A horse engine (also called a horse power or horse-power) is a (now largely obsolete) machine for using draft horses to power other machinery. It is a type of animal engine that was very common before internal combustion engines and electrification. A common design for the horse engine was a large treadmill on which one or more horses walked. The surface of the treadmill was made of wooden slats linked like a chain. Rotary motion from the treadmill was first passed to a planetary gear system, and then to a shaft or pulley that could be coupled to another machine. Such powers were called tread powers, railway powers, or endless-chain powers. Another common design was the horse wheel or sweep power, in which one or several horses walked in a circle, turning a shaft at the center. Mills driven by horse powers were called horse mills. Horse engines were often portable so that they could be attached to whichever implement they were needed for at the time. Others were built into horse-engine houses. Horse engines were used to power farm implements and industrial processes. Examples of machines that were powered with a horse engine include gristmills (see horse mill), threshing machines, corn shellers, feed cutters, silo blowers, grain grinders, pumps, and saws such as bucksaws and lumber mill saws. They could also be used interchangeably with other forms of power, such as a hand crank, stationary engine, portable engine, or the flat belt pulley or PTO shaft of a tractor, which eventually replaced them on most American and European farms. Today there are still a few modern versions used by Amish people that assist in farm chores and that power machine shops via line shafts. Designs, terms, and output The term "horse power" probably predates the name of the horsepower unit of measurement. (For reference the 1864 Webster's Dictionary defines horse-power as “A machine operated by one or more horses; a horse engine.") The word "power" in late-19th-century American English, for example, was often used for any example in the whole category of power sources, including water powers, wind powers, horse powers (for example, sweep powers), dog powers, and even (in a few cases) sheep powers; in the Pennsylvania Oil Country during that era, sweep-style powers run by steam engines and gas engines to power oil derricks were called "powers" in the local vocabulary, just as horse powers on farms