Power for Technology: Wind
Power for Technology: Wind
Windmills. Medieval Europeans who did not live near rivers or streams capable of turning waterwheels harnessed the wind to power their mills. In the Low Countries, for example, the average elevation is only a few feet above sea level (and many areas where land was reclaimed are actually at mean sea level—that is, below storm-surge levels). While water is plentiful there, it is not suited to powering mills. They do have steady winds coming off the North Sea, however, and the windmill became their principal means of power. Windmills began to appear in Europe during the 1180s, and seem to have come from Asia Minor. The earliest known windmills were horizontal mills used by the mid tenth century in Anatolia, a high, arid region in modern Turkey, which, like the Low Countries, has little water-power potential, but a great deal of wind.
Horizontal Windmills. There are two types of windmills, horizontal and the vertical. The horizontal windmill may have been modeled on the horizontal watermill, for both use a single vertical shaft that is directly connected to the millstone. The horizontal windmill works much like the propeller on top of a child’s beanie or a pinwheel turned on its side. Also like a horizontal waterwheel, this kind of windmill tends to be modest in size and power output.
Vertical Windmills. Vertical windmills are much larger and more powerful than horizontal windmills, and they were more common throughout Europe. A vertical windmill has several blades, or sails, mounted on a horizontal shaft and set on a tall tower, where they could turn in the wind. The sails were sometimes made of wood, but frequently they were made of cloth, often in a triangular shape that was efficient at catching the wind. Gearing connected the main axle to the millstone, which was usually placed on the second floor of the tower, so that the ground flour could fall to the lower floor, where it was sifted and bagged.
Catching the Wind. There is a variable in wind power that does not exist in water power. Unlike a river, which always flows in the same direction, the wind can blow in many directions. In many places the wind direction changes from day to day and from season to season. A horizontal windmill can be powered by winds from any direction, but a vertical mill needs to be facing directly, or nearly directly, into the wind. Thus, millwrights need a way to turn vertical windmills into the wind. Moving an entire building was a challenge for medieval Europeans, and their attempts to solve the problem greatly increased their facility with machine design, especially in the areas of gearing and self-regulating, or automated, mechanisms.
Post mills. The earliest solution was the post mill, which was in use by the last third of the twelfth century. A small wooden house that contains the millstone and has the sails attached to the peak of its roof is placed on top of a vertical post that has two crossed timbers attached to its base (much like an old-fashioned Christmas-tree stand) and struts connecting these timbers to the middle of the post. This structure is buried in a small mound to ballast it, and the house sits on a bearing on top of the post. A long tiller can then be used to revolve the whole house on the bearing so it faces the wind. Post mills have several drawbacks. There is a limit to weight a miller can safely perch on the post, and the constant pivoting of the structure creates a large amount of wear, causing frequent breakdowns. They are also cumbersome to operate. Tillers were sometimes thirty, forty, or even fifty feet long, and, to provide enough
leverage to turn the house, millers frequently used horses or oxen to pull the tiller. While the post mill was an effective solution to providing power where watermills were not an option, its drawbacks inspired medieval millwrights to look for a better solution.
Turret Mills. Their solution was the turret mill, which was probably in use by the early fourteenth century. The base of the turret mill is built solidly on the ground, and only the top of its tower turns to face the wind. To operate the turret mill, medieval technicians invented a rotating gear assembly to attach where the sail axle meets the main driving shaft, smooth bearings to allow the turret to turn easily, and self-acting mechanisms to keep the sails facing the wind without any effort on the part of the miller. The technical knowledge necessary for creating the gearing system for the vertical mill came from earlier medieval inventions of gears for things such as clocks, winches, and watermills. Bearings had been one of the great limitations of machinery since ancient times. Placing the turret on wheels arranged in a circle and riding on a wooden track (or one of cut stone if the tower is built of stone) allows it to turn. Then the main difficulty was making sure that the top of the mill did not get blown off in high or even moderate winds. Its weight solves this problem to some extent, as does inclining the sail axle a few degrees to make the thrust from the sails act slightly downward.
Wear and Tear. As with watermills, windmills are subject to wear and tear. The constant force of wind can shred the sails or tear a windmill to pieces in a season or two. Gale-force winds can knock out a mill in a day. Windmills, however, were easier to protect than water-mills. If a storm were approaching, the miller could remove the sails from their arms. Post mills were more vulnerable to gales than vertical mills. As millwrights built larger and larger windmills, they often built their towers of stone rather than wood.
Richard Holt, The Mills of’Medieval England (Oxford: Blackwell, 1988).
Edward J. Kealey, Harvesting the Air: Windmill Pioneers in Twelfth-Century England (Berkeley: University of California Press, 1987).
John Langdon, “The Birth and Demise of the Medieval Windmill,” History of Technology, 14 (1992): 54ndash76.
Elizabeth Bradford Smith and Michael Wolfe, eds., Technology and Resource Use in Medieval Europe: Cathedrals, Mills, and Mines (Alder-shot, U.K. &Brookfield, Vt.: Ashgate, 1997).