They had continued the search for fully ten minutes, and the professor, occasionally flashing his pocket torch, was carefully examining the long grass within a radius of some twelve of fifteen feet of the spot where the fire had been. Wise man that he was, he carried out his final investigation to the leeward of the fire, trusting that the breeze might have carried some paper fragment, used in lighting a pipe or starting the fire, in that direction. Nor was he disappointed. He was just about to conclude his search, however, when his sharp eyes caught sight of a piece of half burnt and twisted paper hidden away amongst the longer grass. "Donnerwetter!" he exclaimed under his breath, as he flashed his torch upon the paper for a second. "I thought so; here is evidence enough for an execution." "What is it, mein herr?" asked the mechanic, hastening to his side. "Do you see that?" said his companion, untwisting the paper once again and flashing a light upon it. "Ja! ja!" replied the other as he strained his eyes in the attempt to decipher the handwriting on the half-burnt sheet. "But I cannot understand it, for it is in a foreign language." "It is part of a small fragment of an envelope, and the writing, which is in English, is certainly almost undecipherable, but I can distinguish the letters ’...eane’." "Ach, Himmel! That is Keane!" replied Strauss. "He is one of the aerial police, is he not?" "You are right, Fritz. This letter was addressed through the English post to Captain Keane, one of Tempest’s best men, if not indeed his most brilliant ’brain-wave,’" hissed the professor. "Donner und blitzen! Then he has come here to search for the _Scorpion_, and the aerodrome." "Yes, but look, he only left London a few hours ago, for here is the London postmark in the corner, bearing yesterday’s date." "And his companion? Who is he?" asked the mechanic. "It must be that other scout pilot, Sharpe; they work together. But, mark my word, Friedrich Strauss, they are mistaken if they think to find an easy victim in Professor Rudolf Weissmann. I’ll teach them to track me like a murderer through the Schwarzwald. They have come to the Black Forest, and here they shall stay." And for once, the quiet, mild-mannered professor jerked out his words with unusual vehemence. The mechanic saw that his chief was deeply agitated by this sudden discovery, which confirmed all his recent fears, and to allay his feelings, he said, "But they will never find the aerodrome, Professor, or, if indeed they find it, they will never enter it alive; think of the preparations you have made for all uninvited guests," and the speaker shuddered, for he knew something of the terrors of that "death-circle" in the lonely forest. "Bah! it is my secret they want, the secret of that mysterious power which drives the _Scorpion_." "Uranis?" ventured the other.