Steam Power's Long Run: Hero's Aeolipile to Watt and the Rankine Cycle

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Steam began as a novelty: in first-century Roman Egypt, Hero of Alexandria described the aeolipile, a sphere spun by jets of steam, and later inventors such as Taqi al-Din in 1551 and Giovanni Branca in 1629 used steam to turn spits and models. Real demand came from Britain's coal mines, which flooded as shortages of wood pushed miners deeper. Jerónimo de Ayanz patented a steam pump for mines in 1606, Thomas Savery built the first commercial one in 1698 with boilers that tended to burst and a lift of about 30 feet, and in 1712 Thomas Newcomen's atmospheric engine condensed steam under a piston so air pressure did the work, wasting fuel by cooling the cylinder each stroke.

Between 1763 and 1775 James Watt, repairing a Newcomen model at the University of Glasgow, added a separate condenser that cut coal use by about 75%, then geared the engine for rotary motion so factories no longer needed rivers. When his patent expired in 1800, high pressure took over: Richard Trevithick ran the first full-scale railway locomotive at Penydarren in 1804, George Stephenson followed with Locomotion and the Rocket, and ships adopted compound engines. Charles Parsons's turbine replaced the piston, and the Rankine cycle now runs coal and nuclear plants, condensing steam so a pump can return it to the boiler as liquid.

• Newcomen's engine relied on 14.7 pounds per square inch of air pressure pushing into a partial vacuum.
• A lead fusible plug in the firebox crown melted if water ran low, venting steam to douse the fire before the boiler failed.
• Superheated boiler water flashing to steam expands about 1,600 times, which is why early explosions leveled buildings.
• Trevithick's 1804 locomotive hauled 10 tons of iron and 70 passengers.
• Pumping condensed water back into a power plant boiler uses only about 1% to 3% of the turbine's output.