[32] "Architecture Hydraulique," 1734. The Edmonstone colliery was licensed, in 1725, to erect an engine, not to exceed 28 inches diameter of cylinder and 9 feet stroke of piston, paying a royalty of £80 per annum for eight years. This engine was built in Scotland, by workmen sent from England, and cost about £1,200. Its "great cost" is attributed to an extensive use of brass. The workmen were paid their expenses and 15_s._ per week as wages. The builders were John and Abraham Potter, of Durham. An engine built in 1775, having a steam-cylinder 48 inches in diameter and of 7 feet stroke, cost about £2,000. Smeaton found 57 engines at work near Newcastle in 1767, ranging in size from 28 to 75 inches in diameter of cylinder, and of, collectively, about 1,200 horse-power. Fifteen of these engines gave an average of 98 square inches of piston to the horse-power, and the average duty was 5,590,000 pounds raised 1 foot high by 1 bushel (84 pounds) of coal. The highest duty noted was 7.44 millions; the lowest was 3.22 millions. The most efficient engine had a steam-cylinder 42 inches in diameter; the load was equivalent to 9-1/4 pounds per square inch of piston-area, and the horse-power developed was calculated to be 16.7. Price, writing in 1778, says, in the Appendix to his "Mineralogia Cornubiensis:" "Mr. Newcomen's invention of the fire-engine enabled us to sink our mines to twice the depth we could formerly do by any other machinery. Since this invention was completed, most other attempts at its improvement have been very unsuccessful; but the vast consumption of fuel in these engines is an immense drawback on the profit of our mines, for every fire-engine of magnitude consumes £3,000 worth of coals per annum. This heavy tax amounts almost to a prohibition." Smeaton was given the description, in 1773, of a _stone_ boiler, which was used with one of these engines at a copper mine at Camborne, in Cornwall. It contained three copper flues 22 inches in diameter. The gases were passed through these flues successively, finally passing off to the chimney. This boiler was cemented with hydraulic mortar. It was 20 feet long, 9 feet wide, and 8-1/2 feet deep. It was heated by the waste heat from the roasting-furnaces. This was one of the earliest flue-boilers ever made.