The waterwheel 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 moves the paddles, and the consequent rotation of the wheel is transmitted to the machinery via the shaft of the wheel. Water Wheels 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. 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. The earliest known description of a vertical water wheel is found in the writings of a Roman engineer by the name of Vituvius who lived in the Augustan age around 11 B.C. However, water wheels remained few and far between and were rarely employed as the two great sources of power in that day and age were men and animals. However, in the 4th century A.D. the Romans used 16 overshot water wheels to power a large flour mill at Arles in Southern France. When Rome was under siege in the year 537 AD, the Goths shut off the aqueducts whose water drove the city's gristmills. Belisarius, the Bzyantine general defending the city, ordered floating mills installed close to the Tiber bridges, whose piers constricted and accelerated the current. Two rows of boats were anchored with waterwheels suspended between them. The arrangement worked so well that cities all over Europe were soon copying it. While the Romans continued to progress, it was the Persians who first used the power of wind and water to operate mills.

In ancient China, waterpower was also applied at an early date to grinding grain. Large rotary mills appeared in China about the same time as in Europe (2nd century BC). For many centuries Europe relied heavily on slave and donkey powered mills; in China the waterwheel was a critical power supply. One of the most intriguing applications was for iron casting. According to an ancient text, in 31 AD the engineer Tu Shih "invented a water-powered reciprocator for the casting of iron agricultural implements." Smelters and casters were "instructed to use the rushing of water to operate their billows. Chinese waterwheels were typically horizontal. The vertical wheel, however, was known. It was used to operate trip hammers for hulling rice and crushing ore. The edge-runner mill was another commonly used crushing device. With the latter a circular stone on edge running around a lower millstone was used to pulverize. The edge runner appeared in China in the 5th century AD. Both the trip hammer and edge runner were not used in Europe until eight centuries later. In medieval Europe, social and economic conditions increased the need for replacing manual labor with powered machines. The Domesday Book, a survey prepared in England in 1086 AD, lists 5,624 water mills (this number is low since the book is incomplete). A century earlier, fewer than 100 mills were counted. Tidal mills were apparently a medieval invention. They were first mentioned in the 12th century in both England and France. Their numbers increased every century until modern times. These mills were constructed in low-lying areas near the ocean. Dams containing swinging gates were built along shallow creeks. As the tide came in, the gates swung open inwardly. Water filled the area behind the dam. When the tide turned, the gates swung shut, forcing the water to flow seaward through the mill race of the tidal mill. The obvious disadvantage to tidal mills is that the time of the tides shifts every day. Thus the millers had no choice but to work hours dictated by the tides. These mills seem only to have been used to grind grain (although the water wheels on London Bridge were definitely affected by tidal action on the Thames River). There were never many of them when compared to "ordinary" water wheels. During the late Middle Ages, the increasing demand for metals drove miners deeper into the earth. Old mining methods were no longer adequate. Miners started using water wheels to pump water from the mines, grind ore, run bellows at the blast furnace, and operate hammers at the metal smith's forge. Among the other uses of water wheel technology included rice husking, paper making, and pulping sugar cane. The usual method of adapting water wheels for such purposes was to extend the axle and fit cams to it. The cams caused trip-hammers to be raised and then released to fall on the material. The first water wheels can be described as a grindstone mounted atop a vertical shaft whose vanes or paddles lowered and dipped into a rapid stream; the wheel was horizontal. As early as the first century, the horizontal water wheel, which is terribly inefficient in transferring the power of the current to the milling mechanism, was being replaced by water wheels of the vertical design. One of the early design must have been where the one rim of the water wheel had gear teeth mounted in its outer face and that turned a lantern pinion that ran a single pair of millstones. This was a very common design for centuries. When a mill wanted to run another pair of millstones they simply added another water wheel. It was easier to construct another water wheel than try and design a mill setup where a single water wheel could operate more than one pair of millstones.

