The Sweeps
The shuttered sweeps fitted to Shipley Mill are of the type known as Patent Sails. Invented by an engineer called William Cubitt in 1807, their big advantage is that they can be adjusted without the need to stop the Mill. 
Each of the four sweeps carries a set of shutters, which can be
opened and closed like a Venetian blind, by the movement of the uplongs, long wooden rods to which the shutters are connected. At the centre of the four sweeps, the uplongs are coupled to the spider, which connects through links and triangles to the striking rod, an iron bar that goes right through the windshaft which carries the sweeps. 
The speed of the Mill can be regulated to some
extent by altering the weight; a heavy weight will hold the shutters tightly closed in a light breeze, while a lighter weight can be used in a strong wind to allow the shutters to open slightly in the gusts. Ten to twelve turns per minute of the sweeps is a good.