In the history of technology, waterpower has a surprisingly long career. Developed in Antiquity, the water wheel became the main source of power during the Middle Ages and remained so until the 19th century, when it was replaced by the steam engine. Waterpower still plays an important role today in many countries, where it is used for generating electricity, although the water wheel has now been replaced by the much more efficient water turbine.
The main application of waterpower for centuries was the grist mill, which grinds grain into flour. Water mills appeared throughout the Roman Empire during the third and fourth centuries, but the fall of the Roman empire prevented their widespread use in later centuries.
Much of the technology of water wheels from the Roman and the Hellenistic civilizations was taken over and perfected in the Islamic world. Several of their types of irrigation systems were powered by water wheels. In Europe, water wheels came into widespread use only during the tenth century. The Domesday Book (eleventh century) lists 5624 water mills in England, which corresponds to about one water mill per 300 inhabitants.
The early water wheels were mainly of the undershot type; water passed under the wheel, driving the lower paddles. Undershot wheels are easy to install and they appeared in many locations on rivers and streams. Many of these water mills were mounted on barges, making their operation independent of the water level of the river. This type of floating mill was first invented by the Roman general Belisarius in 537. He installed floating mills on the Tiber when the Goths, besieging Rome, had cut the city’s water supplies. In cities it was preferable to install floating mills under bridges because of the faster flow of water between the arches. The number of water mills built under bridges in Paris was so great at one time that they seriously impeded boat traffic on the Seine.
During the late Middle Ages, water wheels became the power source for a wider range of applications, including pumps, hammers, grinding wheels, saws, and lathes.
The overshot wheel also appeared in the late Middle Ages. Overshot wheels are more efficient because they use more energy in the flow of water. Their efficiency was further increased by replacing the paddles by buckets, whereby the weight of water collected in buckets adds to the driving force acting on the wheel. John Smeaton calculated in the 18th century that an undershot wheel uses 22 percent of the energy in the flow of water, while an overshot wheel uses 63 percent.