The wind-driven wheel, or wind wheel, consists of a number of evenly bal¬anced vanes or sails which radiate from a shaft. When the wind strikes the sails, the wind wheel rotates, which turns the shaft. This turning force is transmitted by gears from the shaft to the parts of the machine that does the work.
The principle of the windmill has been known since ancient times, but little is known of its use before the 12th century. It was during that time that windmills began to be widely used in Europe.
The earliest kind of windmill, which is called the post or German mill, had a fixed vertical post around which the entire mill rotated so that the wind wheel might face into the wind. The other principal kind of windmill called the tower, turret, or Dutch mill, had its wind wheel and shaft set in a rotating turret mounted on a stationary tower.
The horsepower developed by these windmills ranged from two to eight horsepower for the German type, and from six to fourteen horsepower for the Dutch type. About half the horsepower developed was lost in transmission, so they weren't very efficient machines. But they were used in great numbers until steam power was developed in the 18th century. There were as many as eight thousand windmills in the Netherlands alone in use in the 17th century!
A much smaller type of windmill, used for pumping water, was very popular in the United States at the end of the 19th century. They were used to pump water for livestock, household use, or for irrigation.
