A swimming machine is a resistance swimming apparatus, often self-containe, enabling the swimmer to swim in place. This may be accomplished either by accelerating the water past the swimmer or by supporting the swimmer, either in water or on dry land. The first type, known as a countercurrent swimming machine, usually consists of a water tank at least twice as long and about one and a half times as wide as an average person with the limbs extended. The swimmer swims unrestrained against an adjustable stream of water set in motion by means of mechanical devices, such as jets, propellers or paddle wheels.