[Illustration: TREVITHICK'S DOLCOATH 76-INCH CYLINDER PUMPING ENGINE, ERECTED IN 1816, CEASED WORKING 1869. _a_, steam-cylinder, 76 inches in diameter, 9-feet stroke; _b_, steam-jacket; _c_, steam expansion-valve, 11 inches diameter, double beat; the upper beat 11 inches diameter, the under beat 9-1/2 inches, valve 8 inches long; _d_, expansive cam on plug-rod; _e_, plug-rod for moving the gear; _f_, expansive horn; _g_, equilibrium valve, 13 inches in diameter, single beat moved by a tooth-rack and segment; _h_, exhaust-valve, 14-1/2 inches in diameter, single beat moved by a lever and link; _i_, equilibrium-valve handle; _j_, exhaust-valve handle; _k_, Y-posts for carrying the gear arbors; _l_, main beam in two plates of cast iron; _m_, parallel motion; _n_, feed-pump rod; _o_, air-pump bucket-rod, the pump, 2 feet 9 inches diameter; _p_, the main pump rods.] [Illustration: CYLINDER, MAIN BEAM, AND PUMP-ROD OF DOLCOATH 76-INCH CYLINDER ENGINE.] In 1867 the writer was a member of the Dolcoath Managing Committee, when it was determined that the old engine of 1816 should be replaced by a new one. The cylinder sides were reduced in thickness by half an inch; the steam-pipes and nozzles were thinned by rust and decay; the valves and gear-work remained in good order. Captain Josiah Thomas, the present manager of the mine, offered to sell this old engine at scrap price, that it might be stored in the Patent Museum at Kensington as a memento of the early high-pressure expansive steam pumping engine. [Illustration: BOILERS ERECTED IN 1811 IN DOLCOATH, USED IN THE BOULTON AND WATT 63-INCH ENGINE, THEN IN THE NEW 76-INCH UNTIL 1869. _a a_, two wrought-iron cylindrical boilers, 5 feet in diameter, 18 feet long, with internal fire-tube, oval, 3 feet 4 inches by 3 feet; _b_, a boiler, 6 feet 2 inches diameter, 22 feet long, cylindrical tube, 4 feet diameter in the fire-place, the remainder 3 feet; _c_, brick bridge; _d_, fire-bars; _e_, brick external flues under boiler; _f_, brick side-flues; _g_, ashes, or other non-conductor; steam 30 to 50[**unclear] lbs. on the inch above the atmosphere.] The steam-cylinder of 1816 was cast in South Wales; the beam still working in the new engine of 1869 was cast in the foundry of the Williams' at Perran. John West replaced the original flat expansive steam-valve with a double-beat valve; the gear was principally made by him on the mine, and remained in good working to the last. This double-beat valve is the first the writer has met with; it is of the same form as the modern double-beat valve; an earlier plan was to have a small valve on the top of the main valve. The steam in ordinary working was shut off when the piston had moved from an eighth to a quarter of its stroke.