Kew Bridge Steam Museum -- Boulton and Watt 64 inch Cornish Beam Engine

The Boulton & Watt 64 inch Cornish beam engine was built in 1820. It was moved to Kew in 1840. For over one hundred years until1943 it was in operation at Kew pumping water for London. In 1975 it was restored and returned to steam. It continues to be run at Kew on selected steam days throughout the year.
Cylinder diameter 64 inches, the beam weighs 15 tons, water output 130 gallons per stroke. A Lancashire boiler built in 1927 provides steam at 40psi.
The term "Cornish" refers to the operating cycle of these engines. Some were made in Cornwall and many were used to pump out the constant ingress of water that affected Cornish mines. The main characteristic of a Cornish engine is that pumping is done by a falling weight which is lifted by the engine. This weight is positioned above the pump, which is linked to a beam, with the piston attached to the opposite end of this beam. The weight is lifted by a combination of steam pressure above, and vacuum below, the piston. During the pumping stroke, as the weight falls, the piston returns to the top of the cylinder because an equilibrium valve opens to allow steam to pass from above to below the piston.
Kew Bridge Steam Museum has the largest collection of Cornish engines in the world, including this, the world's oldest known working beam engine.