The is an ancient device that uses flowing or falling water to create power by means of a set of paddles mounted around a wheel. The force of the water moved the paddles, and the consequent rotation of the wheel is transmitted to machinery via the shaft of the wheel. The first reference to its use dates back to about 4000 B.C., where, in a poem by an early Greek writer, Antipater, it tells about the freedom from the toil of young women who operated small handmills to grind corn. They were used for crop irrigation, grinding grains, supply drinking water to villages and later to drive sawmills, pumps, forge bellows, tilt-hammers, trip hammers, and to power textile mills. They were probably the first method of creating mechanical energy that replaced humans and animals.

There are three main kinds of waterwheels. One kind is the horizontal waterwheel, in which water flows from an aqueduct and the forward action of the water turns the wheel. Another kind of waterwheel is the overshot vertical waterwheel, in which water floes from an aqueduct and the gravity of the water turns the wheel. The last kind is the undershot vertical waterwheel, a large vertical waterwheel placed in a stream is turned by the river’s motion.

The first waterwheels can be described as a grindstone mounted atop a vertical shaft whose vaned or paddled lower end dipped into a swift stream, the wheel was horizontal. As early as the first century, the horizontal waterwheel, which is terribly inefficient in transferring the power of the current to the milling mechanism, was being replaced by waterwheels of the vertical design.

Waterwheels were most often used to power different types of mills. A waterwheel and combination is called a watermill. An early horizontal-wheeled watermill used for grinding grain in Greece was the called Norse . In Syria, the watermills were called “noriahs”. They were used for running mills to process cotton into cloth.

A modern invention based on the same principles as the waterwheel is called the hydraulic . The  is a rotary engine that uses the flow of fluid (either gas or liquid) to turn a shaft that drives machinery. Hydraulic turbines are used in hydroelectric power stations. Flowing or falling water strikes a series of blades or buckets attached around a shaft. The shaft then rotates and the motion drives the rotor of an electric generator.

Waterwheels and Mills
A history of waterwheels, includes diagrams of how waterwheels work.

Of the three distinct types of water mills, the simplest and probably the earliest was a vertical wheel with paddles on which the force of the stream acted. Next was the horizontal wheel used for driving a millstone through a vertical shaft attached directly to the wheel. Third was the geared mill driven by a vertical waterwheel with a horizontal shaft.

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