"Prior to the invention of your petitioner's boiler, the most striking defect observable in every steam-engine was in the form of the boiler which in shape resembled a tilted wagon; your petitioner's invention consists principally in introducing the fire into the midst of the boiler, and in making the boiler of a cylindrical form, which is the form best adapted for sustaining the pressure of high steam, and does not require half of the materials, nor does it occupy half the space required for any other boiler, and greater duty can be performed by an engine with this boiler with less than half the fuel, than by any engine without it, and is the only one that can be used with success in steam-vessels, as none of the old boilers could have withstood a pressure of above 6 lbs. on the inch, much less a pressure of 60 lbs. or even of 150 lbs. when necessary."
A report of the Royal Mail Steam Packet Company in 1871 states, "by placing compound engines in the 'Tasmania,' they had reduced the consumption of coal to one-half the former quantity, doubled her capacity for freight, and increased her speed."[154] Presuming that the compound engines of the 'Tasmania' are like other engines known by that name, having high-pressure steam in a comparatively small cylinder from which it expands in a larger one, tubular boilers, surface condensers, and screw-propeller, the saving admitted in the 'Tasmania' is just what Trevithick's petition pointed out forty-three years ago—to lessen by one-half the weight, space, and fuel in marine steam-engines—his opinion being founded on the experience of a lifetime, for as early as 1804 he wrote on the question of compound engines, "I think one cylinder partly filled with steam would do equally as well as two cylinders;"[155] and again in 1816, describing expansion, "The engine is now working with 60 lbs. of steam, three-quarters of the stroke expansive, and ends with the steam rather under atmosphere strong;"[156] and in the same year worked the expansive compound engine at Treskerby.[157]
[CHAPTER XXVI.]
TUBULAR BOILER, SUPERHEATING STEAM, AND SURFACE CONDENSER.
"Hayle Foundry, December 14th, 1828.
""Mr. Gilbert,
"Sir,—On my return from London five weeks since I was disappointed at not finding you in Cornwall. I have made inquiry into the duty performed by the best engines, and the circumstances they are under, from which it appears to me there is something which as yet has not been accounted for, particularly in Binner Downs engines. A statement was given to me by Captain Gregor, the chief agent and engineer of the mine, which appears so plain that I cannot doubt the facts, though they differ very widely from all former opinions. There are two engines, one of 42 inches diameter, the other of 70 inches diameter, 10-feet stroke.