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Wooden hollow-post windmills were used for lifting water and irrigation. One of the early hollow-post windmills in South Africa was built by J.C. Poortermans in 1848 at the home of a Mr. Brink at Saldanha Bay. (Not the illustration on the left).
It consisted of a vertical hollow post which was supported by sloping ladders against two opposite sides and a sloping metal strut against each of the other sides. The head of the post carried four latticed sails which, by means of a cranked shaft, lifted and lowered a long steel rod passing down the hollow post to the pump or other lifting mechanism below.
This type of mill is supported and pivoted on a single stout upright post, so that the whole “buck” or body of the mill is rotated in order that the sails could be faced square into the wind at all times.
The post-mill spread rapidly throughout Western Europe, as far south as the river Loire. 
Probably the only post-mill erected in South Africa was Heath’s Mill near Grahamstown, which was built some time before 1817, when it was sold to Piet Retief by a Mr. Heath, the builder and owner.
The post-mill is described by John Reynolds as  ‘one of the most daring and ingenious works of the medieval carpenter’. Basically it consists of a small timber framed building housing the mill machinery and carrying the sails. The building is balanced and pivoted on a massive vertical central post set on a pair of cross-trees, the ends of which are carried on four upright stone or brick pillars. The main post is further supported by four raking struts.   
Across the top of the main post is a stout horizontal beam, the ‘crown tree’, which carries the entire weight of the buck and rotates on a gudgeon at the top of the main post.  
Access to the buck is by a flight of wooden steps which are hinged and could be lifted clear of the ground. A heavy beam, the ‘tail pole’, projected through the steps, and when the miller wished to turn the buck into the wind he lifted the steps and pushed the tail-pole, thus rotating the buck into the desired position. The steps are then lowered and the tail-pole anchored to a post