll wait here, then. You'll arrest them if they come?"

"That's a bit irregular, sir," said the inspector, rubbing his chin. "You saw them do the job?"

"Well, no, I didn't."

"Then you can't be sure of 'em?"

"I'm afraid I can't, but there wouldn't be two sets of foreigners on motor cycles. You could detain them on suspicion, couldn't you?"

"I might, if you would take the responsibility."

"Willingly. I'll keep a look-out then."

It occurred to Burton that the men might leave the cycles and approach on foot, so he closely scrutinised all the passengers of foreign appearance who passed on the way to the boat. None of them answered to Micklewright's description.

"Haven't you got any clue to their identity, sir?" asked the inspector, who remained at his side.

"None; it happened during my absence. They tied up my friend and gagged him. I came across country in my flying machine yonder."

"They'll lose this boat for certain," said the inspector, as the steamer's warning siren sounded. "You're sure they are Frenchmen?"

"Yes; well, they left a French newspaper behind them."

"Do you happen to have it with you?"

Burton drew the crushed paper from his pocket, and handed it to the policeman, who unfolded it, and displayed a torn sheet, with only the letters IND remaining of the title.

"That's the Indépendance Belge," said the inspector at once. "I expect they're Belgians, and aren't coming here at all. Ostend's their mark, I wouldn't mind betting."

"Via Dover, of course. Is there a boat?"

"One at 4.30, sir. I'm afraid they've dished you."

"I'm not so sure about that," said Burton, glancing at his watch. "It's now 4.20; this boat's off. If the Ostend boat is ten minutes late too I can get to Dover in good time to have it searched."

"Then if I were you I'd lose no time, sir, and I hope you'll catch 'em."

Burton raced back to the boat that had brought him ashore. In five minutes he was on his own vessel, in two more he was in full flight before the favouring wind, and at 4.35 he dropped on the water in the lee of the Admiralty pier at Dover. But he had already seen that he was too late. The boat, which had evidently started on time, was at least half a mile from the pier.

"Yes, sir, I did see a big foreigner go on board at the last minute," said the policeman of whom Burton inquired ten minutes later. "He was carrying a small brown leather hand-bag. I took particular note of him, because he blowed like a grampus, and took off his hat to wipe his head, he was that hot."

"Was he bald?"

"As bald as the palm of your hand. A friend of yours, sir?"

"No," said Burton emphatically. "He's got away with a secret worth thousands of pounds--millions perhaps, to a foreign navy."

The policeman whistled.

IV

Burton stood looking at the diminishing form of the steamboat. The constable touched his sleeve.

"You see that gentleman there, sir?" he said.

Following his glance, Burton saw a slim youthful figure, clad in a light tweed suit and a soft hat, leaning over the rail.

"Well?" he asked.

The constable murmured a name honoured at Scotland Yard.

"Put the case to him, sir," he added; "he can see through most brick walls." Burton hastened to the side of the detective.

"A man on that boat has stolen the secret of the new explosive for the British Navy," he said without preamble. "Can you stop him?"

The detective turned his keen eyes on his questioner and looked hard at him for a moment or two.

"Tell me all about it, sir," he said.

Burton hurriedly related all that had happened. "A cable to Ostend would be enough, wouldn't it?" he asked in conclusion.

"I'm afraid it would hardly do, sir," replied the detective. "Your description is too vague. Tall man about forty, bald, with a hand-bag--there may be dozens on the boat. It would be too risky. We have to be careful. I saw a notorious diamond thief go on board, but I couldn't arrest him, not having a warrant, and nothing certain to go upon. You had better go to the police station, tell the superintendent all you know, and leave him to communicate with the Belgian police in due course."

"And give the thief time to get rid of the stuff! If it once passes from his hands the secret will be lost to us, and any foreign