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A watermill is a machine constructed by connecting a water wheel to a pair of millstones. Like windmills, watermills were a common system for milling flour until the arrival of steam and electrical power in the last two centuries.
Strictly speaking, a pair of millstones forms a mill, and it is possible to have more than one mill under the same roof, though the term watermill is commonly used to refer to the building housing the milling machinery, as well as the machinery and millstones inside.
The technology behind the watermill is somewhat older than that of the windmill. Harnessing a flow of water to drive machinery is not new. The ancient Greeks used primitive water wheels, and the Romans are known to have improved the technology. They were responsible for the introduction of the watermill to many of the countries of the Roman Empire.
Operation of a watermill
The method of operation of a typical watermill is as follows: water is diverted from a river or millpond to a water wheel, along a channel known as a millrace or mill leet. A sluice gate is used to open the channel and so start the water flowing and the water wheel turning. Apart from in some very early watermills (so called Norse Mills ), the water wheel is mounted vertically (i.e. edge-on) in the water. A large gear-wheel called the pit wheel is mounted on the same axle as the water wheel and this drives a smaller gear-wheel (the wallower) on a main driveshaft running vertically from the bottom to the top of the building. This system of gearing ensures that the main shaft turns faster than the water wheel, which typically rotates at 10 rpm, or so.
The millstones themselves turn at around 120 rpm. They are laid one on top of the other. The bottom stone, called the bed, is fixed to the floor, while the top stone, the runner, is mounted on a separate spindle, driven by the main shaft. A wheel called the stone nut connects the runner's spindle to the main shaft, and this can be moved out of the way to disconnect the stone and stop it turning, leaving the main shaft going to drive other machinery. This might include driving a mechanical sieve to refine the flour, or turning a wooden drum to wind up a chain used to hoist sacks of grain to the top of the mill house.
The grain is lifted in sacks onto the sack floor at the top of the mill. The sacks are emptied into bins, where the grain falls down through a hopper to the stones on the stone floor below. The flow of grain is regulated by shaking it along a gently sloping trough (the slipper) from which it falls into a hole in the centre of the runner stone. The milled grain (flour!) is collected as it emerges from the outer rim of the stones and it gets fed down a chute to be collected in sacks on the ground or meal floor.
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