A couple of years back, an old mate gave me a very nicely built model of a Cornish boiler. The Cornish boiler was invented by Cornish engineer, Richard Trevithick in about 1803. 

He utilised the cylindrical barrel with an internal cylindrical furnace tube running the full length of the boiler, and he then used the furnace gasses to return under the barrel and along in the reverse direction on each side and thence to the chimney. (See Drawing No 1) In one fell swoop he had revolutionised the steam boiler, with pressures from 40 to 120psi in 1803!