Steam engine governor

The two balls in the middle of this photograph are a steam engine governor. I've known about them for years but hadn't realised they were such a big deal. Apparently they were a James Watt innovation because his rotary steam engines had to be kept at a constant speed. The engine spins the ball mechanism as well as driving its load. If it goes too fast, the balls gain energy and move outwards. This pulls on the levers at the top of the ball mechanism which reduces the size of the fuel inlet. Less fuel in means the engine slows down. The balls spin slower and gravity pulls the balls down. The levers open the hole and more fuel goes through... I was amazed to find that the maths of it was done by James Clerk Maxwell and that Gibbs based his thermodynamic analysis on it. http://en.wikipedia.org/wiki/Governor_(device)