[jamesakirk]

Norbert Wiener discussed this in Cybernetics:

"Another example of a purely mechanical feedback system — the one originally treated by Clerk Maxwell — is that of the governor of a steam engine, which serves to regulate its velocity under varying conditions of load. In the original form designed by Watt, it consists of two balls attached to pendulum rods and swinging on opposite sides of a rotating shaft. They are kept down by their own weight or by a spring, and they are swung upward by a centrifugal action dependent on the angular velocity of the shaft. They thus assume a compromise position likewise dependent on the angular velocity. This position is transmitted by other rods to a collar about the shaft, which actuates a member which serves to open the intake valves of the cylinder when the engine slows down and the balls fall, and to close them when the engine speeds up and the balls rise. Notice that the feedback tends to oppose what the system is already doing, and is thus negative. "

(https://archive.org/details/CyberneticsOrCommunicationAndCon...)

[jhayward]

> consists of two balls attached to pendulum rods and swinging on opposite sides of a rotating shaft

Random aside: This is the origin of the phrase "balls out" to mean going full-blast. When the locomotive was putting out maximum power the spinning balls on the governor would be furthest out, inscribing their largest circle.