The technique of using a windmill to grind corn is a complex one. Not only must the windmill sails be always set to catch the wind but their speed of rotation must be regulated so that the millstones grind at their optimum speed. Originally all these tasks were done by the miller but slowly simple semiautomatic and automatic devices were introduced to take over part of the work.

This change was most marked among British millers of the eighteenth century and it was by them that the most difficult problem, that of keeping the sails revolving at the proper speed, was solved. A centrifugal pendulum was employed to measure the angular velocity of the millstone and, working through additional mechanisms, to adjust the sail setting so as to reduce the sail speed if the angular velocity was too large or increase it if the angular velocity was too small.

This invention was taken up by James Watt and formed one of the chain of ingenious improvements to Newcomen's steam engine to which we referred in Chapter 42. Initially enclosed, to hide the secret from the eyes of Watt's competitors, the rotating centrifugal pendulum was soon proudly displayed as the visible symbol of man's control over steam.

Naturally, attempts were made to produce new or improved versions of ‘Watt's governor’, but not all were successful.