Before many months Black, the discoverer of latent heat, then lecturer on chemistry, and Robison, then a student, afterwards professor of natural philosophy, became his intimate friends, and with them he often discussed the possibility of improving the steam-engine, of which at that time Newcomen’s was the most advanced type. The engine was then applied only to pumping water,—chiefly in the drainage of mines; and it was so clumsy and wasteful of fuel as to be but little used. Some early experiments of Watt in 1761 or 1762 led to no positive result, but in 1764 his attention was seriously drawn to the matter by having a model of Newcomen’s engine, which formed part of the college collection of scientific apparatus, given him to repair. Having put the model in order, he was at once struck with its enormous consumption of steam, and set himself to examine the cause of this and to find a remedy.

In Newcomen’s engine the cylinder stood vertically under one end of the main lever or “beam” and was open at the top. Steam, at a pressure scarcely greater than that of the atmosphere, was admitted to the under side; this allowed the piston to be pulled up by a counterpoise at the other end of the beam. Communication with the boiler was then shut off, and the steam in the cylinder was condensed by injecting a jet of cold water from a cistern above. The pressure of the air on the top of the piston then drove it down, raising the counterpoise and doing work. The injection water and condensed steam which had gathered in the cylinder were drained out by a pipe leading down into a well.

Watt at once noticed that the alternate heating and cooling of the cylinder in Newcomen’s engine made it work with tedious slowness and excessive consumption of steam. When steam was admitted at the beginning of each stroke, it found the metal of the cylinder and piston chilled by contact with the condensed steam and cold injection water of the previous stroke, and it was not until much steam had been condensed in heating the chilled surfaces that the cylinder was able to fill and the piston to rise. His first attempt at a remedy was to use for the material of the cylinder a substance that would take in and give out heat slowly. Wood was tried, but it made matters only a little better, and did not promise to be durable. Watt observed that the evil was intensified whenever, for the sake of making a good vacuum under the piston, a specially large quantity of injection water was supplied.

He then entered on a scientific examination of the properties of steam, studying by experiment the relation of its density and pressure to the temperature, and concluded that two conditions were essential to the economic use of steam in a condensing steam-engine. One was that the temperature of the condensed steam should be as low as possible, 100° F. or lower, otherwise the vacuum would not be good; the other was, to quote his own words, “that the cylinder should be always as hot as the steam which entered it.” In Newcomen’s engine these two conditions were incompatible, and it was not for some months that Watt saw a means of reconciling them. Early in 1765, while walking on a Sunday afternoon in Glasgow Green, the idea flashed upon him that, if the steam were condensed in a vessel distinct from the cylinder, it would be practicable to make the temperature of condensation low, and still keep the cylinder hot. Let this separate vessel be kept cold, either by injecting cold water or by letting it stream over the outside, and let a vacuum be maintained in the vessel. Then, whenever communication was made between it and the cylinder, steam would pass over from the cylinder and be condensed; the pressure in the cylinder would be as low as the pressure in the condenser, but the temperature of the metal of the cylinder would remain high, since no injection water need touch it. Without delay Watt put this idea to the test, and found that the separate condenser did act as he had anticipated. To maintain the vacuum in it he added another new organ, namely, the air-pump, the function of which is to remove the condensed steam and water of injection along with any air that gathers in the condenser.

To further his object of keeping the cylinder as hot as the steam that entered it, Watt supplemented his great invention of the separate condenser by several less notable but still important improvements. In Newcomen’s engine a layer of water over the piston had been used to keep it steam-tight; Watt substituted a tighter packing lubricated by oil. In Newcomen’s engine the upper end of the cylinder was open to the air; Watt covered it in, leading the piston rod through a steam-tight stuffing-box in the cover, and allowed steam instead of air to press on the top of the piston. In Newcomen’s engine the cylinder had no clothing to reduce loss of heat by radiation and conduction from its outer surface; Watt not only cased it in non-conducting material, such as wood, but introduced a steam-jacket, or layer of steam, between the cylinder proper and an outer shell. All these features were specified in his first patent, in words which have been quoted in the article Steam-Engine, vol. xxii. p. 475 (q.v).

