Windmill

A windmill is a machine that captures the energy of moving air.

This mechanical force is used to perform physical tasks like grinding grain into flour or pumping water. These machines are known as gristmills when they are used for milling. Windmills represent a vital way humans have used natural energy to automate difficult labor. They have been essential tools throughout the high medieval and early modern periods. By converting wind into mechanical motion, they provide a steady source of power for many industries.

The basic mechanism of a windmill relies on the interaction between wind and sails.

Most sails consist of a lattice framework. A miller can cover this frame with sailcloth or reed matting. When the wind hits these surfaces, it creates a force that causes the sails to rotate. This rotation turns a central shaft, which transfers the power to internal machinery. In some designs, the miller can adjust the amount of cloth to control the power. In colder climates, wooden slats are used instead of cloth because they are easier to handle when frozen.

There are two primary types of windmill designs based on their axis of rotation.

The first is the panemone, or horizontal windmill. These were the first practical windmills, appearing in Persia between 700 and 900 AD. They feature sails that rotate in a horizontal plane around a vertical axis. The second type is the vertical-axle windmill. These appeared in northwestern Europe during the 12th century. These mills rotate their sails in a vertical plane. This design allowed for different ways to capture wind in various geographic regions.

The history of wind power spans thousands of years.

Records show the Babylonian emperor Hammurabi used wind power for irrigation in the 17th century BC. In the first century, Hero of Alexandria described a wind-driven wheel in Roman Egypt. While some of his designs might have been toys, they showed early concepts of wind energy. The Persian panemone mills eventually spread to the Middle East, China, and India. By the 11th century, vertical-axle windmills reached Southern Europe. In the 18th and 19th centuries, engineers in Europe independently developed new vertical-axle models.

European windmill evolution led to several specialized structures.

The post mill was a common early European type. Its entire body, or "buck," sits on a large upright post. This allows the whole structure to rotate so the sails face the wind. As economies grew, the masonry tower mill was introduced by the late 13th century. In a tower mill, only the top cap rotates. This makes the structure much taller and more stable. Taller mills can use longer sails to capture more energy even when winds are low.

Another important development was the smock mill, introduced by the Dutch in the 17th century.

Instead of heavy masonry, a smock mill uses a wooden framework called a "smock." These are often octagonal and can be covered in thatch or metal. Because they are lightweight, they can be built on unstable or wet ground. Some smock mills even include a small turbine at the back. This turbine helps the main mill automatically face the direction of the wind.

Technological improvements have made sails much more efficient over time.

In 1807, William Cubitt invented patent sails. These use a mechanism of connected shutters instead of cloth. This allows the sails to adjust to wind speed automatically. In France, Pierre-Théophile Berton created a system of wooden slats that could be adjusted while the mill was turning. In the 20th century, engineers used aerodynamics from airplane research to improve efficiency. These advancements allowed windmills to work more effectively across different environments.

In some cultures, windmills serve social functions beyond mechanical work.

In the Netherlands, the position of the sails can signal messages to the community. If the sails stop in a "+" shape, the mill is open for business. An "X" shape means the mill is closed. A slight tilt of the blades can signal joy, such as a birth. Conversely, a specific tilt can be used to signal mourning. This turns a machine into a tool for communication.