Friday, 19 February 2010

Windmills


A windmill is a machine which converts the energy of wind to rotational motion by means of adjustable vanes called sails. (pictured right: This Dutch windmill in Amsterdam was built in 1757 and is identified a De 1100 Roe. It is a smock mill of the type called by the Dutch a grondzeiler (ground sailer) since the sails almost reach the ground. The main use is for a grinding mill powered by the wind, reducing a solid or coarse substance into pulp or minute grains, by crushing, grinding, or pressing. Windmills have also provided energy to sawmills, paper mills, hammermills and windpumps for obtaining fresh water from underground or for drainage (especially of land below sea level).

History
The windwheel of Heron at Alexandria marks one of the first known instances of wind powering a machine in history. The first practical windmills were the vertical axle windmills invented in eastern Persia, as recorded by the Persian geographer Estakhri in the 9th century. The authenticity of an earlier anecdote of a windmill involving the second caliph Umar (AD 634–644) is questioned
on the grounds that it appears in a 10th-century document. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were quite different from the later European horizontal-axis versions. (Pictured left: The windmills of Campo de Criptana were immortalized in chapter VIII of Don Quixote.)
Some popular treatments of the subject have speculated that the Afghanistan-style vertical-axle mills were used throughout the Caliphate by the ninth century and spread to Europe through Islamic Spain. This has been denied by the specialist of European medieval technology, Lynn White Jr., who points out that there is no evidence (archaeological or documentary) that the Afghanistan-style vertical-axle windmill spread as far west as al-Andalus, and notes that "all Iberian windmills rotated on horizontal axles until towards the middle of the fifteenth century," which is an indication of their European origin (see below).
A similar type of vertical shaft windmill with rectangle blades, used for irrigation, can also be found in 13th-century China (during the Jurchen
Jin Dynasty in the north), introduced by the travels of Yel├╝ Chucai to Tukestan in 1219.

Fixed horizontal-axle windmills
Fixed windmills, oriented to the prevailing wind were extensively used in the Cyclades islands of Greece. The economies of power and transport allowed the use of these 'offshore' mills for grinding grain transported from the mainland and flour returned. A 1/10th share of the flour was paid to the miller in return for his service. This type would mount triangular sails when in operation.
In northwestern Europe, the horizontal-axle or vertical windmill (so called due to the dimension of the movement of its sails) dates from the last quarter of the 12th century in the triangle of northern France, eastern England and Flanders. Lynn White Jr. claims that the first certain reference to the European horizontal-axle windmill is dated to 1185 in Weedly
, Yorkshire. (This predates Joseph Needham's claim that the earliest known reference is from the 1191 chronicle of Jocelin of Brakelond, in which a Dean Herbert of East Anglia supposedly competed with the mills of the abbey of Bury St Edmunds). These earliest mills were used to grind cereals. The evidence at present is that the earliest type was the sunk post mill, so named because of the large upright post on which the mill's main structure (the "body" or "buck") is balanced. By mounting the body this way, the mill is able to rotate to face the wind direction; an essential requirement for windmills to operate economically in North-Western Europe, where wind directions are variable. (Pictured right: An eight sailed windmill at Heckington, Lincolnshire UK) By the end of the thirteenth century the masonry tower mill, on which only the timber cap rotated rather than the whole body of the mill, had been introduced. In the Netherlands these stone tower like mills are called "round or eight-sided stone stage mills, ground-sailers (windmills with sails reaching almost down to the ground), mound mills, etc." (Dutch: ronde/achtkante stenen stelling molens, grond-zeilers, beltmolens, etc.). Dutch tower mills ("torenmolens") are always cylindrical (such as atop castle or city wall towers). Because only the cap of the tower mill needed to be turned the main structure could be made much taller, allowing the sails to be made longer, which enabled them to provide useful work even in low winds. Such mills often have a small auxiliary set of sails called a fantail at the rear of the cap and at right angles to the sails; this rotates the cap through gearing so the sails face into the wind.

Multi-sailed windmills
The majority of windmills had four sails. An increase in the number of sails meant that an increase in p
ower could be obtained, at the expense of an increase in the weight of the sail assembly. The earliest record of a multi-sailed mill in the United Kingdom was the five sail Flint Mill, Leeds, mentioned in a report by John Smeaton in 1774. Multi-sailed windmills were said to run smoother than four sail windmills. In Lincolnshire, more multi-sailed windmills were found than anywhere else in the United Kingdom. There were five, six and eight sail windmills. (Pictured left: Windmill at the National Ranching Heritage Center in Lubbock, Texas.)
If a four sail windmill suffers a damaged sail, the one opposite can be removed and the mill will work with two sails, generating about 60% of the power that it would with all four sails. A six sail mill can run with two, three, four or six sails. An eight sail mill can run with two, four, six or eight sails, thus allowing a number of options if an accident occurs. A five sail mill can only run with all five sails. If one is damaged then the mill is stopped until it is replaced. Apart from the UK, multi-sail mills were built in Malta and the USA.