The aftermost compartment is called the buoyancy chamber. Besides adding to the floatability of the torpedo, this space also holds the engine shaft and the gear attaching it to the twin propellors. The first Whiteheads were single-screw boats. But the revolution of the propellor in one direction set up a reaction that caused the torpedo itself to partially revolve or heel over in the other, disturbing its rudders and swerving it from its course. This reaction is neutralized by using two propellers, one revolving to the right, the other to the left. Instead of being placed side by side, as on a steamer, they are mounted one behind the other, with the shaft of one revolving inside the hollow shaft of the other, and in the opposite direction. Long after they could be depended on to keep a proper depth, the Whiteheads and other self-propelled torpedoes were liable to swing suddenly to port or starboard, or even turn completely round. During the war between Chile and Peru, in 1879, the Peruvian ironclad _Huascar_ discharged an automobile torpedo that went halfway to the target, changed its mind, and was coming back to blow up its owners when an officer swam out to meet it and succeeded in turning it aside, for the torpedoes of that time were slow and small as well as erratic. Nowadays a torpedo is kept on a straight course by a gyroscope placed in the buoyancy chamber. Nearly every boy knows the gyroscopic top, like a little flywheel, that you can spin on the edge of a tumbler. The upper part of this toy is a heavy little metal wheel, and if you try to push it over while it is spinning, it resists and pushes back, as if it were alive. A similar wheel, weighing about two pounds, is placed in the buoyancy chamber of a Whitehead. When the torpedo starts, it releases either a powerful spring or an auxiliary compressed air engine that sets the gyroscope to spinning at more than two thousand revolutions a minute. It revolves vertically, in the fore-and-aft line of the torpedo, and is mounted on a pivoted stand. If the torpedo deviates from its straight course, the gyroscope does not, and the consequent change in their relative positions brings the flywheel into contact with a lever running to the servo-motor that controls the two vertical rudders, which soon set the torpedo right again. [Illustration: Cross-section of a Whitehead Torpedo. Redrawn from the Illustrated London News.