James Watt: The Engineer Who Measured the Power of an Age

Few figures of the eighteenth century left a deeper or more durable mark upon the modern world than the soft-spoken Scottish instrument maker who, between 1763 and his patent of 1769, transformed the steam engine from a wheezing pumping device into the universal prime mover of industrial civilization. The image of James Watt as a daydreaming boy watching a tea kettle boil and intuiting from the dancing of its lid the principle of steam power is a charming Victorian invention with little historical foundation. The real story is more interesting and more revealing. Watt was a slow, patient, melancholic, and stubbornly methodical man, often laid low for weeks at a time by migraine and stomach trouble, given to spells of black depression that bordered on incapacity, and disposed by temperament toward an exacting perfectionism that drove him to spend years refining an idea that lesser engineers would have rushed into the marketplace half-formed. He was a working tradesman before he was an inventor, a mathematical instrument maker by craft, a chemist by avocation, a surveyor and civil engineer by necessity, and finally — after his decisive partnership with the Birmingham manufacturer Matthew Boulton — the most consequential mechanical engineer of his century. The unit of power that bears his name is fittingly the unit that quantifies the very thing he taught the modern world how to count: the rate at which a machine does work. Before Watt, work was the labor of arms and backs, of falling water and rising wind, of horses heaving against traces; after Watt, work was the controlled and metered output of engines fed by coal, and the wealth of nations could be measured in horsepower as surely as in gold.

He was born on 19 January 1736 in the modest port town of Greenock on the south bank of the River Clyde, the son of a Scottish shipbuilder, ship's chandler, town councilor, and treasurer of the burgh; he was reared in the dissenting Presbyterian tradition of the western Lowlands, learned the trade of mathematical instrument making in London and Glasgow, and at the age of twenty-one took up a small workshop and a sub-letter's pair of rooms in the precincts of the University of Glasgow, where Adam Smith, Joseph Black, and the philosopher Thomas Reid were teaching during the heroic decades of the Scottish Enlightenment. He read voraciously, taught himself French and German and Italian in order to consult the technical literature of continental Europe, befriended professors and students alike, and through his consultations with the chemist Joseph Black absorbed the new theory of latent heat that would later inform his fundamental analysis of the wasteful operation of the Newcomen pumping engine. The patent he obtained on 5 January 1769 for "A New Invented Method of Lessening the Consumption of Steam and Fuel in Fire Engines" was the single most important industrial patent of the eighteenth century. Through his partnership from 1775 with Matthew Boulton at the great Soho Manufactory on the outskirts of Birmingham, he supervised, over the following twenty-five years, the construction of nearly five hundred steam engines that drained the deepest tin mines of Cornwall, powered the cotton mills of Lancashire, drove the bellows and hammers of the ironworks of South Wales and the Black Country, and propelled the early commercial steamships that would soon shrink the Atlantic and open the rivers of America to the steam age. By the time his Soho patents expired in 1800, the steam engine had become the irreplaceable instrument of British industrial supremacy, and Britain had become the first nation in human history to transcend the ancient constraints of organic energy and to build its prosperity upon the controlled combustion of fossil fuel.

But the conventional story of Watt as solitary inventor obscures both the social texture of his achievement and the depth of his other contributions. He was, throughout his life, a member of a remarkable circle of natural philosophers and industrial improvers — the Lunar Society of Birmingham, which gathered around Boulton at the full moon on Mondays and which numbered among its dozen or so habitual members the potter Josiah Wedgwood, the physician and natural philosopher Erasmus Darwin (grandfather of Charles), the dissenting chemist and theologian Joseph Priestley, the geologist James Keir, the metallurgist John Roebuck, and a shifting penumbra of correspondents that reached from Benjamin Franklin in Philadelphia to Antoine Lavoisier in Paris. Through this circle Watt's engine drew its first capital, its skilled mechanics, its political friends in Parliament, and the cultural credit that allowed industrial improvers to be received as members of the Republic of Letters rather than merely as ingenious mechanics. Watt himself contributed to that circle's intellectual exchange far more than the steam engine. He invented (or shares the credit for inventing) the technique of letter-copying that became the standard office procedure throughout the English-speaking world until photocopying replaced it in the mid-twentieth century. He devised the first attachment for direct mechanical drawing of engine-indicator diagrams, by which the changing pressure of steam inside a cylinder could be plotted graphically against the position of the piston, giving engineers for the first time a precise and quantitative window into the thermodynamic operation of their machines. He played a substantial part in the chemical controversies of the 1780s, and was a serious contender (along with Henry Cavendish and Lavoisier) for the discovery of the composition of water. He participated in canal and harbor surveys across Scotland and northern England, designed the bleaching with chlorine that revolutionized the linen industry, and produced architectural drawings, hydraulic calculations, and improvements in pottery glazing that would be the work of a respectable career on their own. The slow, melancholy, depressive instrument maker of the rue d'Argyll in Glasgow turned out, when his temperament permitted, to be one of the most versatile minds of his century.

Yet for all the breadth of his accomplishments, it is the steam engine that defines him and that justifies the lavish memorials raised to his memory after his death — the statue in Westminster Abbey, the colossal seated bronze in St. George's Hall in Liverpool, the monumental column in Greenock, the dozens of schools and engineering institutes and public houses across the British Isles that bear his name. When the British Association for the Advancement of Science formalized the system of mechanical units that would become the international standard, the unit of power was named the watt by an act of conscious historical homage: a watt is the rate of doing work equal to one joule per second, and by that measure the worldwide electrical generating capacity of the early twenty-first century amounts to roughly six trillion watts, or six terawatts. The unit and the man are everywhere. Yet in his lifetime he was, by his own repeated description, an anxious tradesman, plagued by debt, by litigation against patent infringers, by family bereavement, by recurrent ill-health, by a wife (his first) who died in childbirth and a second wife who outlived him by a decade, by sons he loved and could not always shield from the consequences of their political enthusiasms. The portraits we have of him in middle age show a face of unmistakable gentleness, deeply lined, with high cheekbones, a delicate long nose, and large, melancholy gray-green eyes; in old age he appears in caricature and in dignified academic portraits as a benevolent patriarch, but the patriarchy was earned at the price of decades of patient struggle. The biography that follows traces that struggle in detail — the early years in Greenock, the apprenticeship in London, the long Glasgow phase, the decisive collaborations of the 1760s, the partnership with Boulton, the great expansion of the steam-engine business, the personal sorrows and friendships of his middle years, the chemical controversies and political crises through which he passed, the comfortable retirement at Heathfield outside Birmingham, and the worldwide diffusion of his ideas in the century after his death in 1819.

Birth and Family in Greenock

James Watt was born on the morning of 19 January 1736 in the family house on Dalrymple Street, Greenock, a busy port and ship-building town on the south bank of the Firth of Clyde some twenty-five miles down the river from Glasgow. Greenock in the 1730s was a place of perhaps three thousand inhabitants set between the steep wooded hills of the Renfrewshire shore and the broad, salty tide of the lower Clyde. Its prosperity depended on the herring fishery, on the building and repair of wooden trading vessels, on the importing of timber from the Baltic and tobacco from Virginia, and on the modest re-export trade through which Scottish merchants forwarded West Indian sugar, rum, and molasses to the markets of northern Europe. The town had grown rapidly in the half-century since the Union of 1707 had opened the protected colonial markets of the British Empire to Scottish enterprise, and James Watt was born into a family that had risen with the rising fortunes of the Clydeside burghs.

His grandfather Thomas Watt had been a teacher of mathematics, surveying, and navigation in the neighboring parish of Cartsburn, where he had run a small private school that prepared young men for the practical professions of an Atlantic port. Thomas Watt was a man of some local distinction — he had been a kirk elder, a member of the burgh council, a friend of professors at the University of Glasgow, and a survivor of the bitter sectarian struggles of the seventeenth century that had given the Lowlands their stern Presbyterian temper. He had taught his sons the rudiments of mathematics and the practical applications of geometry to surveying and navigation, and one of those sons, James Watt the elder (the inventor's father), had carried that combination of practical learning and mercantile ambition into the next generation.

James Watt the elder, born in 1698, was by trade a shipwright, joiner, ship's chandler, and provider of nautical instruments to the small fleet of trading vessels that put in at the harbor of Greenock. He maintained a workshop on the quayside where ropes, blocks, anchors, compasses, quadrants, and the wooden bricks of the carpenter's trade were stored and repaired; he served on the town council; he was elected for two terms as treasurer of the burgh; and he was the proprietor, jointly with several of his fellow citizens, of a small share in a trading sloop that ran to the Caribbean. He was, by the standards of provincial Scotland, a man of comfortable middling station — neither a merchant prince of Glasgow's tobacco aristocracy nor a struggling laborer, but a master tradesman of the rising port whose household servants, brick-built house, and provision of formal schooling for his children placed him among the respectable bourgeoisie of the Lowlands.

The inventor's mother, Agnes Muirhead Watt, was the daughter of a family of small landholders in the neighboring parish of Kilmacolm; she had been educated, at least to the standard of literacy in English and a little French, at a parish school. She was, by every account, a woman of warm, vigorous temperament with a particular interest in her children's education, and she had absorbed something of the cultivated sociability of the Glasgow merchant class through her own urban relations. She was a careful housekeeper, a regular communicant at the West Kirk of Greenock, and a Tory in her domestic politics, though her husband leaned increasingly toward the moderate Whig sympathies of the Glasgow merchants. The marriage produced five children, but only two — the inventor James, who was born in 1736, and his elder brother John, born in 1734 — survived infancy. The early deaths of three siblings, including a younger sister named Margaret who died at the age of seven, marked the household with the kind of recurrent grief that was a familiar feature of eighteenth-century family life and may have contributed to the protective gentleness with which Watt's parents reared their two surviving sons.

Early Health, Education, and Temperament

James Watt was a sickly child. He was subject from infancy to migraine, to digestive complaints of various kinds, and to a chronic toothache that troubled him through long stretches of his school years. Family tradition held that he was so often laid up with these complaints that his mother taught him to read at home rather than send him to the parish school, and that his earliest formal education was an irregular affair of lessons taken between bouts of illness. The picture that emerges from his childhood letters and from the recollections of his cousin Margaret Campbell is of a quiet, withdrawn, but not unhappy boy who spent long hours alone with books, drawings, and small mechanical objects, and who developed early an unusual capacity for patient concentration upon problems that interested him.

The famous anecdote of the tea kettle — the story that James as a boy of six or seven sat by his aunt Mrs. Muirhead's fireside watching the steam push up the lid of the kettle and thence inferred the principle of his future engine — was first set down in print by Andrew Carnegie in his Victorian biography and has the unmistakable savor of nineteenth-century industrial mythology. The boy was not, by any reasonable evidence, thinking about steam pressure at the age of seven. But the anecdote does capture something true: that he was the kind of child who watched ordinary phenomena patiently and asked himself how they worked. His father's workshop on the Greenock quayside was a marvelous laboratory for such a boy. There were tools of every description for working in wood and metal, compasses and sextants and theodolites under repair, sketches and ship's drafts pinned to the walls, and the constant traffic of seamen and shipwrights talking over the particulars of their trades. The young James was given the run of the workshop, was encouraged by his father to take apart and reassemble any instrument he could understand, and was praised when he made small models of cranes, pumps, and capstans from the scraps of wood and brass on the workshop floor. By the time he was twelve he had built a small electrical machine of the kind that natural philosophers used to demonstrate static electricity, and he had begun the lifelong habit of keeping notebooks in which he sketched and annotated mechanical devices that caught his interest.

His formal schooling, when health permitted, took place at the Greenock Grammar School, where he received the standard provincial Scottish education of his class — Latin grammar, a little Greek, English composition, arithmetic, geography, and what passed for natural philosophy. He was not, by his own recollection or by the judgment of his teachers, an outstanding scholar in the conventional curriculum. The dead languages bored him, the rote learning bored him, and the schoolmaster's rod failed to extract from him the kind of quick verbal facility that the eighteenth-century schoolroom prized. But in mathematics, which was taught at Greenock with a strong practical emphasis on surveying and navigation, he excelled, and his interest in mechanical drawing and in the geometry of solids was conspicuous. His best subject, by some distance, was mathematics, and his school exercise book preserved at the Birmingham Reference Library shows clearly drawn diagrams of conic sections, accurate trigonometric calculations, and the kind of methodical neatness in numerical work that would mark his later engineering drawings.

