"This," said the professor, directing attention to the lever, "is the lever which controls the valves of the main engines. I have fashioned and arranged it exactly like the corresponding lever in a locomotive. Placed vertically, thus, the engines remain motionless. Thrown forward, thus, the engines will turn ahead. And thrown backward, thus, they will turn astern. That is simple enough. And so is this," directing attention to a dial on his left hand which stood facing him. The dial had a single hand which was obviously intended to travel over a carefully graduated arc of ninety degrees painted on the dial-face, and which, in addition to the graduations, was marked in the proper positions with the words "Stop;" "Quarter speed;" "Half speed;" "Full speed;" and also with two arrows pointing in opposite directions marked "On" and "Off" respectively. Just beneath the dial was a small wheel with a crank-handle projecting from one of its spokes, and on this crank-handle the professor now laid his hand. "This," he said, "regulates the valve which admits vapour into the engine; and the dial-hand shows the extent to which the valve is opened. Turn the wheel in the direction of the arrow marked `On'--thus, and you admit vapour into the engine. You will observe that, as I turn the wheel, the hand on the dial travels over the arc and indicates the extent to which the valve is open. There; now it is fully open, and the cylinders are full of vapour." Then he quickly reversed the wheel and sent the index hand back to "Stop," keeping a wary eye on his companions as he did so. "These are dangerous things to meddle with," he remarked apologetically. "The engines are of one hundred thousand horse-power; and, full as the ship now is of air at the atmospheric pressure, they would drive her irresistibly along the ground and through all obstacles. I must beg that none of you will meddle with the machinery until you are fully acquainted with its tremendous power." "What is this pendulum-looking affair, professor?" asked the colonel, pointing to a pendulum the point of which hung in a shallow basin-like depression thickly studded with needle-points which the pendulum just cleared by a hair's-breadth.