A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails. The term also refers to the structure it is commonly built on. In much of Europe, windmills served originally to grind grain, though later applications included pumping water and, more recently, generation of electricity. Recent electricity-generating versions are referred to as wind turbines.

Vertical axle windmills

The first practical windmills were the vertical axle windmills invented in eastern Persia by the Persian geographer Estakhri in the ninth century.Ahmad Y Hassan, Donald Routledge Hill (1986). Islamic Technology: An illustrated history, p. 54. Cambridge University Press. ISBN 0-521-42239-6. The authenticity of an earlier anecdote of a windmill involving the second caliph Umar (634–644 AD) is questioned on the grounds of being a 10th-century amendment. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind corn or draw up water, and quite different from the European versions. A similar type of vertical shaft windmill with rectangle blades, used for irrigation, can also be found in 13th-century China (during the Jurchen Jin Dynasty in the north), introduced by the travels of Yelü Chucai to Turkestan in 1219.Needham, Volume 4, Part 2, 560.

Fixed windmills

Windmills that turn to face the wind

In North Western Europe, the horizontal-shaft or vertical windmill (so called due to the dimension of the movement of its blades) dates from the last quarter of the 12th century in the triangle of northern France, eastern England and Flanders. Joseph Needham states that the earliest known reference came in 1191 by a Dean Herbert of East Anglia, who supposedly competed with the mills of the abbey of Bury St Edmunds. These earliest mills were used to grind cereals. The evidence at present is that the earliest type was the post mill, so named because of the large upright post on which the mill's main structure (the "body" or "buck") is balanced. By mounting the body this way, the mill is able to rotate to face the (variable) wind direction; an essential requirement for windmills to operate economically in North-Western Europe, where wind directions are various. By the end of the thirteenth century the masonry tower mill, on which only the timber cap rotated rather than the whole body of the mill, had been introduced. In the Netherlands these stone towerlike mills are called "round or eight-sided stone stage mills, ground-sailers (windmills with long blades/sails reaching almost down to the ground), mound mills, etc." (Dutch: ronde/achtkante stenen stelling molens, grond-zeilers, beltmolens, etc.). Dutch tower mills ("torenmolens") are always cylindrical (such as atop castle or city wall towers). Because only the cap of the tower mill needed to be turned the main structure could be made much taller, allowing the blades to be made longer, which enabled them to provide useful work even in low winds. Windmills were often built atop castle towers or city walls, and were a unique part of a number of fortifications in New France, such as at Fort Senneville.