"A mill race, millrace or millrun, mill lade (Scotland) or mill leat (Southwest England) is the current of water that turns a water wheel, or the channel (sluice) conducting water to or from a water wheel. Compared with the broad waters of a mill pond, the narrow current is swift and powerful. The race leading to the water wheel on a wide stream or mill pond is called the head race (or headrace), and the race leading away from the wheel is called the tail race (or tailrace).
A mill race has many geographically specific names, such as leat, lade, flume, goit, penstock. These words all have more precise definitions and meanings will differ elsewhere. The original undershot waterwheel, described by Vitruvius, was a ´run of the river wheel´ placed so a fast flowing stream would press against and turn the bottom of a bucketed wheel. In the first meaning of the term, the millrace was the stream; in the sense of the word, there was no separate channel, so no race. " - (en.wikipedia.org 14.01.2022)
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