A windmill is a set of rotating blades (often called vanes or sails) that turn in the wind, and convert the energy of the wind into turning energy which can be made to work something useful.
Windmills were often used to grind corn, with the windmill turning a heavy grindstone on top of a stationary stone, but they were just as commonly used to pump water, and even used as sawmills, for iron hammering, and cloth hammering (fulling), and to make paper pulp and crush oilseeds. .
Today windmills are mainly very large and the turning motion is used to work electric generators as a form of renewable energy.
These are called wind turbines.
The earliest know windmill was made by the Greek engineer Heron of Alexandria in the first century
The first practical windmills had sails that spun around an upright shaft (see page 20 of our book Energy>>link at the foot of this page). They were in use in Persia by the 9th century. They were made of six to twelve sails covered in reed matting or cloth material, and used to grind grain or draw up water. Early windmills of this kind were in widespread use across the Middle East and Central Asia, and later spread to China and India.
This design is very easy to make with simple tools, and is no less efficient than ones with horizontal shafts and vertical sails. The reason for putting the sail higher on a vertical shaft is that the speed of the wind is lower near the ground than high in the air, so the sails get more energy if they are placed high above the surface. After a point, wind speeds to not increase as you go higher, and most modern wind turbines are placed as low as possible, while still getting the maximum wind energy. As you can see, that is still pretty high, hence the tall towers!
The horizontal blade technology did not get passed on to people in Europe, who mainly independently invented windmills that turned on a horizontal shaft, the kind we mostly see today.
This was invented in the last quarter of the twelfth century and the earliest proven windmill in Europe was built in 1185, in Yorkshire, England. These earliest mills were used to grind cereals.
These early mills with vertically-turning blades were known as post mills because of the large upright post on which the mill's main body is balanced. By mounting the whole windmill on the post, , the mill could be rotated to face the wind. Modern windmills do this automatically using a small windmill called a fantail. The body also contained all the milling machinery.
These early mills had cloth-covered sails so that the amount of turning force (how much of the sail caught the wind) could be adjusted to increase or decrease the speed of turning.
As steam, and then steam-generated electricity took over from windmills, they fell into disuse except for pumping water on farms. But with the idea of renewable energy, they have been reinvented and called wind turbines, and now generate considerable amount of electricity.
Agricultural wind pumps look different from most windmills because they have many small blades, rather than 3-6 large ones. This means they turn slowly with high force in low winds, and are self-regulating when the wind is high (ordinary windmills have to be stopped).
But they could not be built at the large scale needed for wind turbines
