"[Engine control] requires an intelligent man, an honest man, a sober man, a steady man..."
Isambard Kingdom Brunei, engineer
The steam in early steam engines was used only once; after it had pushed back the piston it was discharged into the atmosphere, A more efficient process allowed the steam to expand in two or more stages. These "compound" engines had two or more cylinders. After the steam had been expanded in the high-pressure cylinder the exhaust steam was then used to push back the piston of a following, larger-circumference, low-pressure cylinder. The two pistons were connected with cranks that enabled them to work at the required different phases. With correct size scaling, the power output per cylinder could be equalized, and the engine ran smoothly. As these systems were
rather complicated, theywere mainly used in industrial and marine engines. Some compound railway locomotives were built, but the tough operating conditions made them difficult to maintain.
Jon athan Hornblower (1753-1815) was originally an employee of Boulton and Watt and designed the first compound steam engine in 1781. Unfortunately, the early compounds were no more economical than simple single-cylinder engines. The concept was then revised by the Cornish engineer Arthur Wolf, who obtained a patent in 1805. Problems with the high- pressure cylinder meant that few such engines were used until the mid-nineteenth century.
