The Cornish Boiler was one of a number of attempts by various steam engineers to overcome the limitations of designs such as the Waggon and the Haystack boilers of the past and thus permit the use of higher steam pressures.
Generally it consists of an iron tube of some 1.5 m (5 ft) in diameter and about 5.5 m (18 feet) long enclosing a single cylindrical furnace tube of 90 cm (3 feet) in diameter. The fire burns on a cast iron grate inside the first 1.8 m (6 ft) of this tube. Gasses from the fire pass down the tube, are diverted at the end into brick flues down the side of the boiler, then back down a central flue under the boiler before escaping to the chimney. The furnace tube served the dual purpose of conducting the heat from the fire into the water surrounding it and preventing the end plates of the boiler from bulging outwards from the pressure of steam.