Water wheels were used to power different types of mills. A water wheel and mill combination is called a water mill. An early horizontal-wheeled water mill used for grinding grain in Greece was the called Norse Mill. In Syria and Jordan, the water mills were called "norias". The noria were probably developed from the Persian water wheels. The Persian water wheels were used to lift water from wells by an endless system of ropes upon which a series of clay pots turn over two pulley wheels. The Persian windmills were built in rows or a series of windmills that contained a number of windmills in one long building. The Persian windmills have a tall narrow slit in the wall to allow the wind to enter and another on the other side for the wind to exit. In side is a tall multi sided vertical wind blade. The wind (that only blows from one direction) comes through the slot in the wall and turns the wind blade. On the bottom of the shaft that goes through the floor and to a room below is a pair of millstones sitting on a shelf or platform. The blade turns one of the round millstones and grinds the grain. On the opposite side of the room is a trough where on one side of the room is a round hole that allows the wind to enter above the trough and the air leaves through a hole on the other side thus forming a simple wind tunnel. This type of water wheel was later used to lift water out of underground mines in Europe.

The Persians believed that wind power was a gift from the gods. It could be used for good or evil. So when the windmills became damaged, broken or in need of repair they would simply abandoned them and build a new windmill due to the simple fact that in repairing them the gods can become angry and cause pain, hurt or kill you. So even today in parts of ancient Persia where these windmills are still used they abandon them when they become in need of repair or rebuilding.

Grist Mills have changed very little in thousands of years. They were simple devices, often a single pair of millstones, which were turned by simple gearing and a water wheel. Often if you wanted to install another pair of millstones you had to add another water wheel. Up until the year 1500 the average diameter of a pair of millstones was 72 inches, but they discovered that bigger was not better and finally the average became a standard of 48 inches in diameter. The Romans had seven grades of flour or products that they would sift from wheat, so white flour was nothing new. You kept within your class and you only ate what your class of people ate. The rich and the affluent classes ate white flour while the poorer peasant classes ate the brown flours. If starvation and malnutrition does not get you then eating this bread might very well finish you off. Brown bread was also termed "dog bread" because often it was not fit to feed dogs. It was not until the year 1700 that machines were developed that could clean the dirt, seeds, fungus, smut, straw, chaff and stones out of grain. The fungus ergot was a problem in rye and opium was a common additive in bread in the middle ages.

The one person who did the most for mills was Oliver Evans (1755-1819). What he did mainly benefited the larger merchant mill that produced commercial white flour. Its biggest benefit was in material handling and how grain and flour is stored and moved. At one time grain and flour was hoisted through trap doors and carried on backs up narrow stairs. Grain and flour was stored in the open on floors. It was not uncommon for someone to compress flour into a barrel with road mud on their boots so the barrel head could be closed.

From classical times, there have existed 3 general varieties of water wheels: the horizontal wheel and 2 variations of the vertical wheel. The horizontal wheel has vanes protruding from a wooden rotor. A jet of water turns the rotor. In modern Europe the design was altered to use water moving axially, like air flowing through a pinwheel, creating a water turbine. Wheels with curved blades onto which the flow was directed axially are described in an Arabic essay of the 9th century. A horizontal wheel turns a millstone directly. The more powerful vertical wheels come in 2 designs: undershot and overshot. The undershot is a paddle wheel that turns under the flow of water current. This technology requires gears to drive a typical millstone. When the levels of rivers fall in the dry season, and their flow diminishes, undershot wheels lose some of their power. The overshot wheel receives water from above, often from specially constructed channels; it adds the impetus of gravity to that of the current. An overshot wheel requires gears and an elevated stream of water. The breast shot water wheel was the most common water wheel type used by American industry up until the French developed the water turbine in the 1840's Also the same requirements for the breast shot are also the same for the water turbine. The requirements for overshots and undershots are different as to the amount of water and required fall. Undershot water wheels were primarily used on boat or floating mills and on tidal mills were there is little or no fall but lots of water that pushes the wheel. Millwrights or mill builders tended to only build what they know how to construct. So if they were trained only to construct overshot or undershot water wheels that is what they built when they came to America. So it was not uncommon to have undershot water wheels with a 40 foot fall. Undershot water wheels are the least efficient of all water wheels. Breast shot and undershot water wheels tend to be larger to make up in torque the short comings they have with less fall than an undershot. I have never seen a saw mill being built with an undershot water wheel. Along the coast of Maine, tidal powered saw mills are quite common. A modern invention based on the same principles as those of the water wheel is called the hydraulic turbine. The turbine is a rotary motor 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.

With global warming and the increasing cost of utilities, the water wheel may well be making a big comeback although it has never really left us.