Middle Eastern origins. The earliest known references to windmills occur in the work of Arab writers in the 9th century AD and refer to a Persian millwright in AD 644. These mills were in Seistan on the borders of Persia and Afghanistan and were of a horizontal type, with sails radiating from a vertical axis standing in a fixed building that had diametrically opposite openings for the inlet and outlet of the wind. Each mill drove a single pair of stones directly, without the use of gearing. The first mills were built with the sails below the millstones, as in the horizontal water mills from which they were derived. But in presently existing mills the sails are placed above the stones.
By the 13th century, horizontal mills were known in northern China, where they were in use by the 16th century to evaporate seawater for salt production. The horizontal mill was used in Crimea, in most of the west European countries, and in the United States, though very few remain. Its most satisfactory manifestation is in S-Rotor (by S.J. Savonius in Finland) now used for ventilating purposes.
Early European windmills. The idea of windmills reached Europe via the Arabs, interpreted in the form of an upside-down Roman vertical waterwheel, with sail in place of the waterwheel to drive millstones through a pair of gears. The mill was mounted on a post so that it could be faced square into the wind from any quarter. These post mills first appeared in France in 1180, in England in 1191, and in crusader Syria in 1190. The earliest known illustration was in the Windmill Psalter made in Canterbury, England, about 1260.
Post mills have a wooden, boxlike body, which contains the millstones and machinery and carries the sails. The upright post is supported by a four-legged trestle; the post top sockets into a transverse beam, like the top stroke of T, from which the mill body is supported. In some early mills, the post and its supports were buried in the ground, and the mill was then known as a “sunk-post mill”. Later the substructure was often protected by a “roundhouse”, which served for storage. Access to the mill body is by a ladder at the tail of the mill. Inside the mill, the gear mounted on the wind shaft, which carries the sails, is usually a brake the wheel with a contracting brake acting on the rim to stop the mill. A heavy lever has to be raised to release the brake, an early example of a “fail-safe” device.
At the beginning of the 14th century the “tower mill” was developed in France. Its millstones and gearing are placed in a fixed tower with a movable roof, or “cap”, which carries the sails and can be turned on a track, or “curb”, on top of the tower. Brick and stone towers are usually round; timber towers are usually octagonal and tapering and are known in England as “smock mills”. The earliest known illustration of a tower mill in a French Psalter of about 1420. Both types of mills were found throughout Europe and built later in America by European settlers.
The “hollow-post mill”, invented in the Low Countries at the beginning of the 15th century for drainage purposes, was the first windmill to have a two-step gear drive. An upright shaft, with a pair of gears top and bottom, passes through the hollow post and drives a “scoop wheel”, like the paddle wheel of a ship, which scoops water up from a lower to a higher level. This indirect drive was adopted slowly in tower and, eventually, in post mills and led to the development of windmills for other purposes. The hollow-post mill was also used in France and Finland for grinding corn. Small mills were used for pumping in salt evaporation beds in England, France, and nearly New England. In 1592 Cornelis Cornelisz in the Netherlands built the first wind driven sawmill, mounted on a raft so that it could be turned into the wind. This was developed into paltrok sawmill, mounted on a ring of rollers and supported on a low brick base; it was peculiar to the Zaan district of the Netherlands. Other types of mills include the fixed-tower mills at the top of escarpments in Crete, Rhodes, and Turkey; and pivot mills, like post mills, but with a short pivot fixed into the post housed in the solid masonry base and socketing into a bearing fixed below the bottom floor. These could be found in Brittany, the Madeira Islands, and Portugal.
As first both post mills and the caps of tower mills were turned into the wind manually with the aid of a tailpole. Later, small posts were set around the mill and a winch, either portable or fixed to the tailpole, were used to wind up a chain hitched to one of the posts. Winches were also put in the caps of tower mills, engaged with geared racks on the curb, and operated either from inside, or from the ground by means of an endless chain over a wheel.
In 1745 Edmund Lee of England invented the fantail, consisting of a ring of five to eight vanes mounted on the ladder or tailpole of a post mill at right angles to the sails, and connected by gearing to wheels running on a track around the mill. When the wind veers, it strikes the sides of the vanes and turn them, causing the track wheels to turn the mill body until the sails are square into the wind again. Fantails are also applied to tower mills, driving to a rack on the curb, and are an early example of an automatic feedback device.
DEVELOPMENTS THROUGH THE 19TH CENTURY
Improved sails. The sails of a mill are mounted on an axle or “wind shaft” inclined upwards at an angle to the horizontal. Early sails had sail cloth spread on wooden frames; in areas with abundant wood the frames were covered with removable wooden sections. Sometimes fixed wooden sails like propeller blades were used. In Portugal, the Mediterranean islands, and Turkey, jib sails were and still are used, 4 to 12 per mill. The cloth are wrapped around lain poles and braced to the poles next to them, with each pole braced to a bowsprit on the front of the wind shaft.
