Special Terms. Steam:The invisible vapor into which water is changed when it is heated to the boiling point

Steam: The invisible vapor into which water is changed when it is heated to the boiling point.

Boiler: A vessel in which water is boiled to produce steam.

Condense: To return to a liquid state, as when steam reverts to water. The process is condensation and a device that causes the process to take place is a condenser.

Vacuum: A space devoid of matter. Vacuums are partial, not com­plete.

Valve: A device that opens or closes to control the flow of liquids or gases.

Piston: A rod or disk that can move up or down within a hollow cylinder.

Walking Beam: Alever that works on the seesaw principle, with in­put on one side and output on the other side of the fulcrum.

Crankshaft: A shaft driving or driven by a crank.

Sun-and-Planet Gears: An arrangement in which a smaller gear (the planet) rotates around a larger gear (the sun).

Safety Valve: Avalve that opens automatically when steam or other pressure exceeds a predetermined amount.

Internal Combustion Engine: A heat engine in which combustion (burning of fuel) takes places within a cylinder, as in engines that burn gasoline. Steam engines are not internal combustion engines.

Turbine: A rotating machine with blades set and kept in motion water, steam, or gas.

Generator: A machine that produces energy by rotating through it magnetic field; the shaft of the generator is often turned by a turbine.