A windmill is a machine that harnesses wind energy for a purpose like grinding grain, pumping water, or generating electricity. Some people prefer to use the term “wind turbine” to refer to one that is used for electricity generation, differentiating energy-generating machines from those designed for other functions. While the popularity of windmills waned with the rise of other sources of energy, they continue to be widely found around the world, and especially in remote areas, these devices sometimes provide the only source of energy for the surrounding community.
The earliest documented attempt at building a windmill occurred in Ancient Greece around the first century CE. Apparently, the technology didn't catch on, because the Persians developed their own version 400 years later, coming up with versions that were seen by Europeans during the Crusades. Some of the Crusading knights brought the technology back to Europe with them, and Europeans started building these devices in the 12th century, making several adaptations to the Persian design. The Chinese also appear to have picked up the technology from the Persians.
The basic form hasn't changed since the first century. It has a set of blades or slats mounted on a central axis that is designed to revolve as the slats are pushed by the wind. The axis connects to a system of gears to turn machinery that can do things like operate a millstone for the purpose of grinding grain or pumping water.
Many windmills are designed to rotate, so that the operator can take advantage of prevailing winds. Early European designs were very simple, consisting of little more than long poles attached to the blades, but in the 14th century, people started to construct towermills, which had a fixed tower base and a rotating top. The development of the tower mill allowed for the construction of facilities directly beneath the blades, maximizing energy efficiency.
These structures are a distinctive part of the landscape in many regions of the world, as by their very nature they are tall and quite imposing. Those in some regions have become particularly famous. Holland, for example, is dotted with windmills, many of which continue to be used to control floodwaters in low-lying areas, and those in La Mancha in Spain are famous thanks to the novel Don Quixote, in which the eponymous protagonist is seen mistaking them for giants and attempting to fight them.