His mother died in 1755, when James was nineteen, and her death was a shattering loss to the household. The father's commercial fortunes had already begun to decline. The merchant ship in which James the elder was a part owner had been wrecked on a return voyage from the West Indies the previous year, with the loss of nearly all the capital invested; a second venture had failed shortly after; and the chandlery business on the quayside was being pressed by debts of various kinds. The family's modest competence was eroding, and it became clear by the end of 1755 that the young James could not look to his father for the kind of financial support that would have allowed him to attend the University of Glasgow as a regular student. A trade would have to be learned. The trade that he and his father agreed upon, after a good deal of discussion, was that of mathematical instrument maker — a craft that combined the precise work in brass and ivory of the clockmaker with the geometrical knowledge of the surveyor and the optical understanding of the lens grinder, and that promised, in a port town like Greenock or a university town like Glasgow, a reasonable income for a careful workman.

London and the Apprenticeship in Mathematical Instruments

In June 1755, even before his mother's final illness, James Watt was sent up to Glasgow with the hope of finding a master with whom he could be apprenticed. Glasgow, however, had no mathematical instrument maker of the first class — the craft in Britain was concentrated in London, where the great instrument-making firms of Adams, Sisson, Bird, Ramsden, and Dollond supplied the navigators, the astronomers, and the surveyors of the British Empire and of much of continental Europe. After a few weeks of unsuccessful inquiry in Glasgow, Watt determined to go to London. The journey, in the summer of 1755, was an undertaking of some consequence — four hundred miles by sea or by jolting coach over the rutted roads of the eastern Lowlands and the northern English shires, with the obligatory layovers at Berwick, Newcastle, York, and Stamford, and the final approach to the capital by way of the Great North Road through Highgate. Watt traveled the first stage by sea from Greenock to Leith, the second by coach from Edinburgh to London, and arrived in the metropolis on 5 July 1755, in possession of a small purse of his father's saving and a letter of introduction to the Glasgow tobacco merchant John Marr, who kept a counting-house in the City and who was instructed to assist the young Scotsman in finding a master.

The early weeks in London were difficult. The masters of the great instrument-making firms were reluctant to take on a Scottish apprentice for the conventional seven-year term, in part because of the rules of the City companies (which excluded non-freemen from full apprenticeship), in part because Watt, at nineteen, was already too old for a regular apprenticeship under the customs of the trade. After some weeks of inquiry, Marr secured for Watt an irregular arrangement with John Morgan, a mathematical instrument maker of Cornhill, who agreed to take Watt for one year as a working assistant in return for a payment of twenty guineas. The arrangement was technically illegal under the rules of the City of London — a non-apprentice was not supposed to work at a regulated trade within the City — and it meant that Watt would be denied the kind of formal certification that ordinarily followed a completed apprenticeship. But Morgan was an able workman, and he would teach Watt as much in one intensive year as a regular apprentice might learn in two or three of the rather casual seven.

The year in Morgan's workshop in Cornhill was the most concentrated period of skill acquisition in Watt's life. He worked, by his own account in a letter to his father, from six in the morning to nine at night, six days a week, with one hour for his midday dinner. The work was the patient hand-finishing of brass and ivory components for sextants, theodolites, parallel rulers, telescopes, navigational quadrants, and the small portable barometers that were just then coming into use among scientific gentlemen. He learned to file and drill brass to the tolerances of a few thousandths of an inch; he learned to engrave fine graduations on the limbs of measuring instruments; he learned to grind and polish lenses of moderate quality; he learned the tricks of straightening rules, of adjusting verniers, and of inlaying ivory scales. By the end of the year he could produce, from raw materials and on his own, any of the standard instruments of the trade. He could also draw any of them with the kind of precise mechanical accuracy that would later mark his engine drawings.

The cost of the London year, in money and in health, was considerable. His father's remittances scarcely covered his lodging and his meals; he ate cheaply, lived in a garret in Finch Lane, and rationed his clothing and his books with monkish economy. He suffered, by the spring of 1756, from increasingly severe headaches and from a recurrence of the stomach trouble that had plagued him in childhood. By the summer of 1756 his health had broken down completely, and he was advised to return to Scotland before he should fall seriously ill. He left London in the third week of July 1756, eleven and a half months after his arrival, with no formal certification but with the practical mastery of a skilled instrument maker, the equipment necessary to set up a workshop of his own (he had bought tools in London at considerable expense), and a determination to establish himself in some Scottish town as a mathematical instrument maker on his own account.

Glasgow and the University Years

The natural destination for a Scottish instrument maker of Watt's training and ambition was Glasgow, the second city of Scotland and the seat of the country's most distinguished university outside the capital. Watt arrived in Glasgow in August 1756, intending to set up a shop in the city center, and at once ran into the obstacle that he had not anticipated. The Glasgow guild of hammermen — the corporation that controlled all metalworking trades in the city — required that anyone setting up shop in the regulated crafts within the burgh either be a member of the guild (which required either a city apprenticeship or the payment of a substantial fee) or have been born within the burgh. Watt was neither. The hammermen, when he applied for permission to open a workshop, refused him the privilege of carrying on his trade within their jurisdiction.

What might have been a defeat became, by one of the lucky accidents that marked Watt's life, the foundation of his career. The University of Glasgow, on the western edge of the medieval town center, occupied a small precinct of buildings around the old college quadrangle off the High Street; its precinct was a privileged enclave outside the jurisdiction of the burgh guilds, and the university authorities had the right to license tradesmen to work within the college bounds without interference from the city corporations. Watt's case came to the attention of the professor of natural philosophy, Robert Dick, through the intercession of Robert Dick's pupil Andrew Anderson, who was a cousin of one of Watt's Glasgow relations. Dick took the young instrument maker under his wing, examined his work, found it of excellent quality, and proposed to his fellow professors that the university appoint Watt as official mathematical instrument maker to the institution, with the right to maintain a shop within the college precinct.

The proposal was accepted. In 1757, James Watt, twenty-one years old, was installed in two small rooms in the college precinct (a workshop and a sleeping chamber) and given the title of Mathematical Instrument Maker to the University of Glasgow. He was provided with a modest salary, drawn from the university's account for the maintenance of its scientific apparatus, and was given the responsibility for repairing, calibrating, and (when required) constructing afresh the instruments of the various university departments. He was also permitted to take in private work from outside the university — to make and sell instruments to customers throughout the western Lowlands and to ply such other trades as might offer themselves to a skilled workman with a steady hand and a calibrated eye.

The little workshop in the college precinct became, over the next ten years, one of the most consequential rooms in the history of technology. Watt repaired the university's telescopes and the academic quadrants and the lecture-room demonstration apparatus; he made and sold to gentlemen and tradesmen a steady stream of compasses, sextants, theodolites, parallel rulers, and surveying chains; he took up, as a sideline, the repair and tuning of musical instruments, becoming in time a skilled maker of organs and of a particular kind of single-action harpsichord; he taught himself the rudiments of organ building from the published treatises of the German master Andreas Werckmeister and built an organ for the Mason's Lodge of St. John in Glasgow that was considered one of the finest small organs in the country. He surveyed and drew up plans for canals and harbors in the Highlands and along the western coast. He read, in the evenings, the works of Newton, of Desaguliers, of Boerhaave, of Smeaton, and of the principal continental natural philosophers, teaching himself French, German, and Italian in order to follow the technical literature of the European Enlightenment.

The most important of his Glasgow friendships was with the professor of medicine and chemistry, Joseph Black, the discoverer of carbon dioxide (which he called fixed air) and the first systematic analyst of latent and specific heats. Black was eight years older than Watt, a quiet, courteous, immensely learned man whose lectures on chemistry were already drawing students from across Britain to the University of Glasgow. Watt repaired Black's laboratory apparatus, was permitted to attend his lectures, and was drawn into a friendship that would last for the next forty years. From Black he learned the chemistry of his day, including the new theory of latent heat that Black was developing in the late 1750s — the theory that a definite quantity of heat was required to convert ice to water without changing its temperature, and a much greater quantity to convert water to steam, and that this heat was somehow "latent" in the substance until it was given up again when the substance was condensed. Without the concept of latent heat, Watt's later analysis of the wastefulness of the Newcomen engine would not have been possible, and Black always claimed, with characteristic generosity, that he had played a part in the genesis of the separate condenser by providing Watt with the theoretical framework within which the wastage of steam in the Newcomen design could be understood and quantified.

Through Black and through the natural philosopher John Robison (then a young student of Black's, later professor of natural philosophy at Edinburgh), Watt was drawn into the lively intellectual life of the university. He attended public lectures, supped at the Anderston Club where the moderates of the Glasgow faculty gathered weekly for conversation and conviviality, became acquainted with Adam Smith (who held the chair of moral philosophy from 1751 to 1763 and whose Lectures on Rhetoric and Belles Lettres Watt heard with admiration), and through these connections was admitted, despite his trade and his lack of a university degree, to the fellowship of the Glasgow Enlightenment. He was, in this period, a melancholy and self-doubting young man — his letters from the late 1750s and early 1760s are full of complaints about his health, his finances, the difficulties of his trade, the slowness of his progress in becoming a man of substance — but he was being shaped, by the daily contact with men like Black, Robison, Smith, and Reid, into the kind of cultivated mechanic who could think theoretically about practical problems and who could move with equal ease in the world of the workshop and the world of the lecture hall.

The Newcomen Engine and the Winter of 1763–1764

In the late autumn of 1763, the professor of natural philosophy John Anderson — Robert Dick's successor in the chair — brought to Watt's workshop a small working model of the Newcomen atmospheric engine that the university had acquired for the purpose of demonstrating the principles of steam power to its students. The model, which had been built some years before by the London instrument maker Sisson, had never worked satisfactorily. It would condense its steam in the cylinder, draw the piston down by atmospheric pressure, and then refuse to repeat the cycle more than two or three times before the steam ran out and the boiler had to be replenished. Anderson, who had heard from Robison that Watt was the most ingenious mechanic in the precinct, asked Watt whether he could put the model in working order in time for the spring term's lectures. Watt agreed, took the model into his shop, and began the long course of investigation that would, within five years, transform the design of the steam engine and the economic future of his country.

The Newcomen engine of 1712, designed by the Dartmouth ironmonger Thomas Newcomen and his partner John Calley, was the first commercially successful steam-powered machine. Its principle was the so-called "atmospheric" cycle, in which a vertical brass or iron cylinder open at the top contained a heavy piston connected by chain to one end of a great wooden beam pivoted at its center; the other end of the beam carried the rod of a pumping mechanism that descended into a mine shaft. Steam was admitted to the cylinder below the piston at a pressure scarcely above the atmosphere; once the cylinder was full of steam, a jet of cold water was injected, which condensed the steam back to water and produced a partial vacuum below the piston. The atmospheric pressure on the upper face of the piston then drove it down, lifting the pump rod at the far end of the beam and raising water from the mine shaft. The piston was then drawn back up by the weight of the pump rod, fresh steam refilled the cylinder, the cold-water injection condensed it again, and the cycle continued at a speed of perhaps a dozen strokes a minute.

The Newcomen engine had served the British mining industry for half a century by the time Watt began to study its operation. It had pumped the deep tin mines of Cornwall, the lead and copper mines of Derbyshire, the coal pits of Tyneside and Yorkshire and Staffordshire, and the salt-water clay pits of Cheshire. Several hundred Newcomen engines were in operation across the kingdom in 1763, and the design had been exported to Belgium, Hungary, France, Sweden, and (under Henry Beighton's improvements and James Brindley's later refinements) had become the standard tool of European deep-mine drainage. But it suffered from a single grave defect that nearly limited its commercial reach: it was prodigally wasteful of coal. The same cylinder was alternately heated by steam and cooled by the injection of cold water — a cyclical heating and cooling of a substantial mass of iron that consumed an enormous quantity of heat (and therefore of coal) at every stroke. The Newcomen engine could be profitable at the pit head of a coal mine, where the fuel was effectively free, but it was barely economic at the metal mines where coal had to be hauled long distances, and it was wholly impracticable for industrial applications away from coal fields. Watt, examining the Anderson model in the winter of 1763–64, came to understand the source of the wastage with a precision that no previous engineer had achieved.

He began with the model itself, and his first practical inquiries showed that the trouble was not in any failure of construction. The Sisson model was well-made; the trouble was that, being a model, its cylinder was relatively small in proportion to the heat that had to be poured into it and that it lost between strokes. Watt repaired the boiler, adjusted the valves, lagged the cylinder with cloth to reduce its losses, and got the model working in a fashion within a few weeks. But the quantity of steam it consumed per stroke was, by his careful measurements, several times the volume of the cylinder — the model wasted most of its steam in reheating the iron walls of the cylinder after each cold-water injection. Working from those measurements, Watt began the slow analytical work that would occupy him for the next two years.