This patent was not obtained till January 1769, nearly four years after the inventions it covers had been made. In the interval Watt had been striving to demonstrate the merits of his engine by trial on a large scale. His earliest experiments left him in debt, and, finding that his own means were quite insufficient to allow him to continue them, he agreed that Dr Roebuck, founder of the Carron iron-works, should take two-thirds of the profits of the invention in consideration of his bearing the cost. An engine was then erected at Kinneil, near Linlithgow, where Roebuck lived, and this gave Watt the opportunity of facing many difficulties in details of construction. But the experiments made slow progress, for Roebuck’s affairs became embarrassed, and Watt’s attention was engaged by other work. He had taken to surveying, and was fast gaining reputation as a civil engineer. In 1767 he was employed to make a survey for a Forth and Clyde canal, —a scheme which failed to secure parliamentary sanction. This was followed during the next six years by surveys for a canal at Monkland, for another through the valley of Strathmore from Perth to Forfar, and for others along the lines afterwards followed by the Crinan and Caledonian Canals. He prepared plans for the harbours of Ayr, Port-Glasgow, and Greenock, for deepening the Clyde, and for building a bridge over it at Hamilton. In the course of this work he invented a simple micrometer for measuring distances, consisting of a pair of horizontal hairs placed in the focus of a telescope, through which sights were taken to a fixed and movable target on a rod held upright at the place whose distance from the observer was to be determined. The micrometer was varied in a number of ways; and another fruit of his ingenuity about the same time was a machine to facilitate drawing in perspective.

Meanwhile the engine had not been wholly neglected. Watt had secured his patent; the Kinneil trials had given him a store of valuable experience; Roebuck had failed, but another partner was ready to take his place. In 1768 Watt had made the acquaintance, through his friend Dr Small, of Matthew Boulton, a man of energy and capital, who owned the Soho engineering works at Birmingham. Boulton agreed to buy Roebuck’s share in the invention, and to join Watt in applying to parliament for an Act to prolong the term of the patent. The application was successful. In 1775 an Act was passed continuing the patent for twenty-five years. By this time the inventor had abandoned his civil engineering work and had settled in Birmingham, where the manufacture of steam-engines was begun by the firm of Boulton and Watt. The partnership was a singularly happy one. Boulton had the good sense to leave the work of inventing to Watt, in whose genius he had the fullest faith; on the other hand, his substantial means, his enterprise, resolution, and business capacity, supplied what was wanting to bring the invention to commercial success.

During the next ten years we find Watt assiduously engaged in developing and introducing the engine. Its first and for a time its only application was in pumping; it was at once put to this use in the mines of Cornwall, where Watt was now frequently engaged in superintending the erection of engines. Further inventions were required to fit it for other uses, and these followed in quick succession. Watt’s second steam-engine patent is dated 1781. It describes five different methods of converting the reciprocating motion of the piston into motion of rotation, so as to adapt the engine for driving ordinary machinery. The simplest way of doing this, and the means now universally followed, is by a crank and fly-wheel; this had occurred to Watt but had meanwhile been patented by another, and hence he devised the “sun and planet wheels” and other equivalent contrivances. A third patent, in 1782, contained two new inventions of the first importance. Up to this time the engine had been single-acting; Watt now made it double-acting; that is to say, both ends of the cylinder, instead of only one, were alternately put in communication with the boiler and the condenser. Up to this time also the steam had been admitted from the boiler throughout the whole stroke of the piston; Watt now introduced the system of expansive working, in which the admission valve is closed after a portion only of the stroke is performed, and the steam enclosed in the cylinder is then allowed to expand during the remainder of the stroke, doing additional work upon the piston without making any further demand upon the boiler until the next stroke requires a fresh admission of steam. He calculated that, as the piston advanced after admission had ceased, the pressure of the steam in the cylinder would fall in the same proportion as its volume increased,—a law which, although not strictly true, does accord very closely with the actual behaviour of steam expanding in the cylinder of an engine. Recognizing that this would cause a gradual reduction of the force with which the piston pulled or pushed against the beam, Watt devised a number of contrivances for equalizing the effort throughout the stroke. He found, however, that the inertia of the pump-rods in his mine engines, and the fly-wheel in his rotative engines, served to compensate for the inequality of thrust sufficiently to make these contrivances unnecessary. His fourth patent, taken out in 1784, describes the well-known “parallel motion,” an arrangement of links by which the top of the piston-rod is connected to the beam so that it may either pull or push, and is at the same time guided to move in a sensibly straight line. “I have started a new hare,” he writes to Boulton in June of that year; “I have got a glimpse of a method of causing a piston-rod to move up and down perpendicularly by only fixing it to a piece of iron upon the beam, without chains or perpendicular guides or untowardly frictions, arch-heads, or other pieces of clumsiness. I think it a very probable thing to succeed, and one of the most ingenious simple pieces of mechanism I have contrived.”

