 This had been invented by Christiaan Huygens in the 17th century to regulate the gap between millstones in windmills depending on the speed of rotation, and thereby compensate for the variable speed of grain feed.With the invention of the low-pressure stationary steam engine there was a need for automatic speed control, and James Watt’s self-designed "conical pendulum" governor, a set of revolving steel balls attached to a vertical spindle by link arms, came to be an industry standard.This was based on the millstone-gap control concept.