He measured, with the careful instrument-maker's hand that was his greatest physical gift, the quantity of cold water that had to be injected to condense a given volume of steam. He compared the result with the heat capacity of the same quantity of water at ordinary temperatures, and saw at once that the heat required to convert water to steam was many times the heat required to warm water by a few degrees. He had Joseph Black, by this time installed at the University of Edinburgh as professor of medicine and chemistry but still in regular correspondence with Watt, explain to him the theory of latent heat in some detail; Black sent him papers and lecture notes and described the experiments by which he had measured the latent heat of vaporization. Watt absorbed these ideas, applied them to the operation of the engine, and reached the conclusion that the prodigal coal consumption of the Newcomen engine arose from a simple but devastating cause: that the cylinder, which ought to contain the steam at the temperature of boiling water throughout the stroke, was alternately cooled to nearly the temperature of cold water and then reheated to the temperature of steam at every cycle, and that the quantity of heat (and therefore the quantity of steam, and therefore the quantity of coal) required to reheat the cylinder at each stroke was several times the quantity required merely to fill the cylinder with the steam that did the actual work.

If only the cylinder could be kept hot throughout the cycle, the wasted heat could be saved and the engine made much more economical. The problem was that the cylinder had to be cold for the condensation step; it could not be kept hot if the steam in it were to be condensed by the injection of cold water. The riddle was how to condense the steam without cooling the cylinder. Watt turned this question over in his mind through the spring and summer of 1764, sketching tentative solutions, working through them on paper, often returning to the model to test particular configurations. Nothing satisfactory presented itself. He was, by his own later testimony in conversation with John Robison, on the point of giving up the problem in despair and returning to the more profitable work of his trade when the decisive idea came to him.

The Separate Condenser and the Walk on Glasgow Green

It happened on a Sunday afternoon in the early spring of 1765 — Watt always remembered it as a Sunday in the middle of May, but the dating has been disputed by his biographers, and the precise day will probably never be settled. He was, on his own account, "walking on a fine Sabbath afternoon" in the open space of Glasgow Green, the broad public common between the College and the Clyde, where he was in the habit of taking the air when the weather permitted. He had been thinking, off and on, about the Newcomen problem for many months. The thought struck him suddenly that the difficulty might be solved by performing the condensation of the steam not in the working cylinder itself but in a separate vessel — what he would later call the condenser — connected to the cylinder by a valve, so that the steam, when ready to be condensed, would rush from the cylinder into the condenser (which could be permanently kept cold) and be condensed there, leaving the cylinder unaffected. By making the condenser sufficiently cold and maintaining a high vacuum in it (which could be done with the help of a small air pump worked by the engine itself), one could obtain the necessary vacuum below the piston while keeping the cylinder always at the temperature of the steam. The cyclical heating and cooling of the iron walls would be eliminated; the heat consumption of the engine would be reduced — Watt estimated, on the spot — by something like three-quarters.

The recollection of the moment that Watt set down in old age, in conversation with his biographer Robert Hart, is one of the most famous passages in the literature of invention. "I had not walked further than the golf-house," Watt is reported to have said, "when the whole thing was arranged in my mind." It was the kind of sudden flash of insight that engineers and natural philosophers know — the moment when the long subliminal turning over of a problem yields, without warning, a complete solution that the mind has been assembling without conscious effort. Watt walked back to his workshop, took out his notebooks, and began the systematic exposition of the new principle. By the evening he had a clear mental design; by the following week he had drawings; within the month he had constructed, from a soldered tin canister bound with a wooden frame, a small working model of an engine fitted with a separate condenser, and demonstrated to himself that the principle worked exactly as he had imagined it would.

The separate condenser was the central idea of the modern steam engine and the foundation of every subsequent improvement in the technology. By itself it would have made Watt's reputation in the history of mechanical engineering. But over the months that followed, he extended the basic idea by a series of subsidiary improvements that together transformed the Newcomen design into the modern engine. He realized that, since the cylinder no longer had to be cooled by cold water, it could be closed at the top and the piston driven down by steam pressure rather than by the atmosphere — eliminating the loss of heat from the piston's exposure to the air and allowing the engine to be operated at higher steam pressures. He realized that the cylinder ought to be enclosed in an outer steam jacket, through which steam from the boiler would flow, to keep its walls at the temperature of the working steam throughout the stroke. He realized that the small air pump connected to the condenser could be driven by the engine itself, eliminating the need for an external mechanism. By the end of 1765, on the basis of further experiments with successive models built in his Glasgow workshop and at a slightly larger scale in the workshop of his friend the iron-founder John Roebuck at Kinneil in Linlithgowshire, he had worked out the complete design of what would later be called the Watt double-acting condensing steam engine.

John Roebuck and the First Patent

By the spring of 1766 Watt understood clearly that what he had invented would, if it could be made to work at full size, transform the steam-engine industry. He understood equally clearly that he could not finance the development of a full-scale prototype from his own resources. The construction of even a small full-scale engine would cost several hundred pounds — a sum well beyond Watt's modest income from instrument making and beyond the resources of his ailing father — and the development of the engine to commercial reliability would cost several thousand more, with no guarantee of return. He needed a partner with capital and with an industrial base from which to launch the new technology.

The partner who presented himself was Dr. John Roebuck of Birmingham, a physician and natural philosopher turned industrial entrepreneur who had founded the Carron Iron Works near Falkirk in 1759 and was, by 1766, one of the most ambitious industrialists in Scotland. Roebuck had been introduced to Watt through Joseph Black, was familiar with Watt's reputation as the best instrument maker in the country, and had himself been thinking about the possibility of applying steam power to the drainage of the deep coal pits at his Bo'ness colliery, on the Firth of Forth a few miles from Kinneil. When Watt described to him the principle of the separate condenser and demonstrated the small model, Roebuck saw at once both the technical merit of the idea and its commercial promise. He proposed a partnership: Roebuck would provide the capital required to develop a full-scale engine, would lodge Watt in a workshop attached to Kinneil House (the manor that Roebuck rented from the Duke of Hamilton, a few miles from the Carron works), and would share with Watt the proceeds of a patent on the invention. In return, Roebuck would take two-thirds of the profits of the patent, and Watt would take the remaining third.

The arrangement, viewed in retrospect, was uneven — Roebuck was acquiring the most valuable industrial property of the century at a discount — but Watt at the time had no leverage with which to bargain for better terms, and Roebuck was assuming a substantial financial risk for a development that, in 1766, was not yet certain of success. They concluded the partnership in the summer of 1766 and began the construction of the first full-scale prototype at Kinneil House in the autumn of that year.

The work at Kinneil proved more difficult than either Watt or Roebuck had anticipated. The available facilities for casting and machining the precise cylinders that the new engine required were inadequate. The cylinder bored for the first Kinneil prototype, supplied by a foundry in Carron, was so far out of round that the piston could not seal against it; Watt was reduced to packing the piston with cloth, leather, paper, and tow in successive attempts to make it gas-tight, but the leaks persisted and the engine could not develop the steady vacuum on which its operation depended. The boiler was inadequate, the steam pipes leaked, the air pump worked imperfectly. The cumulative effect of these mechanical difficulties was that the first Kinneil engine, although it did run after a fashion, never came close to the efficiency that Watt had calculated for it on paper, and could not be exhibited as a commercial demonstration of the new principle.

In the midst of these difficulties, Watt applied — on the strength of the small models and of the partial success of the Kinneil prototype — for a patent on the separate condenser principle. The application was prepared in late 1768 and granted on 5 January 1769 as patent number 913 of the Great Seal of Great Britain, for "A New Invented Method of Lessening the Consumption of Steam and Fuel in Fire Engines." The patent specification, which Watt drafted himself with characteristic thoroughness, described the principle of the separate condenser, the enclosed cylinder with its steam jacket, the use of steam rather than atmospheric pressure to drive the piston, the small air pump for maintaining the vacuum, and the various subsidiary features that constituted his improvement of the Newcomen design. The patent was granted for the standard fourteen-year term, expiring in January 1783 — though, as we shall see, it would be extended by special act of Parliament in 1775 to a term of twenty-five years, expiring at last in 1800.

The patent of 1769 was the most important industrial property of the age, but its commercial value would not be realized for several years. The Kinneil prototype continued to disappoint. Roebuck, by 1772, was in serious financial difficulty — the Bo'ness colliery had proved less productive than he had hoped, his other speculative ventures were going badly, and his creditors were beginning to press him for repayment of large loans. The bankruptcy of the firm of Roebuck and Garbett in 1773 swept away most of Roebuck's industrial empire and left the steam-engine patent as one of his few remaining valuable assets. The patent rights, however, were of little use to Roebuck's creditors because no working engine of the new design had yet been demonstrated; the value of the patent was contingent upon further development that Roebuck no longer had the means to fund.

The Birmingham Connection and the Partnership with Matthew Boulton

Roebuck's bankruptcy might have been a catastrophe for Watt. It was instead the prelude to the most fortunate professional connection of his life. Among Roebuck's principal creditors was Matthew Boulton, the proprietor of the Soho Manufactory in Birmingham — a magnificent factory complex of brass and silver work, ornamental ware, and small metal goods that Boulton had been building since 1761 and that had become, by the 1770s, the largest and most modern industrial establishment in England. Boulton was a man of large ambition and large geniality, six years older than Watt, the son of a Birmingham toy-maker who had transformed his father's modest business into an industrial empire; he was already a friend of Roebuck's, knew Watt's reputation through their mutual connections, and had become aware some years earlier of the steam-engine project at Kinneil. He had visited Watt at Kinneil in the spring of 1767, had been shown the model and the partial prototype, and had immediately recognized the potential of the invention.

When Roebuck's bankruptcy in 1773 made it possible for Boulton to acquire Roebuck's two-thirds share in the patent, he moved quickly. By an arrangement settled with Roebuck's creditors, Boulton took over Roebuck's interest in the patent in lieu of a debt of about twelve hundred pounds that Roebuck owed him, and proposed to Watt a new partnership on the basis that Watt should move to Birmingham, take up residence at Soho, and develop the engine with the resources of the Soho Manufactory behind him. Watt, after some hesitation — his first wife Margaret was reluctant to leave Scotland, and Watt himself was at this stage in his life so subject to depression that the prospect of a major move filled him with anxiety — agreed in the late summer of 1774. He arrived at Soho in May 1775, after some delays caused by the death of his wife in childbirth in September 1773 and the arrangements that had to be made for the care of his two surviving children.

The partnership of Boulton and Watt, formally constituted in 1775 by articles of association, was one of the great industrial collaborations of the modern era. The terms were generous to Watt: Boulton took two-thirds of the profits in compensation for the capital and the facilities he was contributing, and Watt took the remaining third, but the arrangement was clearly understood by both partners as a true collaboration in which each brought irreplaceable abilities. Watt was the engineer of genius, the inventor and improver and detail-perfectionist of the design; Boulton was the businessman of equal genius, the marketer and political operator and capital raiser without whom the engine could not have reached its customers. The Soho Manufactory, with its skilled metalworkers, its modern shops, its access to capital from Boulton's other businesses, and its political connections in Birmingham and in London, provided the industrial base from which the steam engine would be diffused across Britain and the world.

The first task of the new partnership was to secure the legal foundation of the business. The patent of 1769 had been granted for fourteen years and would expire in January 1783, by which time the engine would scarcely have begun to recoup the costs of its development. Boulton, working through the political connections of his fellow Lunar Society members and through his own friends in Parliament, secured an extension of the patent by special act of Parliament in May 1775 — a twenty-five-year extension that pushed the expiry to 1800. The act of 1775 was a remarkable parliamentary intervention on behalf of a single industrial property, and it provoked considerable controversy at the time and in subsequent decades; it has been criticized by historians of industry as a monopoly grant that may have retarded the development of competing engine designs. But it was indispensable to the financial viability of the Boulton and Watt partnership, and it provided the period of guaranteed commercial protection during which the engine could be developed to its full potential and the partnership could establish itself as the dominant supplier of steam power in Britain.

The second task was to develop the engine to the point at which it could be sold to paying customers. The Kinneil prototype had been disassembled and shipped from Scotland to Birmingham, where it was rebuilt at Soho with improved components and was put through extensive trials in the autumn of 1775 and the spring of 1776. The cylinder problem that had plagued Watt at Kinneil was solved by a critical technical development that had emerged independently in the late 1760s and early 1770s in the iron foundries of the West Midlands: John Wilkinson's invention of the precision cylinder-boring machine. Wilkinson, the ironmaster of Bersham in Denbighshire and Bradley in Staffordshire, had patented in 1774 a horizontal boring machine in which the cutter was guided by a rigid bar running through the cylinder, producing cylinders of much greater accuracy than had previously been possible. Wilkinson's first commercial application of his machine was to the boring of cannon for the Board of Ordnance, but he agreed in 1775 to bore cylinders for the Boulton and Watt engines and produced for them, beginning that year, cylinders that were round to within a few hundredths of an inch — an accuracy entirely sufficient for Watt's design.

