"Because delay is dangerous. There is always the danger that 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.