# Steam Engine

> Heat turned into motion

- Date: 1712
- Era: Industrial age
- Domain: [Energy & Prime Movers](https://lastcodex.org/en/domains/energy/)
- Rebuild stage: IV (Science & industry)
- Difficulty: 5/5
- URL: https://lastcodex.org/en/inventions/steam-engine/

A steam engine turns the heat of a fire into movement by using steam to drive a piston. Thomas Newcomen's engine of 1712 pumped water out of mines, and James Watt's improvements from 1765 onwards made steam power cheap enough to drive the factories of the Industrial Revolution.

![Steam Engine · Heat turned into motion](https://lastcodex.org/_astro/plate.B21HsZfP_Z1KcMmY.webp)

## Legend

1. Boiler
2. Cylinder
3. Piston
4. Great beam
5. Injection water cistern
6. Pump rods

## What it is

A steam engine is a machine that uses steam from boiling water to push a piston, and so turns fuel into mechanical work. The first ones pumped water out of mines; later engines drove spinning mills and other machinery, and propelled ships and locomotives.

The engine on the plate is the type Thomas Newcomen built in 1712. It is called an atmospheric engine, because the work is done by the pressure of the air outside, while the steam is only used to make a vacuum. The boiler works barely above atmospheric pressure, so it is far less likely to burst than the high-pressure boilers of the nineteenth century.

## How it works

![Hero's aeolipile: a bronze sphere spinning on its pivots above a heated cauldron, with steam jets on opposite sides](https://lastcodex.org/_astro/v1.CIzEdvek_ZV4zFy.webp)
*Hero of Alexandria's aeolipile, in the first century AD, spun on jets of steam but did no useful work.*

The engine is built around a vertical cylinder, open at the top, with a piston inside. The piston hangs by a chain from one end of a great timber beam that rocks on the wall of the engine house. From the other end hang the pump rods, which go down the shaft to the pumps at the bottom of the mine.

The heavy pump rods pull their end of the beam down, which lifts the piston, and steam from the boiler below fills the cylinder. A valve shuts off the steam. A jet of cold water from the cistern sprays into the cylinder, and the steam condenses at once into a little water, leaving a partial vacuum. The air above the piston now presses down harder than the thin steam left beneath it. On a cylinder half a metre across, the pressure difference could in theory reach about 20 kilonewtons, the weight of two tonnes; the useful force was well below that. The piston drops, the beam rocks, and the pump rods rise, lifting water out of the mine.

![A centrifugal governor with two iron balls on hinged arms, a sliding collar and a lever to the valve](https://lastcodex.org/_astro/v2.CxD_wzqV_Z28XSxx.webp)
*On later Watt engines, the governor's balls fly outwards as the engine speeds up and partly close the steam valve.*

When steam is let in again, it drives air out through a small snifting valve, while the condensed water drains away down a separate pipe with a non-return valve. The cycle repeats about twelve times a minute. A rod hanging from the beam, the plug tree, carries pegs that open and close the valves by themselves.

At every stroke the cold spray chills the cylinder, and the next charge of steam must heat it again before it can fill it. Most of the coal is spent on that reheating.

James Watt's answer was to condense the steam in a separate vessel, kept cold, while the cylinder stayed hot. He later closed the top of the cylinder, used steam pressure on both sides of the piston and added gears to give rotary motion, so the engine could drive machinery as well as pumps.

## How to build it

An atmospheric engine needs a coppersmith or boilermaker, a founder able to cast and bore a cylinder, a carpenter for the beam and a mason for the house.

**Make the boiler.** Historically this was a riveted copper or wrought-iron shell with a domed top, set in brickwork over a firegrate, with a weighted safety valve, cocks to check the water level and a feed pipe. A working reconstruction needs a boiler designed, tested and inspected by a qualified boilermaker; never improvise one.

**Cast and bore the cylinder.** The cylinder stands just above the boiler, carried on heavy beams set in the walls, and is joined to it only by a short steam pipe. Its bore must be round and smooth along its whole length, or the piston will leak; accurate boring was one of the hardest problems of the eighteenth century.

**Fit the piston.** Pack the edge of the piston with rope or leather and keep a thin layer of water on top to seal it.

**Raise the beam.** Make the beam of oak, with curved arch heads at each end so the chains pull straight up and down. Rest its central pivot on a thick wall.

