[Illustration: TREVITHICK'S CYLINDRICAL BOILER FOR WHEAL PROSPER ENGINE, 1811.
Detail of Boilers:--_a_, two wrought-iron boilers, 3 feet in diameter, 40 feet long, using steam of 100 lbs. on the square inch above the atmosphere; _b_, cast-iron manhole door and safety-valve; _c_, ash-pit; _d_, fire-place; _e_, flues, the fire going first the whole length under the bottom of the boiler, then back again over the top, and into the chimney; _f_, brickwork; _g_, ashes or other convenient non-*conductor of heat; the fire-place ends of the boilers were 15 inches lower than the opposite ends, increasing the safety, with less liability to prime, and greater surface for superheating.]
"When a boy I was placed as an apprentice or learner with Captain Trevithick, before he left Cornwall for London. On his return to Cornwall, about 1810, he employed me to erect his first high-pressure expansive pole pumping engine at a mine in Gwythian.
"The pole was 16 inches in diameter; the stroke was very long, but I do not exactly recollect the length. It had a condenser and air-pump. There were two boilers made of wrought iron, 8 feet in diameter and 40 feet long. The fire was placed under them at one end, and flues went round them. A feed-pump forced water into the boilers; each had a safety-valve with a lever and weight. The steam in the boiler was 100 lbs. to the square inch. The pole was raised by the admission of the strong steam under its bottom. The steam-valve was closed at an early part of the stroke, and the steam allowed to expand; at the end of the stroke it was reduced to 20 lbs. or less, when the exhaust-valve allowed the steam to pass to the condenser, and the pole made its down-stroke in vacuum. A balance-bob regulated the movement of the engine.
"Trevithick's character in those days was, that he always began some new thing before he had finished the old."[23]
[Footnote 23: Captain Samuel Grose's recollections. 1858. Gwinear.]
Captain Artha, one of his assistants, said:--
"I erected several of Captain Trevithick's pole-engines. My brother Richard worked the one at Wheal Prosper when first erected. The pole made an 8-feet stroke. The case was fixed over the engine-shaft on two beams of timber from wall to wall. A cross-head was bolted to the top of the pole, and from it two side rods descended to a cross-piece under the pole-case, from which the pump-rod went into the shaft. A connecting rod worked a balance-beam, which worked the air-pump, feed-pump, and plug-rod for moving the valves. The steam, of a very high pressure, worked expansively."[24]