Steam power developed over the past 2000 years from a simple turbine described in the 1st Century to steam piston systems used as mine pumps in the early 1700's, James Watt's improved engine in 1765, steam-powered paddle-wheel boats, and then steam-powered vehicles and locomotives, including Richard Trevithik's first railway in 1804 and the Baltimore and Ohio Railroad's Tom Thumb in 1830, with locomotives growing larger over the next 130 years to culminate in Union Pacific's 4-8-8-4 Big Boy. In a modern steam locomotive, a fireman and an engineer operate the engine by generating steam in the firebox, where wood, coal, or later oil fuel heats gases that pass through boiler and flue tubes surrounded by water in the boiler; the steam collects in the steam dome, then the engineer opens the throttle so steam flows through the dry pipe and superheater tubes into the steam chest, where the valve spindle directs pressure to either side of the piston. The piston's motion drives connecting rods, the main wheel, and side rods, while exhaust rises through the blast pipe and smoke stack, drawing more air through the firebox. Speed, direction, and efficiency are controlled by the throttle lever and reversing lever, or Johnson Bar, whose action through the Walschaerts Gear, radius arm, expansion link, eccentric crank, eccentric rod, union link, and combination lever governs how steam is alternated between the sides of the piston and how efficiently power is used.