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.