Still a later invention was the throttle-valve and centrifugal governor, by which the speed of rotative engines was automatically controlled. One more item in the list of Watt’s contributions to the development of the steam-engine is too important to be passed without mention: the indicator, which draws a diagram of the relation of the steam’s pressure to its volume as the stroke proceeds, was first used by Boulton and Watt to measure the work done by their engines, and so to give a basis on which the charges levied from their customers were adjusted. It would be difficult to exaggerate the part which this simple little instrument has played in the evolution of the steam-engine. The eminently philosophic notion of an indicator diagram is fundamental in the theory of thermodynamics; the instrument itself is to the steam-engineer what the stethoscope is to the physician, and more, for with it he not only diagnoses the ailments of a faulty machine, whether in one or another of its organs, but gauges its power in health.

The commercial success of the engine was not long in being established. By 1783 all but one of the Newcomen pumping-engines in Cornwall had been displaced by Watt’s. The mines were then far from thriving; many were even on the point of being abandoned through the difficulty of dealing with large volumes of water; and Watt’s invention, which allowed this to be done at a moderate cost, meant for many of them a new lease of life. His engine used no more than a fourth of the fuel that had formerly been needed to do the same work, and the Soho firm usually claimed by way of royalty a sum equivalent to one-third of the saving—a sum which must have been nearly equal to the cost of the fuel actually consumed. Rival manufacturers came forward, amongst whom Bull and Hornblower are the most conspicuous names. They varied the form of the engine, but they could not avoid infringing Watt’s patent by the use of a separate condenser. When action was taken against them on that ground, they retaliated by disputing the validity of the fundamental patent of 1769. In the case of Boulton and Watt v. Bull the court was divided on this point, but in an action against Hornblower the patent was definitely affirmed to be valid by a unanimous finding of the Court of King’s Bench. This was in 1799, only a year before the monopoly expired, but the decision enabled the firm to claim a large sum as arrears of patent dues. In connexion with these trials Watt himself, as well as his early friends Black and Robison, drew up narratives of the invention of the steam-engine, which are of much interest to the student of its history.

Before Watt’s time the steam-engine was exclusively a steampump, slow-working, cumbrous, and excessively wasteful of fuel. His first patent made it quick in working, powerful, and efficient, but still only as a steam-pump. His later inventions adapted it to drive machinery of all kinds, and left it virtually what it is today, save in three respects. In respect of mechanical arrangement the modern engine differs from Watt’s chiefly in this, that the beam, an indispensable feature in the early pumping-engines, and one which held its place long after the need for it had vanished, has gradually given way to more direct modes of connecting the piston with the crank. Another difference is in the modern use of high-pressure steam. It is remarkable that Watt, notwithstanding the fact that his own invention of expansive working must have opened his eyes to the advantage of high-pressure steam, declined to admit it into his practice. He persisted in the use of pressures that were little if at all above that of the atmosphere. His rivals in Cornwall were not so squeamish. Trevithick ventured as far as 120 lb on the square inch, and a curious episode in the history of the steam-engine is an attempt which Boulton and Watt made to have an Act of Parliament passed forbidding the use of high pressure on the ground that the lives of the public were endangered. The third and only other respect in which a great improvement has been effected is in the introduction of compound expansion. Here, too, one cannot but regret to find the Soho firm hostile, though the necessity of defending their monopoly makes their action natural enough. Hornblower had in fact stumbled on the invention of the compound engine, but as his machine employed Watt’s condenser it was suppressed, to be revived after some years by Woolf. In one of his patents (1784) Watt describes a steam-locomotive, but he never prosecuted this, and when Murdoch, his chief assistant (famous as the inventor of gas-lighting), made experiments on the same lines, Watt gave him little encouragement. The notion then was to use a steam-carriage on ordinary roads; its use on railways had not yet been thought of. When that idea took form later in the last years of Watt’s life, the old man refused to smile upon his offspring; it is even said that Watt put a clause in the lease of his house that no steam-carriage should on any pretext be allowed to approach it.

