Gasoline-powered engines haven't always been the standard. The first vehicles not to depend on animal power depended instead on steam. Reliable if a bit cumbersome, steam engines were used throughout the eighteenth and nineteenth centuries, and even into the twentieth, to power locomotives, factories, and ships -- including the RMS Titanic and the famed warship the USS Monitor.

Not all steam engines were alike. The Titanic and many later steam-powered ships used so-called turbine engines. The USS Monitor, the Union Army's secret battleship during the early stages of the Civil War, used a much more common type of engine, called a reciprocating, or piston, steam engine. This was the same type of engine used by steam trains.

The principle of the piston steam engine is simple: Water is heated by a coal or wood fire, producing steam. The steam builds up and becomes pressurized inside the engine. Valves at various locations control the movement of the steam through the system. An open valve allows steam to pass; a closed valve keeps the steam from escaping.

Double-acting steam engines, like the one illustrated in this activity, are particularly efficient because they allow steam to act alternately on both faces of the piston. The double-acting steam engine relies on a special type of valve called a slide valve. The slide valve allows high-pressure steam to enter one side of the cylinder, where it then pushes the piston to the opposite side. At the same time, the valve allows the steam that filled the opposite side of the cylinder during the previous stroke to escape through an exhaust pipe. Because the piston's action operates the slide valve, the movement of these two components remains perfectly synchronized.

As the piston moves back and forth in the cylinder, it pushes or pulls a drive rod, which attaches to a flywheel. The drive rod is attached to a point that is off center on the flywheel. As a result, the drive rod's straight side-to-side or up-and-down action causes the flywheel to turn.