Corn mills began with the Romans who spread techniques that helped them to exploit their empire. They built undershot and overshot water mills, one driven by the momentum of flowing water and the other by the weight of falling water. This typical modern example has an undershot drive  with a hopper  for the corn and a chute  conveying it to grindstones . The flour produced fell into a chute  and then poured into a bag .
A water wheel is 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.