Several times Tom turned the propeller around.  Then, with a pop, the engine cylinders began to fire, Tom jumped swiftly back, and the propeller whirred like a mad thing.  At the same time the Sky-Bird gave a start, as though to dash forward; but beyond a steady, slight vibration of her whole body, as Tom slowed down the motor to four hundred revolutions per minute, there was no indication to her inmates that she was straining to get away.  Tom now quietly mounted the step, and came into the cabin, pulling the step up after him and closing the self-locking door.