Small Gods

65

Urn’s steam engines are more or less identical to the ones that were described by Archimedes and used in ancient Ephebe — I mean Greece. These engines also used copper spheres as heating vessels, and these spheres did, in fact, have a regrettable tendency to explode, which is what limited their use until some bright person thought of adding overpressure relief valves.

These steam engines never really caught on, because of various practical problems and the greater cost-effectiveness of slave-power.

The contraption with revolving balls Urn is thinking of in the sentence quoted above was identified by several readers as something called a speed governor, invented by James Watt. This consists of two balls spinning on two opposite movable arms around a rotating central axis. When the centrifugal force gets large enough to lift the balls up, the movement opens a safety valve that lets off the steam, causing the rotation to slow down and the balls to come down again, closing the valve, etc. - a simple but ingenious negative feedback device.