On the expiry in 1800 of the Act by which the patent of 1769 had been extended, Watt gave up his share in the business of engine-building to his sons, James, who carried it on along with a son of Boulton for many years, and Gregory, who died in 1804. The remainder of his life was quietly spent at Heathfield Hall, his house near

Birmingham, where he devoted his time, with scarcely an interruption, to mechanical pursuits. His last work was the invention of machines for copying sculpture,—one for making reduced copies, another for taking facsimiles by means of a light stiff frame, which carried a pointer over the surface of the work while a revolving tool fixed to the frame alongside of the pointer cut a corresponding surface on a suitable block. We find him in correspondence with Chantrey about this machine not many months before his death, and presenting copies of busts to his friends as the work “of a young artist just entering on his eighty-third year.” His life drew to a tranquil close, and the end came at Heathfield on the 19th of August 1819. His remains were interred in the neighbouring parish church of Handsworth.

Watt was twice married,—first in 1763 to his cousin Miss Miller, who died ten years later. Of four children born of the marriage, two died in infancy; another was James, who succeeded his father in business; the fourth was a daughter who lived to maturity, but died early, leaving two children. His second wife, Miss Macgregor, whom he married before settling in Birmingham in 1775, survived him; but her two children, Gregory and a daughter, died young.

Some of Watt’s minor inventions have been already noticed. Another, which has proved of great practical value, was the lettercopying press, for copying manuscript by using a glutinous ink and pressing the written page against a moistened sheet of thin paper. He patented this in 1780, describing both a roller press, the use of which he seems to have preferred in copying his own correspondence, and also the form of screw press now found in every merchant’s office.

In the domain of pure science Watt claims recognition not only as having had ideas greatly in advance of his age regarding what is now called energy, but as a discoverer of the composition of water. Writing to Priestley in April 1783, with reference to some of Priestley’s experiments, he suggests the theory that " water is composed of dephlogisticated air and phlogiston deprived of...

No greater contrast can be found than that which Watteau’s work presents to the painful condition under which he lived his life, a contrast such as that which the boyish glee of Caldecott’s designs showed in comparison to the physical misery against which his gallant spirit made so brave a stand. Watteau sought refuge, as it were, from his bodily pain in that fairyland which he created, where the pompous art of the “Grand Siècle” still cumbered the ground. His work, always conceived in a poetical spirit, lived in spite of the artificial atmosphere of the mock pastoral style of the day, and lives in virtue of the exquisite precision of his observation and of the extraordinary brilliancy and lightness of his art.

In 1706 appeared his Horae Lyricae, of which an edition, with memoir by Robert Southey, forms vol. ix. of Sαered Classics (1834); in 1707 a volume of Hymns; in 1719 The Psalms of David ; and in 1720 Divine and Moral Songs for Children. Various collected editions of his sacred poetry have been published, and in 1869 an edition appeared with music for four voices. Among the theological treatises of Watts, in addition to volumes of sermons, are Doctrine of the Trinity (1726); Treatise on the Love of God and on the Use and Abuse of the Passions (1729); Catechisms for Children and Youth (1730); Essays towards a Proof of a Separate State for Souls (1732); Essay on the Freedom of the Will (1732); Essay on the Strength and Weakness of Human Reason (1737); Essay on the Ruin and Recovery of Mankind (1740); Glory of Christ as God-Man Unveiled (1746); and Useful and Important Questions concerning Jesus Christ (1746). He was also the author of a variety of miscellaneous treatises. His Posthumous Works appeared in 1773, and a further instalment of them in 1779. Several editions of his collected works, with memoirs, have also been published.

The Life and Times of Watts by Milner appeared in 1834; a life is also included in Johnson's Lives of the Poets.