Early cloth-spread sails had a constant angle of weather; sails with a twist like a propeller were developed empirically. The English engineer John Smeaton in 1759 was the first to scientifically investigate the design of windmill sails and waterwheels. He proposed five sails instead of the usual four. Six and eight-sailed mills also were built. Smeaton also introduced cast iron into millwork. In 1772 Andrew Meikle, a Scot, invented his “spring sail”, with shutters like a venetian blind hinged in the sail frame and controlled by a spring on each sail. Each spring had to be individually tensioned to requirements while the mill was at rest; once set, the shutters “spilled the wind” if wind pressure became too high and thus were, within limits, self-regulating.
In 1779 Stephen Hooper of England utilized roller blinds instead of shutters and added remote control. A rod passing through the wind shaft was operated manually by chain from the ground to enable all the blinds to be adjusted simultaneously without stopping the mill; the action was similar to opening and closing an umbrella. In 1807 Sir William Cubitt combined Meikle’s shutters with Hooper’s remote control and, by hanging varying weights on one side of the chain, made regulation automatic. These “patent sails” and the fantail were adopted only in England and northwest Europe.
Air brakes and centrifugal governors. In 1860 R. Catchpole of England successfully applied air brakes to patent sails. The idea was revived in The Netherlands after the application of airfoils to the leading edges of mill sails was initiated in 1927 by A.J. Dekker. Other design of airfoils followed, notably one that automatically limits the maximum speed of the sails. These devices greatly increased the output of mills by enabling them to do useful work in lighter winds. With the same object, E.L. Burne in England at the turn of the 20th century; K. Bilau in Germany after World War I; and G. Ten Have and van Reit in The Netherlands after World War II produced sails hinged on the longitudinal axis, with automatic operation. A sail design used in France featured folding wooden slats similar to a parallel rule and remotely controlled. A feedback mechanism first applied to millstones was the centrifugal governor, invented in England by Tomas Mead in 1787 to maintain the preset gap between millstones, hence controlling the fineness of grinding. With a combination of fantail, patent sails, governor, and the wind-powered sack-hoist the windmill may be said to have been first automatic factory.
Widespread applications. The most important use of the windmill was for grinding corn; in certain areas land drainage and water pumping were equally important. There were, however, many industrial uses: sawing timber; pressing oil from seeds; making paper; polishing stone; grinding paint, snuff, spices, chalk for whiting, and calcined flint for pottery; and, in Finland, making wooden shingles. In the Zaan district of the Netherlands in the 19th century there were at one time as many as 900 corn and industrial windmills, the highest concentration known.
Windmills introduced into other lands by European settlers were used for grinding corn in the United States, Canada, Australia, and New Zealand, while sugarcane-crashing mills were built by the hundreds in the West Indies. Windmills were to be found in most flatlands of Europe. Some of the most curious were the post mills of northwestern Russia, supported by a pyramid of logs and with the wind shaft carrying the sails in the lower-right corner and the drive from below the stones. Horizontal mills were at one time used in Poland and Portugal, although none survive. Horizontal mills were built also on the farms of Nebraska and Kansas for water raising and other purposes, as were the “jumbo” mills, which resembled large, wide, vertical waterwheels with the lower half shrouded in a wooden box only the upper half exposed to the wind.
Invention of the wind pump. A wind pump was introduced in the United States by Daniel Halladay in 1854; fabricated in steel by Stewart Perry in 1883, it was adapted around the World because, though relatively inefficient, it was inexpensive and reliable. The designs consist of a number of small vanes set radially in a wheel. Governing is automatic: a tail vane keeps the wheel facing the wind, while the mill wheel is set in such a way that, as the wind veers, the mill turns on its vertical axis, reducing the effective area and keeping the speed constant.
MODERN DEVELOPMENTS
The windmill has been used as a source of electrical power since P.La Cour’s mill was built in Denmark in 1890, with patent sails and twin fantails on a steel tower. After World War I experiments were carried out with windmills having sails of airfoil section – that is, resembling the blade of an airplane propeller. In 1931 a windmill of this type was built in the Crimea, and the power generated was fed into the low-voltage side of the local supply. Experimental twin-bladed mills were run in the United States in the 1940s, notably one at Grandpa’s Knob, Vermont; in England in the 1950s at Costa Head, Orkney; and in France. The most successful generator was developed in Denmark under J.Juul, with three blades braced to each other and to the bowsprit in front of the wind shaft. In the Netherlands experiments have been carried out by F.G.Pigeaud in adapting old-fashioned corn mills to use surplus energy to generate electricity. An asynchronous motor is used that can drive the mill or be driven as a generator. The gearing incorporates an overrunning clutch so that the motor will not drive the mill sails. The cap is turned by a servomotor controlled by a wind vane. A traditional mill on the island of Texel, The Netherlands, has been converted to generate power only; it can be remotely controlled from the local power station.
The traditional windmill survives in use with increasing difficulty and in diminishing numbers, mainly in areas with a peasant economy. The generation of electricity by wind power has not made the headway that was expected after World War II, but the steel wind pump is wide-spread where electric power is not economically available.
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