A long series of experimental trials was, nevertheless, requisite before the engine could be brought to such perfection as to render it generally available to the public, and therefore profitable to its manufacturers. In January, 1775, six years of the patent had elapsed, and there seemed some probability of the remaining eight running out as fruitlessly. An application which was made for the extension of its term was unexpectedly opposed by the eloquence of Burke; but the orator and his associates failed, and the extension was accorded by Act of Parliament.
The first practical employment of Watt's engines to any considerable extent was in the mining districts of Cornwall, where he himself was, in consequence, compelled to spend much of his time subsequent to 1775. Here he had to contend not only with natural obstacles in the dark abysses of deeply flooded mines, but with a rude and obstinate class of men as deeply flooded by inveterate prejudices. The result in the way of profit was not, however, satisfactory, notwithstanding the service to the mining interest was enormous. "It appears," says Watt, in 1780, "by our books, that Cornwall has hitherto eat up all the profits we have drawn from it, and all we have got by other places, and a good sum of our own money to the bargain."
At this stage Watt himself was more fertile in mechanical inventions than in any other portion of his busy life. Taking his patents in their chronological order, the first (subsequent to that of 1769) was "For a new method of copying letters and other writings expeditiously," by means of copying presses. Of the same date was his invention of a machine "for drying linen and muslin by steam." On October 25, 1781, he took out his third patent (the second of the steam-engine series), "for certain new methods of applying the vibrating or reciprocating motion of steam or fire engines, to produce a continued rotative motion round an axis or centre, and thereby to give motion to the wheels of mills or other machines." One of these methods was that commonly known as the sun-and-planet wheels; they were five in all. A favorite employment of his in the workshops at Soho, in the later months of 1783 and earlier ones of 1784, was to teach his steam-engine, now become nearly as docile as it was powerful, to work a tilt-hammer for forging iron and making steel. "Three hundred blows per minute—a thing never done before," filled him, as his biographer says, with feelings of excusable pride. Another patent in the steam-engine series, taken out in 1784, contained, besides other methods of converting a circular or angular motion into a perpendicular or rectilineal motion, the well-known and much-admired parallel motion, and the application of the steam-engine to give motion to wheel-carriages for carrying persons and goods. To ascertain the exact number of strokes made by an engine during a given time, and thereby to check the cheats of the Cornish miners, Watt also invented the "Counter," with its several indexes. Among his leading improvements, introduced at various periods, were the throttle-valve, the application of the governor, the barometer or float, the steam-gauge, and the indicator. The term during which he seems to have thus combined the greatest maturity with the greatest activity of intellect, and the portion of his life which they comprehended, was from his fortieth to his fiftieth year. Yet it was a term of increased suffering from his acute sick-headaches, and remarkable for the infirmities over which he triumphed; notwithstanding, he himself complained of his "stupidity and want of the inventive faculty."
Watt's chemical studies in 1783, and the calculations they involved from experiments made by foreign chemists, induced him to make a proposal for a philosophical uniformity of weights and measures; and he discussed this proposal with Priestley and Magellan. While Watt was examining the constituent parts of water, he had opportunities of familiar intercourse not only with Priestley, but with Withering, Keir, Edgeworth, Galton, Darwin, and his own partner, Boulton—all men above the average for their common interest in scientific inquiries. Dr. Parr frequently attended their meetings, and they kept up a correspondence with Sir William Herschel, Sir Joseph Banks, Dr. Solander, and Afzelius. Mrs. Schimmelpenninck, who was greatly given to physiognomical studies, has left us this picture of Watt at this period.
"Mr. Boulton was a man to rule society with dignity; Mr. Watt, to lead the contemplative life of a deeply introverted and patiently observant philosopher. He was one of the most complete specimens of the melancholic temperament. His head was generally bent forward, or leaning on his hand in meditation; his shoulders stooping, and his chest falling in; his limbs lank and unmuscular, and his complexion sallow. His intellectual development was magnificent; comparison and causality immense, with large ideality and constructiveness, individuality, an enormous concentrativeness and caution.
"He had a broad Scottish accent; gentle, modest, and unassuming manners; yet, when he entered a room, men of letters, men of science, nay, military men, artists, ladies, even little children, thronged round him. Ladies would appeal to him on the best means of devising grates, curing smoky chimneys, warming their houses, and obtaining fast colors. I can speak from experience of his teaching me how to make a dulcimer and improve a Jew's harp."
In the year 1786, Watt and Boulton visited Paris, on the invitation of the French Government, to superintend the erection of certain steam-engines, and especially to suggest improvements in the great hydraulic machine of Marly, which Watt himself designates a "venerable" work. In Paris Watt made many acquaintances, including Lavoisier, Laplace, Fourcroy, and others scarcely less eminent; and while here he discussed with Berthollet a new method of bleaching by chlorides, an invention of the latter which Watt subsequently introduced into England.
Meanwhile Watt had vigilantly to defend his patents at home, which were assailed by unworthy and surreptitious rivals as soon as it was proved that they were pecuniarily valuable. Some of the competing engines, as Watt himself described them, were simply asthmatic. "Hornblower's, at Radstock, was obliged to stand still once every ten minutes to snore and snort." "Some were like Evan's mill, which was a gentlemanly mill; it would go when it had nothing to do, but it refused to work." The legal proceedings, both in equity and at common law, which now became necessary, were numerous. One bill of costs, from 1796 to 1800, amounted to between £5,000 and £6,000; and the mental and bodily labor, the anxiety and vexation, which were superadded, involved a fearful tax on the province of Watt's discoveries.
With the year 1800 came the expiration of the privilege of the patent of 1769, as extended by the statute of 1775; and also the dissolution of the original copartnership of Messrs. Boulton and Watt, then of five-and-twenty years' duration. The contract was renewed by their sons, the business having become so profitable that Watt and his children were provided with a source of independent income; and at the age of sixty-four the great inventor had personally realized some of the benefits he contemplated.
Henceforth Watt's ingenuity became excursive, discretionary, almost capricious; but in every phase and form it continued to be beneficent. In 1808 he founded a prize in Glasgow College, as an acknowledgment of "the many favors that learned body had conferred upon him." In 1816 he made a donation to the town of Greenock, "to form the beginning of a scientific library" for the instruction of its young men. Nor, amid such donations, were others wanting on his part, such as true religion prescribes, to console the poor and relieve the suffering.