one evening. He was anxious and worried, for something had gone seriously wrong with his plans.

It was his duty to investigate and track down all aerial criminals, whether brigands, smugglers or revolutionists of the Bolshevist type. For this purpose he had been appointed by the Government to the command of the British Aerial Police, whose functions included the patrolling of the routes of the great aerial liners throughout the British Isles, and the All-Red route to Egypt, India, and other British possessions, and the careful guarding and watching of the aerial gateways and ports.

Some of the best scout pilots of the war, including two famous secret service men, named Keane and Sharpe, were detailed to assist him in this important and ever-increasing task, for aerial crime of twenty different kinds was becoming more and more prevalent since the war.

So far his efforts had been conspicuously successful, and he had brought many of the offenders to justice, but at the present moment he had to confess himself baffled--utterly baffled by a series of unfortunate occurrences which it had been beyond his power to prevent.

"There is some master-mind behind all this," he exclaimed to himself, rising suddenly from his chair, and beginning to pace the room, much in the same way that he used to pace his squadron office, in the old days, when, as commander of a squadron of scouts during the Great War, he had attempted to outwit the daring of the German airmen.

"I wonder now--I wonder what happened to that missing German professor!" and Colonel Tempest suddenly halted, and placed his left hand to his forehead, as some powerful, new idea had arrested his mental faculties.

Then, walking across the room swiftly, he switched on a shaded light which illuminated a large map of Germany, showing the aerial routes, the lines of occupation by the Allies, etc.

"It is just possible," he murmured to himself, "that the two things are connected--the disappearance of this eminent scientist and the appearance of this extraordinary flying machine." Then he switched off the light, and returned to the sheaf of papers and documents on his desk. He sorted out one and placed it on top; it was a decoded message, received some days ago from one of his agents at Constantinople. It ran as follows:--

"Mysterious aeroplane, phantom-like in appearance, passed over here yesterday flying at terrific speed. All our signals disregarded. No navigation lights showing. Our fast scouts gave chase but left hopelessly behind. Came from direction of Adrianople, crossed the Bosphorus, and disappeared rapidly flying south-east. Time shortly after sunset.

AERIAL, CONSTANTINOPLE."

"That is three days ago," continued the Colonel, still thinking aloud, "and here are four similar messages from other sources showing quite plainly the route taken. Great Heavens! if I were not tied to my desk in this place, I would take the fastest scout in the country and chase this infernal night-wizard myself."

A soft tap at the door startled the Commissioner, for during the last three days he had become highly nervous; this affair was getting on his mind, but he recovered himself instantly and called out in a deep voice:--

"Come in!"

The door opened softly and his confidential secretary entered, and announced:--

"Two more cables and a wireless message, sir."

"Anything from Keane or Sharpe yet?" demanded the chief.

"Nothing, sir."

"Then what are these confounded things?"

"More about that aerial brigand, sir."

"Let me see them," and Jones handed the messages to his chief.

Consternation and alarm were both visible on the face of Tempest as he read the news.

"So the devil has already got to work, Jones," he remarked, quoting from the sheets, laconic phrases such as "Oil tanks at Port Said burning for three days. Crew of mysterious aeroplane suspected." (Delayed in transit.) "Wireless station at Karachi utterly destroyed, after brief visit by strange airmen." The third was a wireless message which proved most disconcerting of all to the Commissioner. It announced that a silent aeroplane, showing no distinctive marks whatever, passed over Delhi "this afternoon" at a speed estimated at not less than three hundred miles an hour.

The chief of the aerial police leaned back in his chair and groaned.

"Three hundred miles an hour!" he gasped; "but the silent aeroplane idea is a fallacy. It is impossible with any type of internal-combustion engine. It must either have been too high up for the good people of Delhi to hear it, or its engines must have been shut off, or well throttled down. Bah! I know too much about aeroplanes to swallow that." Then rounding upon Jones, who was standing by awaiting instructions, he said sharply:--

"Did that second message go out to Keane?"

"Yes, sir."

"And there's still no reply from him?"

"Nothing whatever, sir."

"H'm. I cannot understand it. Send it out again by wireless telephone; he may be on his way back by aeroplane now, and possibly within reach."

"Right, sir," and Jones disappeared to stab the ether waves again in search of Keane. At that moment the telephone bell on the Commissioner's desk rang. It was the Home Secretary asking for Colonel Tempest, for the same messages concerning the aerial brigand had reached him.

"Hello, Tempest; is that you?"

"Yes; who is that?"

"Lord Hamilton, speaking from the Home Office."

"Oh, yes, my lord."

"I say, Tempest, what is this news just to hand about aerial highwaymen romping half round the British empire, destroying wireless stations, and burning out the big oil tanks along the All-Red Route? I thought you had all these aerial criminals well in hand. There'll be a deuce of a row about all this when Parliament meets in two days' time."

"Well, er--we're doing our best to deal with it, sir, but it will take time to lay these fellows by the heel, I fear."

"Have you got the matter in hand?"

"Yes, sir."

"What have you done? I shall be bombarded with questions shortly; in fact, the Colonial Secretary's here now. He's complaining that the routes are not sufficiently well patrolled. What steps have you taken to deal with these marauders?"

"I've wirelessed to all the aerial stations, to get their fastest scouts out all along the line at once to look for these bandits, and I'm staying on here all night expecting news every moment."

"Very well. Keep me informed of everything that happens. It's becoming very serious. You have full powers to deal effectively with these criminals, and they may be shot down at sight if they don't respond to signals."

Then, as the angry minister rang off, another tap was heard at the