this secret, so jealously guarded, and hidden away in the depth of the Black Forest, may be discovered. You know that Germany, under the Peace terms, is forbidden for the present to manufacture aircraft."

"Yes, yes; we know it only too well."

"Well, even now," continued von Spitzer, "the British Air Police have got wind of the thing, and their agents are in a dozen different parts of Germany trying to fathom the mystery of this phantom aeroplane, but so far they have not succeeded. All the same, it is time for us to get away, and that is why I have confided my plans to you to-day. Do you wish to withdraw?" and there was just a faint suspicion of a sneer in the tone of the speaker's voice, as he said this.

"Withdraw? Ach Himmel, no, a thousand times no! I am ready to start to-day," flashed back the ruffled Carl as he replied.

"Gut!" grunted von Spitzer. "Then you shall see this wonderful thing to-night at sunset; I dare not take you there before, and to-morrow, ach! to-morrow, this great adventure will begin."

CHAPTER II

THE WONDER 'PLANE

The sun was sinking amongst the pines of the Schwarzwald when the three airmen, after traversing for several miles the wild unbroken solitudes of that primeval forest, emerged at length from the dark shadows of the trees on to a little open glade, a natural clearing about two hundred metres in diameter.

"Here we are at last!" exclaimed the chief.

"Himmel! what a perfect little aerodrome," cried the scout pilot.

"But where is the hangar?" asked the more observant Max.

"Hist! Let us wait for the signal," ordered the Rittmeister, waving his companions back to the fringe of the forest.

"But there is not a soul to be seen anywhere," expostulated Carl. "No one ever comes here."

"We must be careful; there is too much at stake," whispered the flight-commander, and then he gave a long, low whistle, repeated twice.

Scarcely had the last sound died away, like the sad piping tone of the woodland robin, than a similar call came in response from the opposite side of the glade.

"Follow me; the way is clear," said the chief as he strode across the clearing towards the spot whence came the signal. And his companions followed him, silently wondering, for, somehow, they felt that they were treading on enchanted ground, and that some interesting dénouement would shortly take place.

As they neared the edge of the forest once more, a movement amongst the trees attracted their attention, and the next instant a solitary figure emerged from the shadows and greeted them. It was the keen, lynx-eyed professor, the great mathematician and engineer; a man about fifty, dressed in a loose working garb, wearing a battered felt hat above his shock of white, wavy hair.

"You are welcome, children of the Fatherland," he said, extending his hand, and fixing the two strangers with his piercing eyes, after this brief salutation.

"I hope we are not late," began von Spitzer, when the first salutation was over and he had introduced his companions.

"The sun is amongst the pines and the shadows of the Schwarzwald deepen," replied the professor, speaking in the language of the forest. "It was the time arranged, but"--and here he paused for a second--"there is no time for delay," and an uneasy look spread over his face.

"You don't mean that----" began the chief, but the genius forestalled him by adding:--

"Yes, strangers have crossed the clearing to-day. For the first time since I came here, I heard strange voices amongst the trees."

"But they found nothing?"

"Nothing!" ejaculated the professor.

"Good! Then my friends may view the aeroplane," said Spitzer.

"Certainly; let them follow me," and through an opening barely fifteen feet wide, the professor led the way to a combined hangar and workshop, carefully camouflaged and hidden away amongst the trees.

The next instant the two young airmen received the greatest surprise of their lives.

"Der Skorpion!" announced the professor.

"Donnerwetter!" came the involuntary cry from both the strangers as their eyes fell upon a new type of aeroplane, with an angry, waspish look about it, that the Bristol Fighter used to wear during the later days of the Great War. Yet it was not a Bristol Fighter by any means, for it was twin-engined, and steel-built throughout, with a central conning-tower, tapering off to a sharp point to improve the stream-line, and a closed-in be-cabined fuselage into which four or six persons might with ease be stowed away.

"But her engines!" exclaimed Max. "How small they are."

"But how powerful!" replied Spitzer. "Each one develops anything up to 400 horsepower."

"Is it possible?" asked Carl, who was already carefully examining the starboard engine, in its covered in and stream-lined casement.

"The propellers are different, too; they're something like the Fokker's, but shorter, and they have a peculiar twist, which I have never seen before. What is that for, Rittmeister?" asked the Gotha pilot.

"For vertical climbs, Max," replied the chief, for while the professor stood by, and looked on, interested and amused at the growing enthusiasm for his idol, the Rittmeister, who had been secretly schooled in the hidden mysteries, explained them point by point, for he was a great mechanic and mathematician was this ex-flight-commander.

"Vertical climbs?" echoed the other. "I thought it was impossible."

"Impossible? Rubbish! Nothing is impossible to the man of science. Have you never heard of the Helicopter?"

"You mean that hybrid mongrel the verdammt Yanks and the Britishers have been experimenting with of late, and which has caused so many accidents?"

"The same; only they went the wrong way about it. This propeller, with this driving power behind it, practically gives the vertical ascent, especially when once flying speed has been obtained."

"Blitz, but it is wonderful!" concluded Max, his enthusiasm growing by leaps and bounds, as he continued his examination.

"Why, the propellers are made of steel, and so are the planes," exclaimed Carl, who was now carefully examining the material of which the aeroplane was made.

"Steel, tempered steel, every bit of it--fuselage, propellers, tail fin, rudders. There's not an ounce of wood about the Scorpion," returned the mentor.

"Then the danger of fire is lessened," ventured Max, whose one dread in the air had always been that of fire.

"That danger is eliminated," replied the chief, in a tone of certitude.

"Except by