Brixton Windmill is a tower mill – a design that was first introduced in the late 13th century. The advantage of a tower mill over the earlier post mill was that it was not necessary to turn the whole body of the mill with all its machinery into the wind.
The windmill is 15 metres (49.5 feet) high and is arranged over five storeys – a modest size compared with England’s tallest tower mill at Moulton in Lincolnshire, which is nine storeys and 30 metres high. The base of Brixton Windmill is 6.6 metres (22 feet) in diameter and 3.65 metres (12 feet) at the top.
Brixton Windmill has four sails, or sweeps, but some millwrights on other mills experimented with more sails to increase efficiency. Some tower mills have six or even eight sails to increase the surface area that catches the wind.
Generally, tower mills were more powerful than watermills but less reliable due to the unpredictable nature of the wind.
The cap (at the top) is an attractive boat-shaped structure – timber framed and weatherboarded.
The cap can be turned using a chain arrangement; you can see the wheel of this chain system at the back of the cap.
South of the River Thames sails were traditionally called ‘sweeps’. The original sails were removed and burnt in 1864. The design of the newly restored sails was based on old photographs of Ashby’s Mill showing two different pairs of sails – common sails and patent sails.
The trellis-like common sail was the standard sail in the 18th century. Canvas sailcloth was laid along each sail frame. In light winds the whole canvas would be unfurled; in stronger winds the sails would be ‘reefed’, or rolled back, so that only a part of the surface was exposed.
The common sail was light, simple, inexpensive and fairly efficient, so its use continued well into the 20th century. However, to adjust them from the ground, each sail had to be stopped in turn in a vertical position. So with four sails, the rotation had to be stopped four times.
The invention of patent sails in 1807, by William Cubitt, eliminated this problem. Patent sails are made up of shutters that can be opened and closed on the same principle as a Venetian blind. This design provided a system for automatically changing the amount of sail exposed as the strength of the wind varied. This saved time and meant that the miller could produce more flour.