THE WATT AND THE TREVITHICK p;NGL\KS 169 



the mine, offered to sell this old engine at scrap price, 

 that it might be stored in the Patent Museum at Ken- 

 sington as a memento of the early high-pressure 

 expansive steam pumping engine. 



BOILEKS EKECTED 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 <! 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; /, brick side-flues; g, ashes, or other non-conductor; steam 30 to 5') l\>e. 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 work- 

 ing 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. 



The G-ons, or Stray Park 63-inch cylinder, survived 

 its companions, the 63 double, and 45 single, for some 

 ten or fifteen years, having beaten both of them in duty. 

 A memorandum in Trevithick's handwriting shows 

 that he in 1798, when designing his large globular 

 boiler with internal flue at the reworking of Dolcoath, 

 tested the relative duty of the Watt 63-inch double and 