The First Commercial Engines

The first commercial Watt engine — that is, the first engine of the new design that was built and sold to a paying customer rather than as a development prototype — was constructed at Soho in the summer and autumn of 1776 and installed at the Bloomfield Colliery near Tipton in Staffordshire, the property of the partnership of Bentley and Co. The engine had a cylinder of fifty inches diameter, a stroke of seven feet, and was rated at about fourteen horsepower. It went into operation in March 1776 and was the subject of an admiring report by William Pratt of the Birmingham Gazette, who described its smooth and powerful operation and noted that it consumed coal at less than a third the rate of a Newcomen engine of equivalent pumping capacity. The Bloomfield engine drew an immediate stream of inquiries from coal owners and metal-mine operators across the kingdom.

The second commercial engine, built later in 1776, was installed at the New Willey ironworks of John Wilkinson in Shropshire, where it was used to blow the bellows of a blast furnace — the first application of the Watt engine to industrial machinery rather than mine drainage. The success of the New Willey installation was decisive in establishing the reputation of the new engine among industrial customers, and orders began to come in steadily from ironworks, copper-rolling mills, paper mills, brass foundries, and the larger textile factories.

The fundamental commercial innovation of the Boulton and Watt partnership was the unique royalty arrangement on which their engines were sold. Boulton and Watt did not, in their early years, actually manufacture complete engines for sale; instead, they supplied the working drawings, the central components (the precision-bored cylinder, the air pump, certain valves and the patented condenser), and the engineering supervision, while the customer arranged for the construction of the rest of the engine (the boiler, the beam, the engine house, the masonry foundations) through local contractors. The customer paid the partnership a royalty equal to one-third of the value of the coal saved relative to a Newcomen engine of equivalent capacity — a calculation that was made by formal indicator tests at the customer's site and was renewed annually for the life of the patent. The royalty arrangement was ingenious because it tied the partnership's revenue directly to the demonstrated commercial benefit of the engine, and it gave both partners and customers a continuing incentive to maintain the engines in efficient operation. But it was also a source of endless administrative complication and of perpetual litigation, since the calculation of the royalty depended on measurements that customers had every incentive to dispute and on legal definitions that infringers were always trying to evade.

By the end of 1781, six years after the formation of the partnership, Boulton and Watt had built or supervised the construction of about forty engines, almost all of them for mine drainage in Cornwall, the Midlands, and the north of England. The engine was a notable commercial success, and the partnership was beginning to repay the investments that Boulton had made in its early years. But Watt had grown dissatisfied with the limitations of the reciprocating-beam engine. The engine in its 1769 form could only produce reciprocating motion — up-and-down strokes of a pump rod or of a bellows — and could not directly drive the rotating shafts of mills and factory machinery. Mills had to be driven by water wheels, by windmills, or by some other source of rotative power; the steam engine, in its 1776 form, was confined to those applications that could use reciprocating motion. To unlock the steam engine's potential as the universal prime mover of industry, Watt would have to extend his design to produce rotative motion suitable for driving the spinning mules of cotton mills, the rolling stands of iron mills, the grinding stones of corn mills, and all the other rotating machinery of the new industrial age. That extension was the central task of his work in the late 1770s and early 1780s.

The Rotative Engine and the Sun-and-Planet Gear

The simplest way to produce rotation from a reciprocating piston is by means of a crank — a right-angle lever attached to a rotating shaft, whose motion converts the up-and-down stroke of the piston into a continuous turning. The crank had been used since antiquity for various small mechanisms, and its application to the steam engine seemed obvious. But there were two obstacles. The first was practical: the crank produces uneven motion (slow at the top and bottom of the stroke, fast in the middle), which without a heavy flywheel produces juddering and stalling. This problem was solved by attaching a sufficiently heavy flywheel to the crankshaft, whose inertia smoothed the rotation. The second obstacle was legal. In 1780, a millwright named James Pickard had taken out a patent (number 1263) on the application of a crank to a steam engine for the production of rotative motion — a patent that Boulton and Watt regarded as a flagrant theft of an obvious principle, but that nevertheless stood in the way of their using the crank without paying Pickard a license fee. (Pickard had, in fact, apparently come to know of the idea through an indiscretion of one of Watt's workmen at Soho.) Watt, who detested patent infringement, refused on principle to license the Pickard patent.

Instead, Watt devised in 1781 an ingenious alternative — the sun-and-planet gear, which produced rotative motion from a reciprocating beam without the use of a crank. In the sun-and-planet arrangement, a gear-wheel (the "planet") was attached to the end of the connecting rod from the beam, and another gear-wheel of the same size (the "sun") was attached to the rotating shaft. The planet gear was prevented from rotating about its own axis by a link to a fixed point on the engine frame, while it was free to swing around the sun gear. As the connecting rod moved up and down, the planet gear rolled around the sun gear in a circle, and the engagement of the two gears transmitted that rolling motion to the sun, which therefore rotated. The arrangement produced twice the revolutions of an ordinary crank for the same number of piston strokes — a useful gearing-up — and entirely avoided the Pickard patent. Watt patented the sun-and-planet gear in 1781 (patent number 1306), and Boulton and Watt began to offer rotative engines based on this design to mill owners and factory operators.

The sun-and-planet engine was the means by which the Watt engine entered the textile mills, the corn mills, the iron-rolling mills, the paper mills, and the other rotative industrial applications that would constitute its largest market over the following decades. The first sun-and-planet engine for industrial use was supplied in 1782 to John Wilkinson's ironworks at Bradley, where it was used to drive a tilt-hammer and a rolling mill; another early installation was at the Albion Mills in London, a large flour-grinding establishment that became, in the late 1780s, one of the technological showpieces of British industry and was visited by, among many others, the king and queen of England, the Russian ambassador, and the future first president of the United States. (The Albion Mills burned down in 1791 in a fire that was widely blamed by London millers on a deliberate act of arson by a competitor — a charge never proven.) By the mid-1780s, sun-and-planet engines were in operation at perhaps thirty industrial sites across the country, and the Boulton and Watt order book had ceased to be dominated by Cornish pumping engines and had come to include a large and growing share of rotative engines for industrial use.

The Double-Acting Engine and the Parallel Motion

Watt extended the rotative engine in 1782 to the double-acting principle. In the engines built up to that date, steam was admitted to only one side of the piston — the underside — and the piston was returned by the weight of the pump rod (in pumping engines) or by the inertia of the flywheel (in rotative engines). The output of the engine was therefore confined to one direction of the piston's travel. Watt realized that, by admitting steam alternately to both sides of the piston (with the corresponding alternate condensation on the other side), the engine could be made to work in both directions of the piston's travel, doubling its output for the same size and the same fuel consumption.

The double-acting principle, patented by Watt in 1782 (patent number 1321), required a more complex valve arrangement than the single-acting engine had used, and it also required a different mechanical connection between the piston rod and the beam. The single-acting engine had used a flexible chain to connect the piston rod to the beam — adequate because the chain only had to transmit a pulling force when the piston was descending under steam pressure. The double-acting engine, in which the piston pushed and pulled the beam in alternate strokes, required a rigid connection between the piston rod and the beam, and the rigid connection had to be guided so that the piston rod traveled in a straight vertical line while the end of the beam swung through an arc of a circle. The geometrical problem of converting straight-line motion of the piston into arc motion of the beam — and back — was one of the most elegant of Watt's mechanical inventions: the parallel motion, patented in 1784 (patent number 1432), which he himself considered his most ingenious mechanical contrivance.

The parallel motion was an arrangement of four links that constrained a point on the linkage to move (very nearly) along a straight line, even as the rest of the linkage swung through an arc. It is the kind of mechanism whose geometric beauty becomes apparent only when one watches it in operation: the linkage swings and turns in a complex way, but the connection point on which the piston rod is attached traces a path that is, to within a thousandth of an inch over the working travel, a perfect straight line. Watt wrote to his son James in 1808 that he was "more proud of the parallel motion than of any other mechanical invention I have ever made" — a remarkable statement from a man who had invented the separate condenser, the steam jacket, the double-acting engine, the sun-and-planet gear, and the centrifugal governor. The parallel motion is still in use in essentially the form Watt patented it in 1784, in walking-beam pumping engines and in certain kinds of mechanical linkages where straight-line motion has to be produced from rotary inputs.

The Centrifugal Governor and the Question of Control

The next problem that confronted Watt was the regulation of engine speed. The single-acting pumping engine had not required precise speed control, because the load — the pumping of water from a mine — varied only slowly and the engine could be allowed to find its own speed. But the rotative engine driving industrial machinery had to maintain a precise and steady speed in the face of varying loads: when the spinning frames or rolling stands or grinding stones of a factory came into and out of engagement, the load on the engine changed suddenly, and an unregulated engine would speed up dangerously when the load dropped and would slow down or stall when the load was increased. The factory operations required, therefore, an automatic regulator that would compensate for changes in load by adjusting the supply of steam to the cylinder.

Watt's solution, devised in 1788, was the centrifugal governor — a device that he adapted from the regulator that had long been used by millwrights to control the speed of windmills. The governor consisted of two heavy balls mounted on arms pivoted at the top of a vertical spindle; the spindle was driven by the engine through a belt or gearing, and the balls swung outward by centrifugal force as the spindle rotated faster. The arms of the balls were linked to a sliding sleeve on the spindle, which in turn was linked through levers to the throttle valve that controlled the supply of steam from the boiler to the cylinder. When the engine speeded up, the balls flew outward, the sleeve rose, the throttle valve closed slightly, and less steam was admitted to the cylinder — slowing the engine. When the engine slowed down, the balls fell inward, the sleeve dropped, the throttle valve opened, and more steam was admitted — speeding the engine up.

The centrifugal governor is one of the most important inventions in the history of automatic control, and it has been studied as the prototype of all feedback control systems by engineers, mathematicians, and historians of technology. It was the first widely used mechanical device that operated on the principle of negative feedback — that is, that adjusted its own operation in response to a measurement of the output it was supposed to be controlling. James Clerk Maxwell, in a famous paper of 1868 titled "On Governors," would use Watt's centrifugal governor as the example by which he developed the mathematical analysis of feedback control systems, and that paper would become one of the foundation documents of modern control engineering. Norbert Wiener, in his 1948 book Cybernetics, would trace the lineage of the entire science of control and communication back to Watt's governor. Watt, who would have understood not a word of Maxwell's differential equations or Wiener's mathematical formalism, had nevertheless built into a brass and iron mechanism a principle of self-regulation that would become one of the central concepts of modern engineering and biology.

The Indicator Diagram and the Birth of Thermodynamics

The final great mechanical contribution of Watt's career was the indicator — a device that drew a graph of the changing pressure of steam inside the cylinder against the position of the piston during a single stroke of the engine. The indicator, devised by Watt and his assistant John Southern at Soho in 1796 or thereabouts, consisted of a small spring-loaded pressure gauge connected to the cylinder, whose movement was recorded by a pencil tracing on a strip of paper that was moved back and forth by a string attached to a fixed point on the engine. The pencil drew a closed loop on the paper — the indicator diagram — whose shape revealed in detail the operation of the engine throughout the stroke: the admission of steam, the expansion of steam after cutoff, the exhaust to the condenser, the compression at the end of the return stroke. The area enclosed by the loop, when measured against the scale of pressure and stroke, gave directly the work done by the engine in a single stroke; the engine's power output could be calculated by multiplying that work by the number of strokes per unit time.

The indicator was, in essence, the first instrument for visualizing a thermodynamic cycle, and it would become the foundation of the experimental study of heat engines that culminated in the second law of thermodynamics. Watt and Boulton, recognizing the indicator's potential for revealing infringers' attempts to copy their engines and for verifying the duty calculations on which their royalties depended, kept the indicator a trade secret of the Soho works for nearly twenty years; the device was not publicly described until the 1820s, by which time Sadi Carnot in France was already developing the abstract theoretical framework that would eventually account for the patterns that the indicator revealed. The combination of Watt's indicator with the mathematical thermodynamics of Carnot, Clausius, Joule, and Kelvin in the nineteenth century produced the modern science of thermodynamics — the science whose foundational diagrams are still drawn in the form of the loops that Watt's indicator first traced on its strip of paper at the Soho Manufactory in the 1790s.

