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No. 1572:
High-Pressure Steam Engines

Today, the first locomotive. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Think about the word you used for a train when you were a child -- the word choo-choo. Choo-choo was the noise made by steam leaving the cylinders, which were mounted down by the wheels. If you've never seen that in real life, you have seen it in movies. A conductor shouts, All aboard, steam gushes about the wheels, and the train starts to move. That choo-choo sound reflects two ideas that converged around 1800, after steam engines had been in use for a hundred years. One was the idea of running steam engines at high pressure; the other idea was using them for transportation.

The first steam car was made in 1769, before Watt, by French military engineer Nicholas Cugnot. Most steam engines were then huge two-story structures. So it's not surprising that Cugnot's car was a brute. It carried four people at two miles per hour. It was meant to pull field artillery, but it was clumsy and cumbersome.

In 1784, William Murdoch, who worked for Watt, used a Watt engine to produce a better car -- lighter and faster. Watt opposed using steam for transportation. He patented the idea only so he could put it on ice. Nor did he like the idea of high-pressure steam, and that's what you ultimately needed for a vehicle. High pressures were dangerous.

Early steam engines all depended to some extent on condensing steam to create a vacuum. Steam not only pushed the piston out of the cylinder. It also sucked the piston into the cylinder as it condensed. Engines were so large because low-pressure steam took up space. When pressure reached fifty or a hundred pounds per square inch, engines could be a lot smaller.

Late eighteenth-century precision boring mills finally made tight-fitting high-pressure pistons possible. Cornish inventor Richard Trevithick and American millwright Oliver Evans both made high-pressure engines just after 1800. So it finally made sense to fit a steam engine into a vehicle. Instead of condensing steam to create a vacuum, builders simply blew spent steam into the atmosphere -- making that choo-choo sound.

Trevithick and Evans both used their engines to drive primitive cars. Then Trevithick realized he could use steam to replace the horses that drew carts on England's rail system. He made the first successful locomotive in 1804. In 1808, he ran a little closed-circuit demonstration railroad in London -- a sort of carnival ride with a locomotive called the Catch-me-who-can. It moved at a swift twelve miles per hour. After that, steam railways spread like ivy, with America close on British heels.

But we were more rash than the English. We drove steam pressures up; and across the land we went. The familiar choo-choo sound of spent steam was, in fact, the sound of new ideas. And the idea of eliminating condensation -- of wasting part of steam's motive force -- was what it took to bind a sprawling continent together.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

For the story of Evans and the high-pressure engine, see, e.g., Pursell, C.W., Jr. Early Stationary Steam Engines in America: A Study of the Migration of a Technology. Washington, D.C.: Smithsonian Institution Press, 1969.

For more on Richard Trevithick, see, e.g., Derry, T. K., and Williams. T. I. A Short History of Technology, New York: Oxford University Press, 1960, 1975.

Lardner, The Rev. Dionysius. The Steam Engines Familiarly Explained and Illustrated, with additions and notes by James Renwick, LL.D. Philadelphia: Carey and Hart, 1836.

This is a greatly reworked version of Episode 109.

Oliver Evans's High-Pressure Columbian Engine

Oliver Evans's High-Pressure Columbian Engine
From the 1832 Edinburgh Encyclopaedia