A few moments after he had passed the home of a youth he knew, he heard a familiar salutation, and turned around to wave his hand in a greeting to this friend, who had come to the front door. As he turned, his eye fell on a slender figure some distance behind, a figure which stepped behind a tree and stopped. "Humph! that's funny," mused Bob. "It looks a lot like that fellow who got out of the taxi back there by our house; I wonder what he's up to, anyhow?" He continued his way, but as he reached the fair-grounds gate and got out his key to unlock it, the whim to look back again seized him. As he turned, his gaze once more rested on the slender form of the wayfarer, who had crossed to the opposite side of the road, and who now, finding himself observed once more, promptly stopped and began to fuss with his shoe-lace. "Say now, this is funny!" ejaculated Bob under his breath, vainly trying again to recall the identity of the lean figure and dark complexion. "I believe that chap is trying to shadow me. I wonder what in the dickens he really is up to?" It was the second time Bob had asked that question of himself, but as he was a poor source of information just then, he was forced to pass into the fair-grounds and relock the gate in as mystified a state of mind as before he put the query. A little later, when he reached the big hangar he whirled about again, as if half expecting to see the stranger still skulking behind him in the grounds. To his relief he did not detect this situation exactly, but he did see a dark face, which had been peering over the top of the highboard fence near the gate, drop down from view on the other side. Bob gave a grunt as he passed into the hangar and took off his coat. "As I live, I believe he's up to some sort of mischief," growled the boy. And when, shortly afterward, John and Paul Ross appeared he told of his experience and repeated his suspicions. "That is funny," asserted John; "Paul and I saw nothing of any such man when we came along, and we passed down the same road. Perhaps he mistook you for somebody else." "I hope so, but I don't like his actions a little bit," declared Bob stoutly. With that he picked up a try-square and pencil and began laying out some work for Paul to cut on the circular saw, while John busied himself at the boring-machine in putting a hole through the center of the big twelve-foot balsa-wood propeller which a little later would be reinforced with a thin jacket of a new metal called "salinamum," which was made chiefly from salt but whose fused components made it as light as aluminum and stronger than tool steel.