The Cornish Pumping War and the Great Patent Trials

The success of the Watt engine in the 1780s and 1790s drew, as commercial success in eighteenth-century England invariably drew, a swarm of imitators and infringers. The Cornish mining district was particularly fertile ground for the would-be improvers and the patent pirates, because Cornwall was the largest single market for the new engine (the deep tin and copper mines of the south-western peninsula required prodigious quantities of pumping), and because the royalties that Boulton and Watt collected from the Cornish mines were the largest single revenue stream of the partnership. The Cornish mine adventurers, who had become accustomed to paying the partnership perhaps a third of their realized savings in coal, increasingly resented those royalties as a tax on their enterprise and looked for any opportunity to evade them.

The two principal Cornish infringers were Edward Bull, a former employee of Boulton and Watt who had returned to Cornwall in 1791 and had begun erecting "inverted" engines (with the cylinder placed directly above the pump rod, so that no beam was required) in apparent reliance on the argument that his design did not infringe the Watt patent; and Jonathan Hornblower, the son of a respected Cornish engine-builder who had taken out his own patent in 1781 on a compound engine using two cylinders in series. Bull and Hornblower, working together and supported by a syndicate of Cornish mine owners, mounted in the early 1790s a sustained attack on the Watt patent — both by erecting engines that infringed it and by seeking to have it declared invalid in the courts.

The legal contest came to a head in a series of suits brought by Boulton and Watt against the Cornish infringers between 1793 and 1799. The decisive trials were Boulton and Watt v. Bull (1793, before Lord Chief Justice Eyre and a special jury) and Boulton and Watt v. Hornblower (1799, before a special court of judges sitting in the Court of Common Pleas). In both cases, after exhaustive argument and the presentation of expert testimony from many of the leading mechanical engineers of the day (including John Smeaton, who appeared as an expert for the defendants), the patent was upheld and substantial damages were awarded against the infringers. The legal victories of 1793 and 1799 secured for Boulton and Watt the revenue from the Cornish trade for the remainder of the patent term, and they established important precedents in English patent law about the validity of patents for principles of operation (as distinct from particular mechanical embodiments) — precedents whose effects would be felt for a century.

The cost of the litigation, however, was substantial — both in legal fees and in the anxieties and labor it demanded of the partners. Watt particularly suffered from the litigation, which engaged his recurrent depressive tendencies and required him to spend long periods in London during the trials, away from his family and from the work at Soho. He wrote to Boulton during the Bull case in 1793 of his "agonies of mind" at the prospect of testifying in court and of the "irksome and abhorrent" character of patent litigation; in another letter he described the trial as "the most painful experience of my life." But Boulton, with his characteristic political and personal energy, managed the litigation skilfully and saw it through to successful conclusion in 1799 — only months before the partnership's patent would expire by its own terms in January 1800.

The Lunar Society and the Intellectual Life of Birmingham

In Birmingham, throughout the years of the partnership, Watt lived in the company of the most remarkable industrial community of the eighteenth century. The Lunar Society of Birmingham — so called because its members met on the Monday nearest the full moon, so that they might have moonlight to ride home by — was a loose dining and discussion club founded in the 1760s by Matthew Boulton, the physician Erasmus Darwin, and the Birmingham manufacturer William Small. By the time Watt arrived in Birmingham in 1775, the society had become the principal social and intellectual institution of the West Midlands manufacturing community, gathering at the houses of its members for monthly dinners at which the leading natural philosophers, manufacturers, and industrial improvers of the region discussed everything from chemistry and electricity to canal engineering, agriculture, classical literature, and politics.

The regular members of the Lunar Society in Watt's time were Boulton, Darwin, the chemist and theologian Joseph Priestley (who lived in Birmingham from 1780 to 1791 and conducted at his Birmingham house the experiments on the composition of air that would lead to his isolation of oxygen and the establishment of the chemical foundations of combustion theory), the potter Josiah Wedgwood (who lived at Etruria in Staffordshire but came to Birmingham for the meetings), the chemist James Keir (who managed Boulton's enterprise for some years and operated a chemical manufactory of his own at Tipton), the geologist and physician William Withering (the discoverer of the medicinal properties of digitalis), the mathematician and astronomer William Herschel (who lived in Bath but corresponded extensively with the Lunar members), the philosopher and mathematician Samuel Galton, the educational reformer Richard Lovell Edgeworth (the father of the novelist Maria Edgeworth, who herself attended several meetings), and a shifting penumbra of visitors that included Benjamin Franklin on his visits to England in the 1760s and 1770s, the future American minister to France Thomas Jefferson, the Swiss geologist Jean-André Deluc, and the German poet and natural philosopher Johann Wolfgang von Goethe.

The Lunar Society had no formal organization, no constitution, no membership rolls, no minutes — and yet for some thirty years it served as the principal forum in which the practical industrial revolution of the West Midlands was discussed and theorized by the men who were making it. The conversations of the Lunar Society members are one of the great unrecorded losses of intellectual history, surviving only in fragmentary references in their letters and in occasional reminiscences set down in old age. But what survives is enough to suggest the extraordinary intellectual energy of their gatherings: discussions of the chemistry of combustion, of the design of canals and harbors, of the prospects of balloon flight (which the members enthusiastically pursued in the 1780s), of the meaning of the discoveries of Lavoisier and Priestley, of the political implications of the American and French revolutions, of the geological time scale, of the principles of evolution (Erasmus Darwin had proposed a theory of organic evolution in his Zoonomia decades before his more famous grandson published On the Origin of Species), of educational reform, of the abolition of slavery, of the steam engine and its industrial uses.

Watt was, by all accounts, one of the most learned and one of the most modest members of the society. He spoke when he had something to say and listened patiently to the disquisitions of his friends; he was particularly close to Erasmus Darwin (who was his family physician and whose poems he greatly admired), to Joseph Priestley (with whom he conducted experiments on the composition of water in 1782–83), and to William Withering (with whom he later quarreled in one of the few personal rifts of his life). Through the Lunar Society he was drawn into the chemical revolution of the 1780s, into the political controversies surrounding the French Revolution, and into the public conversations of the British scientific community in a way that would not otherwise have been possible for a Scottish-born tradesman working in the West Midlands. The Lunar Society made him a recognized figure in British natural philosophy as well as in mechanical engineering, and the friendships he formed there would last him for the rest of his life.

Margaret Miller and the First Marriage

In July 1764, while Watt was still in his Glasgow workshop and before his decisive walk on Glasgow Green, he married his cousin Margaret Miller, the daughter of his mother's brother. Margaret Miller was, by his own description in the letters of his courtship, "a young lady of a most excellent temper and disposition" — quiet, modest, deeply attached to her family, and well-suited to the temperament of her cousin-husband. The marriage was a happy one. They settled in a small house on the Saltmarket of Glasgow, just outside the precinct of the College, and over the next nine years Margaret bore him six children, of whom only two — the son James Watt the younger, born in 1769, and the daughter Margaret, born in 1767 — would survive to adulthood. The deaths of the four other children in infancy or early childhood marked the household with the kind of repeated grief that the eighteenth century imposed on so many parents, and gave both husband and wife the patient, somewhat melancholy temper that visitors to the Saltmarket house remarked upon.

Margaret Miller Watt was a presence of warm steadiness throughout the most difficult years of Watt's early career — the years of the slow development of the separate condenser, of the disappointing trials at Kinneil, of the strained partnership with Roebuck. She nursed her husband through his recurrent illnesses, kept the household solvent on his slender income from instrument making and his fluctuating earnings from surveying contracts, and managed the upbringing of the children with the kind of careful attention that was the great Scottish virtue of her class. Watt's surviving letters from the late 1760s and early 1770s describe her with a tenderness that was unusual in the formal epistolary style of the period; he speaks of her as "my dear Peggy" and refers repeatedly to her good humor, her domestic capability, and her uncomplaining patience in the face of his troubles.

Her death in September 1773 was the most devastating personal loss of Watt's life. She had been delivered of a stillborn child after a difficult labor; childbed fever set in within hours of the delivery; and despite the best efforts of the physicians she lapsed into delirium and died within a few days. Watt was at the time absent from Glasgow on a canal survey in the western Highlands, and the news reached him only a week after her death. He left the survey at once and traveled back to Glasgow in a state of stunned grief that lasted for many months. The letters he wrote in the weeks after Margaret's death are among the most affecting documents in his correspondence; one of them, to his Glasgow friend the chemist John Robison, contains the simple phrase "I have lost the dear partner of my life," followed by several lines of broken sentences in which he attempts to describe the desolation of his household. The two surviving children — James the younger, then four, and Margaret, then six — were placed in the care of Watt's father in Greenock, and Watt himself, suspended between grief and the demands of his still-unfinished engine work, was for some months scarcely capable of work of any kind.

The Widowed Years and the Move to Birmingham

The years between Margaret's death in September 1773 and Watt's arrival in Birmingham in May 1775 were the lowest point of his life. He was thirty-seven years old, a widower with two small children, with a half-developed steam engine that had not yet earned him a penny, with a partner (Roebuck) on the verge of bankruptcy, with a father in declining health and dwindling means, and with health problems of his own that had grown markedly worse in the months after Margaret's death. He took on, in the absence of any return from the engine, a considerable load of surveying and civil engineering work — the survey of the Monkland Canal between Glasgow and the coal fields east of the city, the survey of the Crinan Canal across the Mull of Kintyre, the survey of harbor improvements at Port Glasgow, the survey of road improvements on the Highland trunk routes — and he was, by 1774, recognized as one of the most competent civil engineers in Scotland, though his employment in that capacity was always reluctant and undertaken only as a means of paying his bills while the engine project struggled to find its footing.

It was during this period of widowhood and uncertainty that Boulton, watching the slow collapse of Roebuck's affairs from his Birmingham vantage point, began to negotiate the arrangements by which Watt would move to Birmingham and the partnership would be set on its proper foundation. Boulton's letters to Watt during these months — sympathetic, encouraging, full of practical suggestions for the development of the engine and for the conduct of the partnership — were a critical source of moral support to a man who was, by his own admission, in danger of giving up the engine project altogether. "Take heart, my dear Sir," Boulton wrote in one of these letters, "your engine will yet astonish the world, and Soho is your true home." The promise was not idle. Watt's arrival at Soho in May 1775 was the beginning of his recovery — of his health, of his spirits, of his career, and of his personal life.

Ann Macgregor and the Second Marriage

In July 1776, fourteen months after his arrival at Soho, Watt married Ann MacGregor, the daughter of a successful Glasgow timber merchant and bleacher. Ann MacGregor was eleven years younger than Watt, by all accounts an attractive, intelligent, and capable young woman, who had been known to Watt as a family acquaintance for some years before their marriage. The courtship had been carried on partly by correspondence (Watt was in Birmingham, Ann in Glasgow) and partly during Watt's visits to Glasgow on business. Ann's father, James MacGregor, had been at first hesitant about the match — Watt's prospects at Soho were not yet entirely clear, and the MacGregors were a more prosperous family than the Watts — but Watt's growing reputation as the engineer of the new steam engine, and Boulton's discreet assurances of the partnership's prospects, eventually overcame these hesitations. The marriage was settled in July 1776, and Ann came south to Birmingham to take up residence at Harper's Hill, a comfortable house on the outskirts of the city that Watt had taken on a long lease.

The second marriage was, like the first, a happy one, though of a different temper. Ann was a more vigorous and practical woman than the gentle Margaret had been — she managed the household with brisk efficiency, took a keen interest in her husband's business affairs, was a careful manager of his finances, and was less inclined than her predecessor to the quiet domestic deference that had characterized the first Mrs. Watt. She bore her husband two children: Gregory, born in September 1777, and Janet (always called "Jessy"), born in 1779. Together with the two children of Watt's first marriage — James the younger and Margaret — the household at Harper's Hill comprised four children when the family was reunited, and Ann was, by all accounts, an even-handed and affectionate stepmother to the older children as well as a devoted mother to her own. The marriage would last for the remaining forty-three years of Watt's life. Ann would survive her husband by thirteen years, dying at Heathfield in 1832 at the age of eighty-five.

James Watt the Younger and the Political Crisis

The most important of Watt's children was his eldest son, James Watt the younger, born in February 1769, who was destined to play a substantial part in the family business and in the public life of Birmingham. James the younger was educated first at home and then at the Birmingham Academy and at a school in Geneva, where he was sent in his sixteenth year to complete his studies in modern languages and to acquire the social polish that his Birmingham education had not provided. He returned to Birmingham in 1786, took up a position in the Soho counting-house, and was being groomed by his father and by Boulton to assume in due course the management of the partnership.

