CHAPTER II.


TYPES OF WINDMILL

Setting aside prairie wind pumps, wind turbines and other recent developments, traditional windmills fall into three broad categories, the ‘post mill’, the ‘smock mill’ and the ‘tower mill’.

 
POST MILLS

Post mills were the earliest type of windmill to reach Britain, probably late in the 12th century. They are so named because the body of the mill is supported on and revolves around an upright post. This enables the mill to be turned to face into the wind ― a task called winding or luffing ― where its sails can generate the most torque. [2] Many examples remain throughout Western Europe, those at Pitstone (plate 1), at Brill (plate 4) and at Chinnor (plate 7) being nearest to Tring.

Fig. 2.2: the parts of an open trestle post mill.

The mill’s supporting post needs to be a substantial piece of timber to carry the weight of the mill’s superstructure and machinery.  That which supports Pitstone mill was cut from a single tree, some 17 feet long by 33 inches diameter at its base, comparable in girth to a sailing ship’s mast.  The post is capped with a bearing, which permits rotation and, via a massive wooden cross-beam called the ‘crown tree’, supports the full weight of the mill.

In fig. 2.2, the post appears to rest upon the cross-trees beneath it, but this is not so.  Were it to be, the entire weight of the mill would bear down on their centre, which would eventually fracture under the pressure.  Thus, the post is suspended slightly above the cross-trees by four ‘quarter bars’, which are mortised into it and which also maintain its vertical position.  By this means the mill’s weight is transferred downwards evenly through each of the quarter bars and onto the four brick piers.  The cross-trees, which are held in a permanent state of tension by the outward thrust of the mill’s weight, prevent the quarter bars from spreading, a clever resolution of forces.

Fig 2.3: main post, Pitstone Mill, Buckinghamshire.



Fig. 2.4: base of main post and cross trees, Pitstone post mill, Buckinghamshire.

Many post mills eventually had a brick roundhouse built about their base (plate 4) to protect their timber supports from the weather and to provide a covered loading bay.  Those without a roundhouse are usually described as ‘open trestle’.

Fig. 2.5: open trestle post mill at Brill.
It later acquired a roundhouse.

 
SMOCK MILLS

Due to its distinctive sloping sides, usually six or eight in number, this type of windmill came to be named after the canvas and cotton smocks worn by farm labourers of the time.  As with the earlier post mill, the sides of a smock mill are clad with horizontal weatherboarding, although there are some examples of vertical cladding.  The mill’s wooden body sits on a brick base to protect it against rot and to provide support for its upright posts.  The base can be a substantial structure, that supporting Union Mill (fig. 2.6) being a four-storey building.

Fig. 2.6: Union Mill, Cranbrook, Kent, Britain’s tallest smock mill.
Built in 1814, it is now open to the public.

The main internal structure of a smock mill consists of its cant or corner posts (plate 11), which extend to its full height, converging as they go up.  The cant posts are secured at the bottom to a wooden sill, which is fixed firmly to the brick base.  This joint between post and sill poses a problem; because the cant posts lean inwards they throw the mill’s weight outwards at the sill, where a weak joint can result in posts slipping away and toppling the mill.

Unlike the post mill, in which the whole body of the mill needs to be turned to wind its sails, only the top, or cap, of a smock mill needs to rotate.  The small cap also allows better airflow around the sails, which contributes to a more efficient and stable source of power, especially when winding is performed automatically by a small windmill at the rear of the cap known as a fantail (see fig. 2.6).  Because the body of the mill doesn’t have to be rotated, it can be larger than a post mill, thus allowing more space for machinery.

Smock mills are found throughout Western Europe; in the U.K. they were particularly common in Kent.  The nearest smock mill to Tring is at Lacey Green near Princes Risborough (Chapter XII.).

 
TOWER MILLS

The earliest record of a tower mill in Britain dates from the late 13th century and refers to a mill at Dover, possibly at the Castle.  In that age, windmills were sometimes built on the sides of castles and of towers in fortified towns to protect them from attack.

A tower mill differs from a smock mill in that it is of brick or stone construction and usually round in shape, although there is a fine example of an octagonal tower mill at Wendover (plate 22).  Because of its masonry construction, a tower mill requires increasingly massive walls towards its base to support the weight of the upper storeys; those at Wendover mill are three feet thick at the base.   It is therefore unsurprising that tower mills were much more expensive to construct than wooden post and smock mills, and probably for this reason they did not become prevalent until the 18th and 19th centuries.  However, this type of construction offers the advantage of a more robust frame that can support larger sails and resist harsher weather.

In common with the smock mill, the tower mill is topped by a moveable cap, that at Stembridge Tower Mill at High Ham, Somerset, being the last example of a thatched cap.

The nearest tower mill to Tring is Goldfield Mill (Chapter VIII) on Icknield Way, near to the junction with Miswell Lane.  Slightly further afield are examples of tower mills at Hawridge (Chapter IX) and at Wendover (Chapter X).  These three mills have been converted into private dwellings, but the tower mill at Quainton (Chapter XI) has been restored to working order and is open to the public.