“Ah, ha!” exclaimed George Harper, leaning over Tom’s shoulder, and examining the woodwork above the sergeant’s head. “I guess this explains it. Looks like some sort of a secret door.” He had been tracing the outline of a scarcely perceptible crack which ran from the floor to a height of about three feet, then across for two more feet, where it joined another vertical one which paralleled the first. Also he had found what anyone might have taken for a knot-hole at first glance, but which closer examination showed to have been made, apparently for the purpose of pulling the thing open. “Try it again,” Tom suggested, as George, with one finger crooked into the aperture, gave it a tug without any seeming result. “Try it again; I thought it moved a little.” With considerable effort Harper managed to twist two fingers of his right hand into the hole, and again gave a sudden jerk; but the only result was a slight vibration, while George vigorously rubbed his two pained digits. “Wait a minute,” Ollie suggested, and going across the shed he picked up a piece of iron shaped at the end like a stove poker. “Try this,” he suggested. “Let somebody else do it,” said Harper. “My hand feels as though it was broken.” Tom took the iron, fixed the end into the hole and gave a mighty yank. As the secret door gave way under his weight, Tom suddenly and unceremoniously and without advance preparation took a seat on the floor at the far side of the shed. At any other time his fall might have caused some merriment, but no one paid attention to it now. They were too busy examining a little apparatus in the closet-like aperture in the wall of the shed, which now stood exposed to full view. The mechanism resembled, in fact was almost a replica, of the dial on the front of a combination safe. The only difference was that this disc was marked only with an S at the top, an O at the bottom, and with a line running part way around it, near the outer edge, with an arrow at the bottom end, showing that the small lever over the dial was always moved from the top downward to the right, and then upward on the other side. “S and O,” Tom read off, gazing at the queer thing as he joined the others. “And it points to S now. Why, that seems to be clear. It means ‘shut’ and ‘open.’” “If it’s connected up with any signalling apparatus a complete revolution of that lever also might sound an S.O.S.,” suggested Buck Granger. “Or it might mean ‘slip out’ while the slippin’s good,” put in Ollie. “Oh, it might mean any one of a thousand things, such as ‘stop orating’” George Harper spoke up, impatiently, “but I’m inclined to believe Tom’s right. Let’s try it, anyway.” Tom was just reaching for the small lever when another suppressed moan, unquestionably from directly beneath their feet, arrested his hand.