262 TUBULAR BOILER. CHAP. XITT. 



nious method of stimulating combustion in the furnace, 

 by throwing the waste steam into the chimney after 

 performing its office in the cylinders, thus accelerating 

 the ascent of the current of air, greatly increasing the 

 draught, and consequently the temperature of the fire. 

 This plan was adopted by him, as we have already seen, 

 as early as 1815 ; and it was so successful that he 

 himself attributed to it the greater economy of the 

 locomotive as compared with horse power. Hence the 

 continuance of its use upon the Killingworth Eailway. 



Though the adoption of the steam-blast greatly 

 quickened combustion and contributed to the rapid 

 production of high-pressure steam, the limited amount 

 of heating surface presented to the fire was still felt to 

 be an obstacle to the complete success of the locomotive 

 engine. Mr. Stephenson endeavoured to overcome this 

 by lengthening the boilers and increasing the surface 

 presented by the flue tubes. The " Lancashire Witch," 

 which he built for the Bolton and Leigh Railway, and 

 used in forming the Liverpool and Manchester Railway 

 embankments, was constructed with a double tube, each 

 of which contained a fire and passed longitudinally 

 through the boiler. But this arrangement necessarily 

 led to a considerable increase in the weight of the 

 engine, which amounted to about twelve tons each ; 

 and as six tons was the limit allowed for engines 

 admitted to the Liverpool competition, it was clear 

 that the time was come when the Killingworth loco- 

 motive must undergo a further important modification. 



For many years previous to this period, ingenious 



mechanics had been engaged in attempting to solve the 



problem of the best and most economical boiler for 



, the production of high-pressure steam. As early as 



\1803, Mr. Woolf patented a tubular boiler, which was 



\extensively employed at the Cornish mines, and was 



round greatly to facilitate the production of steam, by 



the extension of the heating surface. The ingenious 



