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, lead ing 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, but introduced a steam-jacket, or layer of steam, between the cylin der proper and an outer shell.
All these features were specified in his first patent (see STEAM ENGINE), which was obtained in January 1769, nearly four years after the inventions it covered 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 he agreed that Dr. John Roebuck, founder of the Carron ironworks, should take two-thirds of the profits of the invention in consideration of his bearing the costs. An engine was then erected at Kinneil, near Linlithgow, and this gave Watt the op portunity of overcoming many difficulties in details of construc tion. Meanwhile he was gaining reputation as a civil engineer. In 1767 he was employed to make a survey for a Forth and Clyde canal, which failed, however, to secure parliamentary sanction. During the next six years he made surveys for canals at Monk land, from Perth to Forfar, and along the lines afterwards fol lowed by the Crinan and Caledonian canals. He prepared plans for the harbours of Ayr, Port-Glasgow and Greenock, for deepen ing 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 to a movable target on a rod held upright at the place whose distance from the observer was to be determined.
In 1768 Watt had met Matthew Boulton, who owned the Soho engineering works at Birmingham. Boulton agreed to take Roe buck's share in the invention, and to apply to parliament for an act to prolong the term of the patent. The application was suc cessful, and in 1775 an act was passed continuing the patent for twenty-five years. By this time Watt had settled in Birmingham, where the manufacture of steam-engines was begun by the firm of Boulton & Watt. The partnership was a happy one. Boulton
left the work of inventing to Watt, in whose genius he had the fullest faith, while he attended to the business side.
During the next ten years Watt developed the engine. Its first and, for a time, its only application was in pumping; it was at once put to this use in the Cornish mines. Further inventions 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 was by a crank and fly-wheel; this had occurred to Watt, but had meanwhile been patented by an other, and hence he devised "sun and planet wheels" and other equivalent contrivances. A third patent, in 1782, contained two new inventions of great 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 con denser. 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 ad mission valve is closed after a portion only of the stroke is performed, and the steam enclosed in the cylinder is then al lowed to expand during the remainder of the stroke, doing addi tional work upon the piston without making any further demand upon the boiler until the next stroke requires a fresh admission of steam. He observed that, as the piston advanced after admis sion 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. Recog nizing that this would cause a gradual reduction of the force with which the piston pulled or pushed against the beam, Watt de vised a number of contrivances for equalizing the effort through out 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 ar rangement of links by which the top of the piston-rod is con nected to the beam so that it may either pull or push, and is at the same time guided to move in a straight line.