W A T T 413 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-con ducting 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 SiEAir-ExGiNE, 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 Iloebuck 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 en gaged 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 de scribes five different methods of converting the reciprocat ing 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, con tained 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 com- .munication 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 intro duced the system of expansive working, in which the ad mission 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 con trivances 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