The big idea
A windmill turns moving air into turning work. Sails or blades are set at a slant, so the wind pushes them round instead of straight back. Gears change the slow, strong turn of the sails into the fast turn of a millstone, a pump or a generator. The mill must face the wind, so the whole body, the cap or the nacelle turns to meet it. The power in the wind grows with the cube of its speed, so a mill needs ways to spill wind in a gale: reefing the cloth, turning out of the wind, or twisting the blades.
Memory trick: “Face the wind, catch the wind, gear it up, grind or pump, and spill the gale.”
A bit of history: Mills with upright shafts and sails behind a wall were grinding grain in eastern Persia by about the 10th century A.D., and some still stand at Nashtifan in Iran. Post mills appeared in north-west Europe late in the 12th century; one at Weedley in Yorkshire is recorded in 1185. Tower mills and smock mills followed, with only the cap turning. Dutch millers used them to drain whole lakes, like the Beemster, dry by 1612. The fantail (1745), spring sails (1772) and patent sails (1807) made mills turn and reef themselves. On American farms, Daniel Halladay's self-governing windmill (1854) and the steel Aermotor (1888) pumped water from wells. Charles Brush made electricity with a windmill in 1888, the Smith-Putnam machine reached 1.25 MW in 1941, and today's three-bladed turbines are the largest windmills of all.
Every miller watched the wind and turned to follow it. The Bible notes the same: The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.
(Ecclesiastes 1:6, KJV)