AND SURFACE CONDENSEll. 321 



similar to those in- Dolcoath, 1 measured at the rate of 

 1000 superficial feet of heating surface for each bushel 

 of coal burnt in an hour, and in round numbers gave a 

 duty of 1500 Ibs. lifted a foot high to each foot of boiler 

 surface. In words not technical, the heat from 1 Ib. of 

 coal gave steam that raised 460 tons weight of water 

 1 foot high. 



The cylinder of this engine used the Watt steam- 

 jacket. The Binner Downs engine was doing not one- 

 half this duty, namely, forty-one millions ; -when brick 

 flues were built around the cylinder, cylinder cover, 

 and steam-pipes, and one or two fire-places, fixed near 

 the bottom of the cylinder, of a size to conveniently 

 burn 5 bushels of coal in twenty-four hours, the heat 

 from which circulated through those flues on its way to 

 the chimney, and increased the duty of the engine by 

 one-half, raising it to sixty-three millions ; in other 

 words, during twenty-four hours of working, 67 bushels 

 of coal in the boiler, and 5 bushels in the cylinder flues, 

 did the same work as 108 bushels in the boiler without 

 the cylinder flues, causing a saving of fifty per cent, by 

 their use. Another startling fact was the greater effect 

 for each foot of heating surface in the steam-cylinder 

 flues than in the boiler flues ; the latter gave a power 

 of 1500 Ibs. raised 1 foot high by a bushel of coal, while 

 the former gave 6000 Ibs. of power from the same 

 amount of coal and heating surface. 



Here was a mystery that Trevithick would not 

 believe until he had seen it with his own eyes: he 

 searched for it for a year or two, and overlooking the 

 fact that the more simply arranged engine of his once 

 pupil, Captain Samuel Grose, was doing more duty than 

 the superheating steam-engine at Binner Downs, he 



1 Sec <lni\vinp;, vol. ii., p. 160. 



