POLE STEAM-ENGINE. 



71 



TREVITHICK'S CYLINDRICAL BOILER FOR WHEAL PBOSPER ENGINE, 1811. 



Detail of Boilers : a, two wrought-iron boilers, 3 feet in diameter, 40 feet long, using steam of 

 100 Ibs. on the square inch above the atmosphere ; 6, 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 ; /, 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 con- 

 denser 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 Ibs. 

 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 ; 



