A wooden or metal wheel with paddles or buckets of some kind attached to the outside so that, when set in a watercourse, it will rotate as a result of pressure from the movement of the water. The energy captured by such wheels in the form of rotary motion is usually transmitted via a connecting rod to some kind of machinery. Three main kinds of vertically set waterwheel can be identified: the undershot wheel, rotated by water passing below the wheel; the overshot wheel, where water is fed onto the top of the wheel, filling buckets which unbalance the wheel, causing it turn; and the breastshot wheel, where water is fed onto the wheel at an intermediate level. Water wheels appear to have been first used in the Graeco‐Roman world during the 1st century bc. The Roman engineer Vitruvius, writing between 20 bc and 11 bc, describes what is essentially an undershot wheel set vertically with a horizontal drive shaft. The use of such water wheels appears to have spread fairly rapidly within the Roman world and beyond, being known in Gaul by ad 370. It is possible that they were preceded by a primitive horizontally set wheel with a vertical shaft, but no certain early evidence for these has been found.