330 TUBULAR BOILER, SITKRIIEATIXG STKAM, 



water, and this amount was further reduced to 13 gallons 

 by superheating the steam ; this roughly agrees with 

 the coal consumed, or in other words, with the amount 

 of heat to be carried off by injection-water : the Watt 

 rule giving 11| bushels as the fair allowance for low- 

 pressure steam vacuum engines, while the high-pressure 

 steam vacuum engine burnt but 4| bushels. This was 

 further reduced to 3 bushels by superheating. Those 

 facts led to the idea that if the steam pressure was suffi- 

 ciently increased, condensation might be carried out 

 without any injection-water, by the transmission of the 

 heat in the stearn through the metal sides of the con- 

 denser. An experiment was at once made by remov- 

 ing the Watt condenser and injection-water, as he had 

 done seventeen years before, 1 using in their stead a 

 thin copper surface-condenser immersed in cold water, 

 producing, within i Ib. on the inch, as good a vacuum 

 as when injection- water was used, leading to the con- 

 clusion, 



" It is my opinion that high steam will expand and contract 

 with a much less degree of heat or cold, in proportion to its 

 effect, than what steam of atmosphere strong will do. I intend 

 to try steam of five or six atmospheres strong, and partially 

 condense it down to nearly one atmosphere strong, and then by 

 an air-pump of more content tnan is usual to return the steam, 

 air, and water hack into the boiler again, and by a great number 

 of small tubes, with greatly heated surface sides, to reheat the 

 returned steam." 



This, in practical words, is the surface condenser by 

 which the used steam is returned to the boiler in the 

 form of water. The more general use of high- pressure 

 steam of 70 or 90 Ibs. to the inch, increasing its expan- 

 sive force on one side of the piston by superheating it 



1 Sec Query 3rd, vol. ii., p. 19. 



