A machine for converting the heat energy in steam to mechanical energy of a moving mechanism, for example, a shaft. The steam engine dominated the industrial revolution and made available a practical source of power for application to stationary or transportation services. The steam power plant could be placed almost anywhere, whereas other means of power generation were more restricted, experiencing such site limitations as an elevated water supply, wind, animal labor, and so on. The steam engine can utilise any source of heat in the form of steam from a boiler. It was developed in sizes which ranged from that of children's toys to 25,000 hp (18.6 MW), and it was adaptable to pressures up to 200 lb/in.² (1.4 megapascals). It reached its zenith in the nineteenth century in stationary services such as drives for pumping plants; drives for air compressor and refrigeration units; power supply for factory operations with shafting for machine shops, rolling mills, and sawmills; and drives for electric generators as electrical supply systems were perfected. Its adaptability to portable and transportation services rested largely on its development of full-rated torque at any speed from rest to full throttle; its speed variability at the will of the operator; and its reversibility, flexibility, and dependability under the realities of stringent service requirements. These same features favoured its use for many stationary services such as rolling mills and mine hoists, but the steam engine's great contribution was in the propulsion of small and large ships, both naval and merchant. Also, in the form of the steam locomotive, the engine made the railroad the practical way of land transport. Most machine elements known today had their origin in the steam engine: cylinders, pistons, piston rings, valves and valve gear crossheads, wrist pins, connecting rods, crankshafts, governors, and reversing gears.