and since the eighteenth century have often been called sweeps.Two problemsTo work efficiently, the sweeps must always face directly into the wind, the direction of which often changes. The sweeps must also be able to deal with changes in the strength of the wind, which are unpredictable. In early mills, both problems required constant attention from the miller, pushing the mill around on a central post to face it into the wind, and changing the area of cloth which made up the working surface of the sails. Two inventions in the eighteenth century made the life of the miller easier. The fantail was invented in 1745. This is like a small windmill fixed to the back of the cap, but the shaft on which it rotates faces across the wind instead of into it. The fan on Stone Cross Mill, seen on the left, has eight paddle-shaped blades. When the sweeps face directly into the wind, the wind tries to blow the blades above the axle backwards, but it also tries to blow the blades below the axle backwards. Because the areas of wood above and below the axle are equal, the forces cancel each other out, and the fan stays still.Should the direction of the wind change, however, balance will be lost. If the wind comes more from your left, the bottom blade will begin to move backwards and the fan will spin. If the wind comes more from you right, the upper blades will begin to move backwards and the fan will spin in the opposite direction. On the right side of the fan's axle is a worm gear, which connects to another worm gear, the wheel of which forms the top of the tower. As the fan spins, the cap of the mill turns on rollers until the fan regains its balance and the sweeps are facing directly into the wind again. In 1772 the spring sail was developed. This works like a venetian blind. The wooden shutters which make up most of the area of the sweeps can be closed, or they can be opened at different angles to reduce the force of the wind on the sweep. This ...