But the years that James the younger spent in continental Europe — first in Geneva, later in extended visits to Paris between 1787 and 1792 — coincided with the great political upheavals of the French Revolution, and James the younger was, in his early twenties, an enthusiastic supporter of the revolutionary cause. He was in Paris in 1792, during the radical phase of the Revolution, where he attended the Jacobin Club, dined with Brissot and Roland, and was, with the Manchester reformer Thomas Cooper, formally presented to the National Convention as a representative of the British "Society for Constitutional Information" — an act of public partisanship that brought him to the unfavorable notice of Edmund Burke, who denounced him by name in a speech in the House of Commons in April 1792, alongside Thomas Cooper, as one of the "young Englishmen who have gone over to mix in the troubles of France." The Burke denunciation was a serious matter for the Watt family. Britain was on the brink of war with revolutionary France; political dissent at home was being increasingly criminalized; and the elder Watt's commercial fortunes depended on his good relations with the British government and with the Birmingham establishment.

The elder Watt was profoundly distressed by his son's political activities. He himself was a moderate Whig in politics, sympathetic in principle to the early stages of the French Revolution but increasingly alarmed by its radical turn, and personally horrified by the September Massacres and the execution of the king in January 1793. He wrote his son a series of urgent letters in 1792 and 1793 urging him to return to England, to break off his connections with the radical elements in Paris, and to take up his place in the family business. James the younger, after some hesitation, returned to Birmingham in late 1792, and his political enthusiasms gradually cooled in the more sober atmosphere of Soho. By the late 1790s he had become a thoroughly Burkean conservative, a defender of the British constitution against the perceived menace of French revolutionary principles, and the head (after his father's gradual retirement) of the increasingly profitable Boulton, Watt and Sons engine works at Soho.

Gregory Watt and the Early Death

The second son of the second marriage, Gregory Watt, was the most brilliant and the most tragic of Watt's children. Born in 1777, he was, from an early age, of conspicuous intellectual ability and of a charm of manner that drew all who met him to his side. He was educated at the Birmingham Academy and then at the University of Glasgow, where between 1793 and 1796 he distinguished himself in classics, in mathematics, and (under the guidance of Joseph Black, who had returned to Edinburgh) in chemistry and geology. While at Glasgow he became close friends with the chemist Humphry Davy, who was then a young apothecary's apprentice in Penzance, and through Gregory Watt's introduction Davy was brought into the orbit of the Pneumatic Institution at Bristol (where he conducted his early experiments on nitrous oxide) and ultimately into the Royal Institution in London, of which he would become the most celebrated professor of chemistry in the first decade of the nineteenth century.

Gregory Watt was the great hope of his father's later years. He combined his father's scientific seriousness with his mother's social grace, and the elder Watt looked to him to carry forward the intellectual life of the family in a more public way than the engineer-businessman elder son would do. But Gregory Watt's constitution was weak. He had contracted consumption — pulmonary tuberculosis — in his late teens, and the disease progressed steadily through his twenties despite long periods of recuperation in the milder climate of Cornwall and on the Mediterranean coast. He died at Heathfield, the house his father had built outside Birmingham, on 16 October 1804, at the age of twenty-seven. The blow to his father was severe. The elder Watt was, at the time of Gregory's death, sixty-eight years old, in retirement from active business, comfortable in fortune but increasingly isolated as the friends of his middle years died around him. The death of Gregory was, his correspondence shows, the deepest grief of his old age. He wrote to a Glasgow friend, three months after Gregory's death, that "the loss of so excellent a son leaves me without the heart for the pursuits that gave my life its purpose." He continued to mourn Gregory for the remaining fifteen years of his own life.

The Literary and Chemical Controversies of Water

While his engine business was being established at Soho through the 1780s, Watt was simultaneously engaged in some of the most important chemical investigations of the decade. The most consequential of these was the question of the composition of water — a question that, in retrospect, marks one of the turning points in the chemical revolution of the late eighteenth century. The phlogiston theory, which had dominated chemistry since the early eighteenth century, held that combustion was the release of a substance called "phlogiston" from a burning body, leaving behind a "calx" or "dephlogisticated" residue. By the late 1770s, however, the new pneumatic chemistry of Priestley, Cavendish, Scheele, and Lavoisier had begun to suggest that the phlogiston theory was inadequate to account for the increasingly precise measurements that were now possible of the gases consumed and produced in combustion.

The composition of water — the question of whether water was, as the ancients and most chemists had supposed, an element, or whether it might be a compound of more elementary gases — was the most important open question in pneumatic chemistry in the early 1780s. Henry Cavendish, the wealthy and reclusive Hertfordshire natural philosopher, had performed in 1781 a series of careful experiments in which he had detonated mixtures of inflammable air (hydrogen) and dephlogisticated air (oxygen) by electric sparks and had observed that the only product of the explosion was water — a result that suggested unambiguously that water was a compound of these two gases. Joseph Priestley had performed similar experiments independently and had reached similar conclusions. Watt, in the course of correspondence with Priestley about these results, had himself worked out a theoretical interpretation in November 1783 and had communicated it in a letter to Priestley, who read it to the Royal Society in April 1784. The letter, which was printed in the Philosophical Transactions for that year, argued that water was a compound of inflammable air and dephlogisticated air "deprived of part of its latent heat" — a formulation that combined the new pneumatic chemistry with the older theory of latent heat that Watt had absorbed from Joseph Black.

The priority dispute over the discovery of the composition of water that followed was one of the most acrimonious controversies of eighteenth-century science. Cavendish published his results in the Philosophical Transactions for 1784 (some weeks after Watt's letter had been read but before Watt's letter was printed); the French chemist Lavoisier, working independently in Paris, claimed in his own Mémoires de l'Académie that he had reached the same conclusion in his experiments of 1783, and his claim was supported (despite Lavoisier's late date of publication) by the prestige of French science. The controversy was particularly bitter because the three principal claimants represented three different theoretical frameworks: Watt and Cavendish were still working within the phlogiston theory (though both were beginning to find it inadequate), while Lavoisier was the great spokesman for the new "oxygen theory" of combustion that would soon supersede phlogiston entirely. The credit for the discovery has been distributed differently by different historians of chemistry; modern accounts generally award priority of the experimental observation to Cavendish and priority of the correct theoretical interpretation (water as a compound of hydrogen and oxygen) to Lavoisier, while acknowledging Watt's contribution to the chemical thinking that led to the discovery. Watt himself, after the bitterness of the immediate controversy had faded, was characteristically generous in his references to Cavendish and Lavoisier and accepted that the priority question would never be settled in his favor. But the controversy left him with a permanent distaste for chemical publication, and he wrote little on chemistry in the remaining decades of his life.

Chlorine Bleaching and the Linen Industry

A more constructive chemical contribution of Watt's middle years was his work on the application of chlorine to the bleaching of linen and cotton. The Swedish chemist Carl Wilhelm Scheele had discovered in 1774 the gas that would later be named chlorine (which Scheele called "dephlogisticated marine acid"), and the French chemist Claude-Louis Berthollet had shown in 1785 that chlorine had remarkable bleaching properties when dissolved in water or in alkali. Berthollet's discovery promised to revolutionize the textile bleaching industry, which had until then depended on a tedious process of exposure to sunlight on bleaching greens that occupied vast tracts of land near the textile mills and took weeks or months to complete.

Watt visited Berthollet in Paris in 1786 and, with characteristic energy, set himself to learn the details of the new process. On his return to Birmingham he wrote to his father-in-law James MacGregor, who was a bleacher at Glasgow, describing the new method, and undertook over the following two years a series of practical trials on commercial linen at MacGregor's bleach field. The trials were successful, and the MacGregor bleach field became the first in Scotland to adopt chlorine bleaching on a commercial scale. The process spread rapidly through the British textile industry over the following decade — taking advantage particularly of the calcium-hypochlorite "bleaching powder" patented in 1799 by Charles Tennant of Glasgow — and revolutionized the economics of cotton and linen bleaching, reducing the time required for the bleaching of a batch of cloth from six months to a few days and freeing the enormous acreage that had previously been devoted to the bleaching greens. Watt's contribution to the introduction of chlorine bleaching has been somewhat obscured by Tennant's later commercial dominance of the field, but he was the principal channel through which Berthollet's discovery reached the Scottish bleaching industry, and his own letters to MacGregor and others in 1786–87 are the foundation documents of British chlorine bleaching.

The Letter-Copying Press

The most widely used of Watt's inventions, after the steam engine itself, was his letter-copying press, patented in 1780 and manufactured at Soho throughout the partnership years. The problem the press addressed was the very mundane one of producing copies of business letters. In the late eighteenth century, the volume of correspondence in commercial firms was growing rapidly, and the customary method of producing copies — having a clerk write out the letter a second time by hand — was both slow and unreliable. Watt's press solved the problem by an ingenious chemical method. The original letter was written in a special ink containing a high proportion of gum (which prevented the ink from drying completely) and pressed against a sheet of very thin, slightly damp tissue paper through a small mechanical press; the ink transferred to the tissue and produced a reverse image that could be read through the back of the tissue, which would be thin enough to be transparent. The result was a permanent copy of the original letter, made in a few seconds, that could be filed and retained in the business records.

The letter-copying press was first marketed by the firm of Watt, Boulton and Keir (a separate firm from the engine partnership, set up specifically to market the press) in 1780 and was an immediate commercial success. It was adopted by mercantile houses, banks, government offices, and the larger professional firms throughout Britain over the following years, and was exported in considerable numbers to the continent and to America. (Benjamin Franklin, returning to America in 1785 after his long diplomatic mission in France, took with him several Watt copying presses, and the press was adopted by Thomas Jefferson and other American statesmen as their standard office instrument; Jefferson's correspondence in the Library of Congress includes thousands of copies made by Watt's press over the four decades of his public life.) The press remained the standard office instrument of the English-speaking commercial world until it was displaced by the typewriter (with its carbon paper for making copies) in the late nineteenth century, and the term "letter-press copy" survived in legal and business usage for many decades after the original device had passed out of use.

CIVIL Engineering and the Argyllshire Canals

In addition to his work in steam power, chemistry, and the letter-copying press, Watt was a practicing civil engineer of considerable reputation throughout his Scottish years and the early Birmingham years. The principal civil engineering works on which he was engaged were the survey of the Monkland Canal (1769–73), which connected the Glasgow market to the Lanarkshire coal fields and which Watt designed in considerable detail before resigning in 1773 to focus on the engine project; the survey of the Crinan Canal across the Mull of Kintyre (1771), which proposed a route that was largely followed when the canal was eventually built between 1794 and 1801; the survey of the Caledonian Canal between Inverness and Fort William (1773), which laid out a route similar to that which Thomas Telford eventually followed in the canal's construction between 1804 and 1822; the survey of harbor improvements at Greenock, Glasgow, Port Glasgow, and Ayr; and the survey of road improvements on the Highland trunk routes.

The civil engineering work was always a sideline for Watt — undertaken to pay his bills while the engine project struggled to find its footing — and he resigned from his last major civil contract (the Monkland Canal) in 1773 with evident relief. But the work he did in these years was of high quality, and the surveys he produced for the Crinan and Caledonian canals were used as the foundation documents for the canals' eventual construction a quarter of a century later. He was one of the founding members of the Smeatonian Society of Civil Engineers (the professional society founded in London by John Smeaton in 1771 and renamed in his honor after his death), and he maintained throughout his life a high reputation among the civil engineers of Britain even after his principal energies had been directed elsewhere.

The Steam Engine as the Prime Mover of Industry

By the 1790s, the Watt engine had become the dominant industrial prime mover of Britain. The total number of Watt engines built by Boulton and Watt and their licensees by 1800 was, by the partnership's own count, four hundred and ninety-six — a number that, while it sounds modest by the standards of subsequent industrial production, represented the very large majority of all steam engines in commercial operation at that date. The Boulton and Watt engines were drainage engines (for Cornish tin and copper mines and for the deeper coal mines of the Midlands and the north), blast-furnace blowing engines (for the ironworks of South Wales, Shropshire, and the Black Country), mill engines (for cotton mills in Lancashire, woolen mills in Yorkshire, flour mills in London and elsewhere), and a growing number of specialized engines for paper mills, breweries, sugar refineries, and other industrial processes.

The engine had transformed the geography of British industry. The Newcomen engine, with its prodigal consumption of coal, had been economical only at the pit head; the Watt engine, with its much lower fuel consumption, could be operated profitably almost anywhere coal could be brought by canal or coastal vessel. This new locational freedom released the textile mills from their dependence on water power and allowed them to be sited in towns where labor was abundant — Manchester, Bolton, Oldham, Glasgow, Belfast — rather than in the river valleys where water power was available. The transformation of Lancashire from a region of scattered weavers into the world's first industrial conurbation owed more to the Watt engine than to any other single technology. Similar transformations took place in the South Wales iron industry (where the Watt engines blew the bellows of the great blast furnaces of Cyfarthfa, Dowlais, and Penydarren that made South Wales briefly the world's leading iron-producing region), in the Midlands metalworking trades, and in the Yorkshire and Northumbrian coal fields.

