directly would help to provide the necessary power. He began to correspond with Watt in 1766, and first met him two years later. In 1769 Watt patented an engine with the innovation of a separate condenser, making it far more efficient than earlier engines. Boulton realised not only that this engine could power his manufactory, but also that its production might be a profitable business venture. After receiving the patent, Watt did little to develop the engine into a marketable invention, turning to other work. In 1772, Watt's partner, Dr. John Roebuck, ran into financial difficulties, and Boulton, to whom he owed £1,200, accepted his two-thirds share in Watt's patent as satisfaction of the debt. Boulton's partner Fothergill refused to have any part in the speculation, and accepted cash for his share. Boulton's share was worth little without Watt's efforts to improve his invention. At the time, the principal use of steam engines was to pump water out of mines. The engine commonly in use was the Newcomen steam engine, which consumed large amounts of coal and, as mines became deeper, proved incapable of keeping them clear of water. Watt's work was well known, and a number of mines that needed engines put off purchasing them in the hope that Watt would soon market his invention. Boulton boasted about Watt's talents, leading to an employment offer from the Russian government, which Boulton had to persuade Watt to turn down. In 1774 he was able to convince Watt to move to Birmingham, and they entered into a partnership the following year. By 1775 six of the 14 years of Watt's original patent had elapsed, but thanks to Boulton's lobbying Parliament passed an act extending Watt's patent until 1800. Boulton and Watt began work improving the engine. With the assistance of iron master John Wilkinson (brother-in-law of Lunar Society member Joseph Priestley), they succeeded in making the engine commercially viable. In 1776 the partnership erected two engines, one for Wilkinson and one at a mine in Tipton in the Black Country. Both engines were successfully installed, leading to favourable publicity for the partnership. Boulton and Watt began to install engines elsewhere. The firm rarely produced the engine itself: it had the purchaser buy parts from a number of suppliers and then assembled the engine on-site under the supervision of a Soho engineer. The company made its profit by comparing the amount of