In the winter of 1763–64 the University of Glasgow handed its resident instrument maker a broken teaching model: a tabletop version of the Newcomen steam engine, used to show students the machine that pumped water from Britain's mines. James Watt, then in his late twenties, was a craftsman of quadrants, scales and flutes rather than an engineer, but he mended the model — and found it would barely run, gulping steam so greedily that its little boiler was exhausted within a few strokes. Most repairers would have shrugged that models simply behave badly at small scale. Watt treated the defect as a question addressed personally to him, and his answer changed the economics of the planet.
Thomas Newcomen's "atmospheric" engine, first set pumping at a Midlands colliery in 1712, had been the workhorse of the coalfields for half a century. It filled a cylinder with steam, sprayed in cold water to condense it, and let the weight of the atmosphere shove the piston down into the resulting vacuum. It worked — and at a pithead, where coal was nearly free, its gluttony scarcely mattered. But every spray of cold water chilled the great cylinder, and every fresh charge of steam had to spend most of itself reheating the metal before it could do anything useful. Watt quantified the waste with an instrument maker's precision, helped by conversations with his friend Joseph Black, the Glasgow chemist whose doctrine of latent heat explained why condensing steam devoured such extravagant quantities of energy.
A walk on Glasgow Green
The solution arrived, by Watt's own telling, on a Sunday walk across Glasgow Green in May 1765: keep the cylinder permanently hot, and condense the steam somewhere else — in a separate vessel, kept permanently cold and connected by a valve. He recalled, decades later, that the whole scheme was arranged in his mind before he had walked as far as the herd's house. The anecdote rests on an old man's memory of his young self, and historians handle it gently; the invention itself is not in dispute. The separate condenser was patented in 1769 under a title of magnificent dullness — "A New Invented Method of Lessening the Consumption of Steam and Fuel in Fire Engines" — and it cut an engine's appetite for coal by roughly three-quarters.
Turning the idea into reliable iron took another decade and two patrons. The first, the industrialist John Roebuck, went bankrupt; his share of the patent passed to Matthew Boulton, the Birmingham manufacturer whose Soho works was already famous for buttons, buckles and ormolu. The partnership sealed in 1775 was a perfect pairing of temperaments: Watt anxious, meticulous and prone to despair; Boulton expansive, charming and rich in nerve. Boulton lobbied Parliament to extend the patent to 1800, and when James Boswell toured Soho in 1776 his host supplied one of the great sales lines of the industrial age: "I sell here, Sir, what all the world desires to have — power." In Cornwall's copper mines, where coal arrived expensively by sea, the firm charged a royalty of one-third of the fuel savings its engines delivered — a price pegged, ingeniously, to the machine's own superiority.
Selling power by the horse
Pumping water was a mining trade; the future lay in rotation — engines to turn the shafts of mills, breweries and factories. A rival, James Pickard, had patented the obvious crank, so Watt sidestepped it with his "sun and planet" gear, then added the double-acting cylinder, in which steam pushed the piston both ways, and the centrifugal governor, whose whirling balls throttled the engine automatically — a machine regulating itself, and an ancestor of modern control engineering. Selling rotary power required a unit that customers understood, and customers understood horses. Watt reckoned a strong dray horse could sustain 33,000 foot-pounds of work per minute, and set the figure deliberately generously, so that no brewer could grumble that the engine underperformed the animal it replaced. Horsepower was born as a marketing device — and it outlived the horses.
Watt retired in 1800 when the patent expired, prosperous and garlanded, to tinker in his attic workshop at Heathfield, outside Birmingham. He had long been a fixture of the Lunar Society, the famous full-moon dining club of Midlands minds — Boulton, Erasmus Darwin, Joseph Priestley, Josiah Wedgwood — and when he died in 1819 he was mourned as a benefactor of the world. In 1882 the British Association attached his name to the scientific unit of power, which is why every lightbulb and phone charger on Earth now quietly cites him. The kettle in your kitchen states its power in watts as it boils — settling, several times a day, an argument that began with a toy engine that would not run.
Quiz nuggets
- Watt hit on the separate condenser during a walk across Glasgow Green in May 1765, after repairing Glasgow University's model Newcomen engine; his patent followed in 1769.
- Newcomen's atmospheric engine dated from 1712; Watt's separate condenser cut its coal consumption by roughly three-quarters.
- The Boulton & Watt partnership began in 1775, and Parliament extended Watt's patent to 1800.
- Watt defined one horsepower as 33,000 foot-pounds per minute — set generously so his engines would beat the horse.
- The watt was adopted as the unit of power by the British Association in 1882, six decades after his death in 1819.