Beyond British shores, the Watt engine was beginning by the 1790s to spread to the continent and to America. The first Watt engines in France were installed at Le Creusot in 1781 (for the iron foundry there) and at Chaillot near Paris in 1781 (for the water-supply works of the city of Paris); engines were exported to Holland, Italy, Spain, Russia, and Sweden during the 1780s and 1790s. The first Watt engine in America — a pumping engine of nineteen-inch cylinder built at Soho in 1801 and shipped to New York in 1802 — was installed at the Philadelphia waterworks, where it became the first municipal water-pumping engine of the new republic. By the time Watt died in 1819, the Watt engine had become an international technology, and the engineering principles he had worked out at Glasgow and Soho were the foundation of the steam engineering tradition that would dominate industrial production through the nineteenth century.

The End of the Patent and the Transition to Retirement

The Watt patent of 1769, extended by act of Parliament in 1775, expired on 31 January 1800. The expiry was an event of considerable consequence both for the Boulton and Watt partnership and for the British steam-engine industry as a whole. For the partnership, the expiry meant the end of the royalty stream that had been the principal source of the engine business's revenue for twenty-five years; in compensation, the partnership had spent the last several years of the patent term building up the manufacturing capacity at Soho to compete with the new entrants who would now be free to build engines of the Watt design without royalty. For the British industry, the expiry meant the long-awaited opportunity for competing manufacturers (chief among them, in due course, the firms of Maudslay and Field in London, of Fenton, Murray and Wood in Leeds, and of Boulton and Watt's various former workmen who set up in business as engine builders on their own account) to enter the market freely. The next twenty years would see a great expansion of the British steam-engine industry and the introduction of important new variants — Richard Trevithick's high-pressure engine, the early railway locomotive of Stephenson, the marine engines of Boulton, Watt and the younger firms — that would build on but go beyond the design that Watt had patented in 1769.

The expiry of the patent coincided, by Watt's deliberate planning, with his own retirement from active business. He had begun by 1795 to withdraw from the daily management of the engine works, leaving the conduct of the business increasingly to his son James the younger and to Matthew Robinson Boulton (the son of Matthew Boulton senior, who had begun to manage his father's affairs as the elder Boulton aged and weakened). By 1800 the management of the firm — now styled Boulton, Watt and Sons — was effectively in the hands of the younger generation, and the elder Watt was free to devote his time to the personal pursuits that had had to wait through the busy years of the engine business.

Heathfield and the Retirement Years

The retirement house that Watt built for himself in 1789–90 was Heathfield, a substantial Georgian country house on the heath that lay between Birmingham and the village of Handsworth, some three miles north-west of the Soho Manufactory. The house was set in extensive gardens that Watt himself laid out, and it included a study at the top of the house — a small, oddly-shaped room under the slates of the roof, lined with shelves and cabinets and reached by a narrow stair — that became Watt's workshop and study for the last thirty years of his life. The workshop survives in essentially its original state, having been preserved by Watt's family and by his descendants after the demolition of the rest of the house in the early twentieth century; it is now installed as a permanent exhibit at the Science Museum in London, where it is one of the most evocative engineering relics of the eighteenth century.

The workshop at Heathfield contained the tools and equipment that Watt used in his retirement years for personal projects — a small lathe of his own design, a set of fine files and gravers, a workbench with vises and clamps, shelves of chemical glassware, cabinets full of mineralogical specimens, drawing instruments, a small library of technical books and journals — and the partly finished projects on which he was engaged at the time of his death. The most ambitious of these later projects was a sculpture-reducing machine — a pantograph-like device that copied a three-dimensional sculptural figure at a reduced scale by means of a probe that traced the surface of the original and a corresponding cutter that worked on the blank — which Watt worked on intermittently from 1809 until his death and which he never brought to commercial perfection. Several copies made by the machine survive (including a small bust of Watt himself that he made as a test of the device), and they are remarkable for the precision of their finish, but the machine was too complex and too laborious to operate to compete commercially with the hand methods of the engravers and the sculptors of the period.

The retirement years were comfortable, productive, and on the whole happy. Watt was, by 1800, a wealthy man — his share of the engine business had earned him over the previous twenty-five years a sum well in excess of one hundred thousand pounds, which he had invested in agricultural estates in Wales and in the funds of the Bank of England — and he was free to pursue his personal interests without commercial constraint. He read widely (his library at Heathfield contained some three thousand volumes at the time of his death, with a particular strength in modern French and German literature, in chemistry, and in the history of music); he conducted occasional chemical experiments; he received the steady stream of visitors who came to pay their respects to the most distinguished engineer of his age; he traveled with his wife to Scotland in the summers, and to the spa towns of England (Bath, Buxton, Cheltenham) when his health required them; he corresponded with the surviving members of the Lunar Society and with younger engineers and scientists who sought his advice.

Among the visitors of his retirement years were many of the leading natural philosophers and engineers of Europe and America. Walter Scott visited him at Heathfield in 1817 and described him, in a famous passage of his journals, as "the most agreeable old man I have ever met with — gentle, unassuming, full of curious knowledge, and possessed of the most extraordinary intellectual range that I have known in any individual." The geologist William Buckland visited him in 1815; the chemist Humphry Davy (the friend of his son Gregory) was a frequent guest; the elder Robert Stephenson (the father of the railway pioneer) came to consult him about the engine of his Killingworth wagonway; the American statesman Albert Gallatin visited him on his diplomatic mission to Britain in 1815 and was charmed by his soft Scottish accent and his unaffected modesty. The visitors uniformly remarked upon the curious combination of celebrity and simplicity in Watt's old age — the great inventor receiving them in his shabby workshop or in his book-lined parlor, dressed in old clothes, slow and gentle in his speech, slightly deaf in one ear, but as keenly interested in the work of younger investigators as if he had been a young engineer himself.

Honors and Public Recognition

The public honors that came to Watt in his retirement years were the natural reward of his contribution to the industrial transformation of Britain. He was elected a Fellow of the Royal Society of London in 1785, a member of the Royal Society of Edinburgh in 1784, a foreign associate of the Institut de France in 1808 (an honor that placed him among the eight foreign members of that body — the most distinguished foreign honor that could be conferred on a man of science), and a member of the Batavian Society of the Netherlands in 1787. He received honorary doctorates from the University of Glasgow in 1806 and from the University of Edinburgh in 1814. He was offered a baronetcy by the British government in 1814, but declined the honor — saying, in the half-apologetic letter he wrote to the Prime Minister Lord Liverpool, that he was too old to assume the new dignities of rank and that he wished to be remembered simply as "a working engineer." (The baronetcy that Watt declined was offered shortly afterward to Humphry Davy, who accepted it.)

The most curious of his retirement-year honors was an invitation extended to him in 1814 by the Russian Academy of Sciences in St. Petersburg, which proposed to elect him an honorary member and at the same time to commission him to design a set of steam engines for the development of the Russian iron industry. Watt declined the engineering commission (he was at this time seventy-eight years old and in declining health) but accepted the academic honor, and he corresponded for some years afterward with the Russian academician Anders Lexell about questions of latent heat and the chemistry of combustion.

The Final Years

The final years of Watt's life were marked by gradual physical decline but by sustained intellectual energy. His wife Ann maintained the household at Heathfield with the same brisk efficiency that she had brought to it forty years before; his son James the younger was a frequent visitor from Soho; his daughter Janet had married in 1801 the surgeon James Miller of Glasgow and had moved with her husband to Edinburgh, but she came south every summer with her growing family to spend several months at Heathfield. The household was prosperous, comfortable, and full of the kind of domestic activity that gave the old engineer the steady framework within which his last creative work could be carried on.

He worked, almost to the day of his death, on the sculpture-reducing machine and on a few other small projects in his workshop. He continued to receive visitors, to read in his library, to correspond with younger engineers and scientists. He attended in October 1818, at the age of eighty-two, the first meeting of the newly founded Institution of Civil Engineers in London — an organization whose founding owed something to his own pioneering work, and whose leading members regarded him as the patriarch of the engineering profession. He was visited at Heathfield in May 1819 by the painter John Constable, who was working on a portrait of him commissioned by the firm; the portrait was completed only a few weeks before Watt's death and shows him seated in the parlor at Heathfield with his hand resting on a small steam engine model — a composition that has been reproduced many times in the iconography of the engineer.

Watt died at Heathfield on 25 August 1819, eight months after his eighty-third birthday. His final illness was brief; he had been declining gradually through the summer, but his mental faculties were essentially undiminished until the last week, when he lapsed into a peaceful semi-consciousness and died in his sleep on the morning of 25 August. He was buried in the parish church of St. Mary's, Handsworth, in a tomb that lies adjacent to that of his old partner Matthew Boulton (who had died ten years before, in 1809) and not far from those of Joseph Priestley (who had died in exile in Pennsylvania in 1804 but whose memorial tablet stood in the same churchyard) and of several other members of the Lunar Society. A marble statue by Francis Chantrey was erected over the tomb in 1825, and a larger memorial — Chantrey's seated figure of Watt — was placed in the north transept of Westminster Abbey in 1834, after a national subscription that raised the very considerable sum required for so prominent a memorial.

Personal Character and Habits

The personal character of James Watt, as it emerges from his vast correspondence, from the recollections of his contemporaries, and from the testimony of his family, was unusually attractive for a man of such professional eminence. He was, throughout his life, a quiet, modest, somewhat shy man whose temperament inclined toward melancholy but who concealed his depressive moods behind a surface of unfailing courtesy. He was not eloquent in conversation — he spoke slowly and deliberately, with a marked Scottish accent that he never lost, and he was reluctant to speak in public — but his conversation, when it could be drawn out, was admired by all who knew him for its accuracy, its breadth of knowledge, and its unaffected modesty.

His habits of work were those of a methodical and patient craftsman. He rose early, took a light breakfast, worked through the morning in his workshop or his study, took a short walk after dinner (the family dinner was at four o'clock in the early Soho years, somewhat later in the retirement years at Heathfield), worked again through the afternoon and early evening, and read or conversed with his family in the late evening. He was not, by the standards of his Birmingham contemporaries, a heavy drinker — he took a glass or two of wine at dinner and occasionally a tumbler of whisky-and-water in the evening, but he avoided the heavy drinking that was a common feature of Birmingham middle-class life. He smoked a pipe occasionally but not regularly. He was, until the last years of his life, an indefatigable walker, taking long walks in the countryside around Heathfield in all weathers, and his health (after the recurrent illnesses of his youth) was generally good once he was settled in the kinder climate of the West Midlands.

His religious convictions were those of a moderate Scottish Presbyterian — broadly Christian in their fundamentals, somewhat skeptical in their theological details, formal rather than fervent in their devotional practice. He attended the parish church of Handsworth on Sundays and contributed generously to its support, but he was not given to public religious profession and was quite tolerant of the dissenting and freethinking opinions of his Lunar Society friends. (Joseph Priestley was a Unitarian whose theological views provoked the mob that burned his Birmingham house in 1791; Erasmus Darwin was a deist; Matthew Boulton's religious views were probably closer to deism than to orthodoxy; Watt himself maintained, throughout these differences, a calm Presbyterian moderation.)

His political opinions were those of a moderate Whig, sympathetic in principle to constitutional reform and to the abolition of slavery, suspicious in practice of the kind of radical political agitation that frightened the propertied classes of his generation. He supported the early stages of the American and French revolutions but was alarmed by their later developments; he supported parliamentary reform but opposed manhood suffrage; he supported the abolition of the slave trade (and his son James the younger was an active member of the Wedgwood family's anti-slavery committee) but was cautious about the consequences of emancipation in the West Indies. His political views were, in short, those of the moderate Birmingham manufacturing class of his time — substantial in their economic interest, cultivated in their intellectual life, philanthropic in their public actions, and Liberal-Whig in their parliamentary alignment.

Legacy and Historical Significance

The historical significance of James Watt may be considered under three headings: technological, economic, and conceptual. The technological significance is the most immediate. The Watt engine, as it was developed between the patent of 1769 and the expiry of 1800, was the foundation of the modern steam-power industry and, through that industry, of the entire industrial transformation of Britain and (within a generation) of much of Europe and North America. The fundamental design principles that Watt established — the separate condenser, the closed cylinder, the steam jacket, the double-acting principle, the parallel motion, the centrifugal governor, the indicator diagram — were the basic vocabulary of the steam engine for the next century and a half, and they survived as the working principles of every reciprocating steam engine that was built down to the displacement of steam by internal combustion and electric power in the early twentieth century.

