Most mineral lodes in Cornwall and west Devon stretch vertically underground far below the natural water table, so mining could only go as deep as the drainage technology available at the time allowed. With the development of the high-pressure steam-pumping engine in the early 19th century, ore bodies could be exploited at much greater depths thereby helping to prolong a mine’s productive life.
Pumping engine houses needed to be large to accommodate steam cylinders which could be up to 100 inches (2.54 metres) in diameter and sturdy engine houses were built to contain them. There are over 200 Cornish engine houses spread across the Cornish Mining World Heritage Site. They stand adjacent to where the main mine shafts were and provide one of the most distinctive displays of industrial buildings anywhere in the world.
Around 3,000 engine houses were built to house beam engines in Cornwall and west Devon.
The Cornish engine house
The main function of an engine house was to provide the framework for the engine it contained. Its basic design was essentially established by Newcomen for his atmospheric engine. The distinctive architecture of Cornish beam engine houses links their landscape context – both in the United Kingdom and overseas – with Cornwall and west Devon mining engineering. More beam engines were installed in Cornwall and west Devon than any other mining region of the world: it is thought that around 3,000 engine houses were built in total to house them.
Local stone was used to build Cornish engine houses. This was sourced from quarries, sometimes from mine waste and often from existing derelict engine houses on the same or adjoining mines. Cut granite was always favoured for the cylinder bedstone, the bob wall and corners – the latter known as ‘quoins’. Gable roofs were covered with Cornish slate and bricks were brought to construct the top most section of the chimney stacks and window and arch details. The necessary strength and size of construction of Cornish engine houses is the principal reason for their survival.
The bob wall
Most surviving engine houses are rectangular in plan with a much thicker wall in the front (the bob wall). This was constructed using massive stones (often cut granite) and was perhaps two-thirds of the height of the other walls. It supported the beam (known in Cornish mining as a bob), which transmitted the reciprocating motion of the piston to the pump rods in the adjacent shaft (in the case of a pumping engine) or to the hoisting or crushing machinery. This wall had to withstand both the weight (that might be over 50 tons for a large pumping engine) and the rocking forces of the bob.
Other design features
The other walls braced the bob wall and helped to take some of the working stresses of the engine. The rear wall (usually with a gable that supported a pitched roof) contained the cylinder opening through which the cylinder, bob, and other large components were brought into the house. There were usually three chambers internally.
Associated structures include: boiler houses, which were usually attached to the engine house as a lean-to building; chimney stacks that were either built-in to a rear corner of the engine house or sometimes detached and connected by a flue; and engine ponds (usually upslope), which stored water for the engine condensers.