The earliest-known references to windmills are to a Persian millwright in AD 644 and to windmills in Seistan, Persia, in AD 915. These windmills are of the horizontal mill type, with sails radiating from a vertical axis standing in a fixed building, which has openings for the inlet and outlet of the wind diametrically opposite to each other. Each mill drives a single pair of stones directly, without the use of gears, and the design is derived from the earliest water mills. Persian millwrights, taken prisoner by the forces of Genghis Khan, were sent to China to instruct in the building of windmills; their use for irrigation there has lasted ever since.

Source: Britannica