Trevithick's engines were very durable, as well as cheap in first cost and in working expense. This famous Dolcoath 76-inch engine remained in constant work night and day for fifty-four years; after which good service the steam-pipes, being thinned by rust, were held together by bands and bolts; the steam-case around the cylinder would no longer bear the pressure of steam; the interior of the cylinder from wear was one inch larger in diameter than when first put in, and had to be held together by strap-bolts. The original boilers were said to remain, only they had been repaired until not an original plate remained; but there they were in the old stoke-hole in 1869, when, from the fear of some part of the engine breaking and causing accident, it was removed.
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 9½ 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, 14½ 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.
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.
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 lbs. on the inch above the atmosphere.