coal used by the machine with that used by an earlier, less efficient Newcomen engine, and required payments of one-third of the savings annually for the next 25 years. This pricing scheme led to disputes, as many mines fuelled the engines using coal of unmarketable quality that cost the mine owners only the expense of extraction. Mine owners were also reluctant to make the annual payments, viewing the engines as theirs once erected, and threatened to petition Parliament to repeal Watt's patent. The county of Cornwall was a major market for the firm's engines. It was mineral-rich and had many mines. However, the special problems for mining there, including local rivalries and high prices for coal, which had to be imported from Wales, forced Watt and later Boulton to spend several months a year in Cornwall overseeing installations and resolving problems with the mineowners. In 1779 the firm hired engineer William Murdoch, who was able to take over the management of most of the on-site installation problems, allowing Watt and Boulton to remain in Birmingham. The pumping engine for use in mines was a great success. In 1782 the firm sought to modify Watt's invention so that the engine had a rotary motion, making it suitable for use in mills and factories. On a 1781 visit to Wales Boulton had seen a powerful copper-rolling mill driven by water, and when told it was often inoperable in the summer due to drought suggested that a steam engine would remedy that defect. Boulton wrote to Watt urging the modification of the engine, warning that they were reaching the limits of the pumping engine market: "There is no other Cornwall to be found, and the most likely line for increasing the consumption of our engines is the application of them to mills, which is certainly an extensive field." Watt spent much of 1782 on the modification project, and though he was concerned that few orders would result, completed it at the end of the year. One order was received in 1782, and several others from mills and breweries soon after. George III toured the Whitbread brewery in London, and was impressed by the engine there (now preserved at the Powerhouse Museum, Sydney, Australia). As a demonstration, Boulton used two engines to grind wheat at the rate of 150 bushels per hour in his new Albion Mill in London. While the mill was not