text: Steam Engine, mechanical device used to transfer the energy of steam into mechanical energy for a variety of applications, including propulsion and generating electricity. The basic principle of the steam engine involves transforming the heat energy of steam into mechanical energy by permitting the steam to expand and cool in a cylinder equipped with a movable piston. Steam that is to be used for power or heating purposes is usually generated in a boiler. The simplest form of boiler is a closed vessel containing water, which is heated by a flame so that the water turns to saturated steam. The ordinary household-heating system usually has a boiler of this type, but steam-generating plants used for power purposes are more complex in design and are equipped with various auxiliary devices. The efficiency of a steam engine is generally low, and therefore, in most power generation applications, the steam engines have been replaced by steam turbines \n\nSteam engines also improved significantly. In early steam engines, steam from the boiler was directed to a cylinder, where it drove the movement of a single piston before it was expelled. These single-expansion engines wasted some of the steam’s energy. More efficient double-expansion engines used the steam expelled from one cylinder to power another cylinder. By 1873 even more efficient triple-expansion engines came into use.