The young aviator left the Interstate plant very thoughtful and serious. Dave decided that he had assumed a big responsibility. He seemed to feel an actual ponderous weight on his young shoulders. A score of theories ran riot through his mind its to the motive for the theft of the Drifter. Then he decided that it must be some professional who had done the act. It was hard to fathom the ultimate plans of such an abstractor, who would not dare to use the machine in any public way and could scarcely sell it. "It's a puzzle, a big, worrying poser," said Dave, walking slowly from the factory grounds. About half a mile city-wards from the plant Dave passed through a square devoted to public park purposes. He sat down on a tree-shaded rustic bench. There, alone, quiet and undisturbed, he set his wits at work. Whoever it was who had committed the theft must have been a professional airman. Dave formulated a plan to ask Mr. Randolph if anybody in Bolton, or any employee of the plant was missing. In case this was not discovered then some stranger must have come to Bolton. There might be a trace found of the party at some of the hotels. "There's a bit of detective work to do by some one besides myself," decided Dave. "I'm going to suggest this plan to Mr. Randolph." "Hello, boss," spoke an approaching voice as Dave got up to return to the plant. He observed a man he had noticed on a bench directly opposite to the one he had occupied sidling towards him. The fellow was ragged and trampish looking. There was a queer leer in his face and his eyes were fixed on the coat Dave wore. "Well, what is it?" inquired Dave. "Excuse a question, matey?" "Oh, that's all right." "Noticed a badge you're wearing," said the tramp. "Oh, that?" spoke Dave lifting his hand to his coat lapel, and wondering at the man been so observant. "Yes--N. A. L.," nodded the tramp. Dave eyed the speaker keenly. At the distance he was, it was doubtful that he could have dearly made out the monogram, yet he named the letters glibly and correctly. "N. A. L." stood for the National Aero League. Dave was not a member and neither was Hiram Dobbs. Mr. King was and during the meets it had become the custom with professionals to furnish their assistants with duplicate badges, which enabled them to enter and leave the aero grounds unchallenged by the gateman, and ticket takers. "You must have pretty good eyes to make out those letters on that badge at a distance," said Dave. "I've seen them before," readily explained the tramp. "Oh, you have?" "Yes, and I've got a badge for sale just like the one you're wearing." CHAPTER XII DAVE'S DISCOVERIES "You have got a badge like mine for sale, you say?" exclaimed Dave. "That's so," bobbed the tramp with a grin. "Where did you get it?" "That don't go with the sale, but I didn't steal it." "You found it, I suppose?" suggested Dave.