Centrifugal governor of a windmill, 1780-1820.
© Science Museum / Science & Society Picture Library
This device was used to control the gap between millstones in a windmill. The windmill's sails powered a rope that ensured the governor was constantly turning. While it was spinning, centrifugal force pushed the two weights outward, raising the bos, which was attached to the millstone. Governors were also used in steam engines to control the flow of steam and maintain a steady engine speed.