Brake Device used to slow or stop the motion of a mechanism or vehicle. Friction brakes, the most common kind, operate on the principle that friction can be used to convert the mechanical Energy of a moving object into Heat energy, which is absorbed by the brake. Friction brakes consist of a rotating part-such as a wheel, axle, disk, or brake drum-and a stationary part that is pressed against the rotating part to slow it or stop it. The stationary part usually has a lining, called a brake lining, that can generate a great amount of friction yet give long wear. The simplest brake form is the single-block brake, a wooden block shaped to fit against the rim of a wheel or drum. In disk brakes, two blocks press against either side of a disk that rotates with the wheel. Drum brakes have two semicircular brake shoes inside a rotating brake drum; when actuated, they press against the inner wall of the drum. Automobiles use hydraulic Pressure to Power disk and drum brakes (see Hydraulic Machinery). Additional braking pressure may be supplied by a power brake, which utilizes the Vacuum created within the running engine to hold a brake shoe away from a drum. The shoe presses against the drum when the vacuum is destroyed. The Air brake, invented (1868) by Westinghouse, George, uses compressed air to power block brakes on trains.