**Fit the valves.** One valve admits steam and another sprays cold water from a cistern placed high in the house; a snifting valve lets out air, and a pipe with a non-return valve drains the condensed water. Work them by hand at first, then from tappets on a rod hanging from the beam.

> **Safety.** A boiler whose safety valve is blocked, wedged or overloaded can explode: never obstruct or isolate the relief path, and never add weight to the valve. Steam and hot water scald badly. Keep well clear of the beam, chains and pump rods while the engine runs, and fence the shaft.

## History and evolution

Hero of Alexandria described a sphere spun by steam jets in the first century AD, but it remained a curiosity. In 1690 Denis Papin showed that condensing steam under a piston could lift a weight. Thomas Savery patented a pump without a piston in 1698. It drew water up by condensing steam in a vessel, then pushed it higher with steam pressure, and it could only manage modest lifts.

![A sun-and-planet gear: a planet gear fixed to the connecting rod turns a sun gear on the flywheel shaft](https://lastcodex.org/_astro/v3.DZLBBIVt_18Vfzb.webp)
*Watt's sun-and-planet gear of 1781 turned the up-and-down motion of the beam into rotation.*

Newcomen, an ironmonger from Dartmouth, and his partner John Calley set up the first working engine in 1712 at a coal mine near Dudley Castle. Because Savery's patent covered raising water by fire, Newcomen's engines were built under it until it expired in 1733. More than a hundred were at work in Britain and Europe by the time Newcomen died in 1729.

In 1763 James Watt, an instrument maker at the University of Glasgow, was asked to repair a model Newcomen engine. Two years later he had the idea of the separate condenser, patented in 1769. With Matthew Boulton he began selling engines in 1776. They used about a quarter of the fuel of a Newcomen engine of the same power. Around 1800, as Watt's patent ran out, Richard Trevithick and others turned to high-pressure steam to build smaller, lighter engines, which soon moved onto rails and into ships.

## Key figures

- First working engine: 1712, Conygree coal works near Dudley, England
- Steam pressure in a Newcomen boiler: About 1 to 2 psi above the atmosphere (0.07 to 0.14 bar)
- Working rate: About 12 strokes a minute
- Newcomen engines built by 1729: More than 100
- Watt's separate condenser: Conceived 1765, patented 1769, about 75% less fuel

## Requires

- [Iron Smelting](https://lastcodex.org/en/inventions/iron-smelting/): Wider adoption around 1200 BCE; ore, charcoal and a bloomery (c. 1,200 BCE)
- [Gears](https://lastcodex.org/en/inventions/gear/): Teeth that trade speed for force (c. 300 BCE)
- [Piston Pump](https://lastcodex.org/en/inventions/piston-pump/): Water raised by valve and piston (c. 270 BCE)

## Leads to

- [Internal Combustion Engine](https://lastcodex.org/en/inventions/combustion-engine/): Small explosions, steady power (1876)
- [Refrigeration](https://lastcodex.org/en/inventions/refrigeration/): Pumping heat out of a box (1834)
- [Steam Locomotive](https://lastcodex.org/en/inventions/steam-locomotive/): Steam power on rails (1804)
- [Steam Turbine](https://lastcodex.org/en/inventions/steam-turbine/): Steam spinning the world's generators (1884)

## Sources

- [Newcomen atmospheric engine](https://en.wikipedia.org/wiki/Newcomen_atmospheric_engine), Wikipedia
- [Watt steam engine](https://en.wikipedia.org/wiki/Watt_steam_engine), Wikipedia
- [Newcomen atmospheric engine (object 1920-124)](https://collection.sciencemuseumgroup.org.uk/objects/co50900/newcomen-atmospheric-engine-atmospheric-steam-engine), Science Museum Group
- [ASME Landmark: Newcomen Engine](https://ethw.org/ASME-Landmark:Newcomen_Engine), Engineering and Technology History Wiki (ASME)
- [A new age](https://blog.sciencemuseum.org.uk/a-new-age/), Science Museum

Fact-checked on 2026-10-11.

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The Last Codex · https://lastcodex.org/en/inventions/steam-engine/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.