The economic significance is more diffuse but no less important. The Watt engine transformed the energy economy of Britain in the late eighteenth and early nineteenth centuries, replacing the limited and variable power of falling water, of windmills, of horses, and of human labor with the controllable and concentrated power of steam. The release of British industry from its dependence on water power allowed the rapid concentration of textile manufacturing into the great mill towns of Lancashire, Yorkshire, and Scotland; the freeing of metal smelting from its dependence on charcoal and limited water-blown bellows allowed the rapid expansion of iron and steel production in the Black Country, in South Wales, and in central Scotland; the freeing of mine drainage from its dependence on the inefficient Newcomen engine allowed the deepening of coal and metal mines to depths that had been previously impossible. The aggregate effect of these transformations was the displacement of Britain from a predominantly agricultural economy (in 1760) to a predominantly industrial one (in 1830) — the original "Industrial Revolution," whose mechanisms have been studied by economists and historians ever since and whose central technological component was, by general consent, the Watt engine.

The conceptual significance is perhaps the most enduring. The Watt engine was the first machine that could be analyzed in terms of input (coal) and output (work), and the first whose efficiency could be measured and improved by the careful study of its operation. The notion of "horsepower" — Watt's own invention, devised as a unit by which the work output of his engines could be compared with the muscular work of the horses they replaced — was the first standard unit of mechanical power, and it remained the unit of power in commercial use until the twentieth century. The notion of "work" as a measurable quantity — distinct from energy, distinct from force, distinct from heat — was developed in the early nineteenth century in considerable part through reflection on the operation of the Watt engine. The notion of "efficiency" as a ratio of useful output to total input was applied first to the Watt engine, and the analysis of its efficiency in terms of the thermodynamic limitations of any heat engine was the central problem from which Sadi Carnot in the 1820s derived the foundation principles of thermodynamics. The notion of "feedback control" was extracted by Maxwell and (later) by Wiener from the operation of the Watt governor and became the foundation of modern control theory and modern cybernetics. The notion of "indicator diagrams" — the graphical representation of thermodynamic cycles — was developed by Watt and Southern at Soho in the 1790s and became the standard tool of the analysis of heat engines.

In each of these conceptual developments, the Watt engine served as the concrete machine on which abstract concepts were developed — the working object whose careful study gave rise to the theoretical sciences of thermodynamics, of control theory, and of mechanical engineering more generally. Watt himself, who was no mathematical theorist and who never developed any of these concepts in their abstract form, would not have recognized the language in which his successors expressed them; but he provided the engineering substrate without which the theoretical developments would have been impossible. The unit of power that bears his name — the watt, defined since the late nineteenth century as the rate of doing work of one joule per second — is the most enduring of the memorials of his contribution. Every electric light bulb, every motor, every electrical generator, every nuclear reactor is rated in units that take their name from the Scottish instrument maker whose patient analysis of the wastage of steam in a Newcomen engine model, two and a half centuries ago, opened the door to the controlled and quantified manipulation of mechanical power.

The Statue in Westminster Abbey and the National Memory

The most striking expression of the national reverence in which Watt was held after his death was the great marble statue by Sir Francis Chantrey that was unveiled in the north transept of Westminster Abbey in 1834 — fifteen years after Watt's death. The statue, which depicts Watt seated in scholarly meditation with a sheet of mathematical drawings on his knee, was commissioned by national subscription at a cost of six thousand pounds (a sum equivalent to several million pounds in modern currency), the subscribers being drawn from the leading manufacturers, scientists, and political figures of the day. The inscription, drafted by Lord Brougham, declared Watt to be "the inventor whose genius gave to his country the steam engine, and to the world the most powerful instrument for the multiplication of human capabilities and the increase of human comfort that has ever been added to the resources of mankind." The grandiloquence of the inscription was characteristic of the early-Victorian style and was the subject of some satirical comment at the time, but it accurately conveyed the sense in which the British public of the 1830s had come to regard Watt as a national hero of the first rank — comparable, in his contribution to the country's greatness, with Marlborough or with Nelson.

The Westminster Abbey statue was followed by a series of other public memorials. The colossal seated bronze in St. George's Hall in Liverpool, unveiled in 1855, was the work of John Gibson; the standing statue in George Square, Glasgow, was unveiled in 1832 (and is one of the earliest of the modern statues in that square, alongside those of Walter Scott, Robert Burns, and the Duke of Wellington); the great memorial column in Greenock, erected on the heights above Watt's birthplace, was unveiled in 1872. Schools and engineering institutes in many British cities were named after him in the nineteenth and early twentieth centuries — Heriot-Watt University in Edinburgh (founded as the Edinburgh School of Arts in 1821 and named for Watt and his Glasgow contemporary George Heriot in 1885), James Watt College in Greenock, the Watt Memorial Library in Greenock, the James Watt building at the University of Glasgow, the James Watt building at Birmingham University. The town of Watt, in the state of Pennsylvania, was named for him by Scottish settlers in the 1820s; so were the now-defunct towns of Wattville (Kentucky), Watt's Folly (Tennessee), and the still-existing Watts (California). His name was given to dozens of streets, parks, public houses, and other minor memorials throughout the British Isles, the United States, Canada, and Australia.

These public memorials reflect, in their cumulative density, the unusual depth of the national affection for Watt — an affection that was based not only on the admiration of his technical achievement but on the perception of his personal character as one of unusual modesty and goodness. In the popular imagination of the Victorian period, Watt occupied a niche similar to that occupied in the American imagination by Benjamin Franklin (a man whom, as it happens, Watt had met and admired) — the practical genius of humble origin whose patient industry had transformed the conditions of life for his fellow citizens. The hagiographical biographies of Watt that were produced in the Victorian period (most notably Samuel Smiles's Lives of Boulton and Watt of 1865) emphasized this aspect of his character and made him one of the principal exemplars of the Victorian creed of self-help and respectable industry.

The Scientific and Engineering Reception

The reception of Watt's work within the technical and scientific community has gone through several phases since his death. The first generation of engineers and scientists who succeeded him — the men who built the railways and the steamships of the early Victorian period — regarded him essentially as a patriarch whose engineering principles they were extending and applying. The Stephensons, Brunel, the marine engineers of Glasgow and the Clyde, the locomotive engineers of Crewe and Doncaster, the great civil engineers of the Victorian transport network — all of them worked within a tradition that traced itself directly to the Watt engine, and they regarded Watt with the kind of professional reverence with which a tradition's founder is conventionally honored.

The second phase, in the late nineteenth and early twentieth centuries, saw the development of the science of thermodynamics by Clausius, Kelvin, Maxwell, and their successors, and the assimilation of the Watt engine into the new theoretical framework. Watt himself, in this phase, came to be regarded as the engineer whose practical work had made the theoretical analysis possible — the indispensable empirical foundation on which Carnot, Clausius, and Kelvin had built. The Maxwell paper of 1868 on governors and the various papers on the thermodynamics of the steam engine that appeared in this period took the Watt machine as the canonical example and built around it the mathematical analysis that engineers would use for the next century.

The third phase, in the mid- and late-twentieth century, saw a more critical historical assessment of Watt's contribution. The classic biographies of the Victorian period (Smiles, Muirhead, the engineering biographies of the early Institution of Mechanical Engineers) had presented Watt as the lone genius who had transformed the steam engine by his unaided insight on Glasgow Green; more recent historical scholarship — particularly the works of L. T. C. Rolt, of Eric Robinson, of A. E. Musson, of Richard Hills, and most recently of Ben Russell — has placed Watt's achievement in a richer institutional and social context, showing the importance of Joseph Black's chemical theories, of Matthew Boulton's commercial enterprise, of John Wilkinson's precision boring, of the Lunar Society's intellectual stimulation, and of the patent system itself in the making of the Watt engine. Watt himself emerges from this more recent literature as a more complex figure — less the solitary genius of the Victorian biographies, more a man of considerable ability working within a fortunate combination of institutional, social, and intellectual circumstances. The shift in interpretation does not diminish Watt's historical importance, but it situates his achievement more accurately within the texture of the eighteenth-century industrial transformation of which he was both an agent and a product.

The Two Hundredth Anniversary and Beyond

The celebrations marking the bicentenary of Watt's patent in 1969 and the bicentenary of his death in 2019 produced renewed scholarly and public attention to his life and work. The 1969 celebrations included major exhibitions at the Science Museum in London (which holds the Watt workshop and many of his instruments) and at the Birmingham Museum and Art Gallery, the publication of several substantial biographies, and a series of academic conferences on the steam engine and its industrial consequences. The 2019 celebrations, which coincided with growing public concern about the climate consequences of the fossil-fuel-burning industrial civilization that the Watt engine had inaugurated, produced a more ambivalent assessment — exhibitions and publications that recognized Watt's enormous contribution to human welfare while also noting that the steam engine had begun, two and a half centuries ago, the cumulative release of carbon dioxide into the atmosphere that has produced the climate crisis of the twenty-first century.

This ambivalent reception is, perhaps, the truest measure of Watt's importance. No invention of the modern era has had a more profound effect on human life than the steam engine — and on the climate of the planet on which human life is conducted. The Industrial Revolution that the Watt engine inaugurated has lifted a substantial portion of humanity out of the grinding poverty of the pre-industrial world, has enabled the modern urban civilization in which most human beings now live, has produced (through the cumulative effects of fossil-fuel combustion) the atmospheric carbon-dioxide concentration that is the principal driver of present-day climate change, and has set the conditions within which all subsequent technologies — the electric motor, the internal combustion engine, the gas turbine, the nuclear reactor, the solar panel, the wind turbine — have been developed. The Watt engine itself is, in the twenty-first century, an obsolete technology, surviving only in museum demonstrations and in a few historical reconstructions; but the world it created continues to shape the conditions of human existence in ways that its inventor could not have anticipated and would surely have viewed with mingled pride and concern.

Conclusion: the Meaning of James Watt

What, finally, does James Watt mean? He was, on the one hand, a particular man — a soft-spoken Scottish instrument maker of melancholy temperament and patient industry, born in a modest port town on the Clyde, trained in a London garret, working through his middle years in a Glasgow workshop and his maturity in a Birmingham factory, dying in a Georgian country house outside Birmingham in his eighty-fourth year. He was, on the other hand, the agent of one of the most consequential transformations in human history — the transition from an organic economy of wood, wind, water, and muscle to an industrial economy of coal-fired heat engines. His name, attached to the unit of power that quantifies the very capacity of machines to do work, is among the most widely used eponymous units in the modern technical vocabulary, recognized in every nation on earth.

His meaning, as the biographical material on the preceding pages has tried to show, lies in the meeting of these two scales — the personal and the historical. The historical scale is grand and impersonal: the transformation of human labor by the controlled combustion of fossil fuel, the multiplication of human productivity by mechanical means, the building of an industrial civilization whose mature form would emerge a century after Watt's death. The personal scale is intimate and human: a slow, patient, depressive, modest man working over many years to understand and to overcome the practical limitations of a particular machine, supported by particular friends and partners, struggling against particular obstacles, marked by particular griefs and joys. The meaning of James Watt is the meaning that this particular man, in this particular set of circumstances, was the human instrument of a historical transformation that has changed the conditions of life for the species.

His own assessment of his life, expressed in many letters to friends and family in his retirement years, was modest. He thought of himself as a "working engineer" — the phrase recurs repeatedly in his correspondence — who had been fortunate to live in a time and place that had given him the opportunity to do work of practical use. He was grateful to his teachers (Joseph Black, the Glasgow professors), to his partners (Roebuck for his early support, Boulton for the great Birmingham partnership), to his customers (the mine owners and manufacturers who had given his engines their commercial trial), to his family (Margaret and Ann, his two wives, and the children of both marriages). He was conscious of the limitations of his own contribution and of the importance of the social and intellectual context within which it had been made. He was, in the manner of his generation of Scottish Presbyterians, suspicious of public praise and uncomfortable with the lavish honors that were paid him in his old age.

But his estimate of himself was, in the conventional way that modesty has of being inaccurate, too low. The "working engineer" of Glasgow and Birmingham was, in fact, one of the most original mechanical minds of his century. The combination of patient analysis, practical craftsmanship, chemical understanding, mathematical competence, and sustained creative imagination that he brought to the problem of the steam engine was rare in any generation and was unique in the second half of the eighteenth century. He took an inefficient pumping device and made it into the universal prime mover of industrial civilization; he gave to the engineering profession the conceptual vocabulary by which the analysis of mechanical work would be carried on for the next two hundred years; he taught the modern world how to count power as it had once counted gold. In doing so he was the instrument of a historical transformation more far-reaching than the most ambitious of the world-changers of his century could have envisaged. The slow, melancholy instrument maker of the College of Glasgow turned out to have done as much to change the world as any single human being in the history of the species. It is a measure of the strangeness of historical agency that this should have been the work of so quiet, so reluctant, and so modest a man.