A steamroller is a form of road roller, a type of heavy construction machinery used for leveling surfaces, such as roads or airfields, that is powered by a steam engine. A steam engine is a heat engine that performs mechanical work using steam as its working fluid. Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Water is heated and transforms into steam within a boiler operating at a high pressure. When expanded through pistons or turbines, mechanical work is done. The reduced-pressure steam is then condensed and pumped back into the boiler. The company Aveling & Porter was the first to successfully sell the product commercially. In 1866 they produced a prototype roller with 3 foot wide rollers fitted to the rear of a standard 12 nominal horsepower traction engine. This experimental machine was described by local papers as "the world's first steamroller" and it caused a public spectacle.