A mechanical device that is designed to use the weight and/or force of moving
water to turn it, primarily to operate machinery or grind grain. The first
water wheels were laid horizontally in moving water, their shafts pointing
upward and surmounted by a millstone. Later water wheels were vertical,
either undershot (in which the water passed under the wheel) or overshot
(in which the water was directed by a sluice on to the top of the wheel).
The latter had the advantage of being operable almost independently of the
level of water in the stream or river that supplied it.