328 TUBULAR BOILER, SUPERHEATING STEAM, 



cylinder, cylinder-top, and steam-pipes heated by flues was 

 300 feet, and caused a saving of 41 bushels of coal in twenty- 

 four hours. A nother engine in the same mine w r as tried, having 

 a 42-inch cylinder ; when the fire was around the cylinder, she 

 worked 100 strokes without injection-water ; the expansion-valve 

 was closed at half-stroke, the steam in the boiler 56 Ibs. on the 

 inch above the atmosphere." 



It is not easy to deal with the important reasonings 

 flowing from those facts, and influencing the form and 

 economy of the steam-engine, nor to show if Trevithick 

 was right in discrediting the laws laid down by Watt. 

 Newcomen's engine had the interior, as well as the 

 exterior of the steam-cylinder exposed to the cooling 

 atmosphere. Watt, by putting a cover on the -cylinder, 

 reduced the loss of the heat from the interior, and by 

 his steam-case hoped to reduce the loss from the exterior, 

 though by it be increased tbe amount of surface exposed 

 to tbe cold. In Trevitbick's early engines the boiler 

 alone exposed heat-losing surface, and tbis was further 

 reduced by its own comparatively small size, the engine 

 and boiler complete not exposing one-quarter of the 

 surface of a Watt low-pressure engine of equal power. 

 One object of the Binner Downs experiment was to 

 further curtail this loss of power by increasing the heat 

 of the steam while in operation in the cylinder, since 

 called superheating steam. 



This principle of giving increased heat to steam, 

 after it had left its state as water, was made practical 

 by Trevithick's boiler at Wheal Prosper in 1810, where 

 the flues having first been carried around the water 

 portion of the boiler, then passed over the steam portion ; ] 

 and again in the upright boiler of 1815, having the 

 upper end of the fire-tube surrounded by steam above 

 the water line. 2 Those early beginnings of super- 



1 See vol. ii., p. 71. * See vol. i., ]. 3.14. 



