Wind power and invention

THE ROAD FROM ROYSTON to Wendens Ambo is both winding and hilly, as well as passing through attractive cultivated countryside. East of the village of Barley (in Hertfordshire), we reached the crest of a hill and saw ahead of us a lovely windmill painted white standing on the side of the next hill.

We stopped in a small car park beside the mill that stands on the western edge of Great Chishill (Cambridgeshire) and slightly below the village. The Great Chishill Mill is currently undergoing restoration, although what we saw of it looked in good condition. The mill was built in 1819 on the site of an older mill. It incorporates some timber from an earlier mill built in 1721. It is a fine example of an open-trestle post mill, one of seven surviving examples in the UK. Of these seven, it is unique in having a fan tail. Let me try to explain this.

The mill housing with its four great sails is mounted high on a central post around which it can rotate. An arm, the ‘tail-beam’, projects from the rear of the mill housing downwards towards the ground. Two wheels are attached to the lower end of the arm. When the wheels are made to move around a circular track in the middle of which stands the base of the post supporting the mill, the windmill can be rotated so as to position it best to benefit from the prevailing wind. Usually, the mill is shifted by hand, but this is not the case at Great Chishill. A second smaller windmill, the fan tail, rotates in a plane perpendicular to that in which the main sails rotate. When the wings of the fantail catch the wind, they rotate. As they rotate, their movement is transmitted via cogs and rods to the wheels attached to the tail-beam that projects from the mill house. The wheels rotate, and thereby turn the main mill sails so that they catch the wind. Thus, the fantail automatically repositions the windmill when the wind changes direction.

Prior to the invention of the fan tail system, shifting the mill around on its post involved heavy manual labour. When Alfred Andrews inherited the Great Chishill mill from his father Job, he installed the fan tail system (www.greatchishillwindmill.com/about-the-windmill.html). Long before he did this, the fan tail mechanism was invented in 1745 by Edmund Lee (died 1763), a blacksmith working near Wigan, England. Although only one of the surviving post mills is fitted with a fan tail, other varieties of windmills can be found fitted with a fantail that repositions the primary sails of the mills.

Great Chishill village is close to the post mill and is well worth a visit. It has a fine parish church, St Swithun, founded in 1136 and some fine old houses. Some of these have thatched roofs decorated with animals made of thatch including a pair of boxing hares, a pheasant, and a cat. Once again, we have set out on a trip, this time to Saffron Walden, and chanced upon something fascinating and quite unexpected along our route.

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