From Graces Guide
c1812 Invented by Richard Trevithick. Until this time boilers were heated by an external fire. This was inefficient, and also potentially dangerous (sediment and scale accumulating at the bottom would impair heat transfer, with the risk of boiler plates overheating and failing).
The Cornish boiler was horizontal, cylindrical with an internal sealed fire tube passing horizontally through the middle. Hot exhaust gases from the fire passed through the tube thus increasing the surface area heating the water and improving efficiency. Gases exhausting to the chimney were usually directed over the lower surface of the boiler shell, contained by brickwork.
The Cornish Boiler is characterized by a single large flue containing the fire. In contrast, the Lancashire Boiler has two horizontal flues. In both types the main flues are sometimes augmented by cross tubes. Having two fires in the Lancashire boiler assisted in minimising the smoke nuisance which occurred with single flues each time fresh coal was added.