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T.E.Crowley (1976)
The typical Cornish engine had its huge beam mounted on the strengthened end wall of the engine house so that the mechanism was indoors and the end of the beam, with pumprods extending down the mine shaft, was in the open air.
The pumprods were composed usually of Oregon pine, in section about 18 inches square and often weighing, in the case of a deep mine, up to a hundred tons. The first pump was in a sump at the bottom of the mine and the water was raised in lifts with intermediate tanks and pumps, worked from offsets on the same pumprods, every few hundred feet.
It was not necessarily pumped to the surface, but often to a tunnel or adit driven to come out on lower ground often miles away. West Cornwall is still riddled with such tunnels, and the construction of them with hand tools was a staggering achievement.
What the engine did was to raise the pumprods and, with them, the plungers in the pumps, thus filling them with water. When the steam valve shut, the rods descended again under their own weight, an ‘equilibrium valve’ opening at the right moment to allow the expanded steam to fill the space below the piston as it rose.
Closure of this valve as the piston approached the top of its stroke cushioned the descent of the rods, bringing them gently to rest so that the pumps could take the water at their own natural speed. This valve was invariably known to the enginemen of old as the ‘Uncle Abram’ valve. The engine then paused at the top of its stroke until the cataract allowed the steam valve to open and repeat the cycle.
The valves were adjustable to cope with whatever quantity of water needed pumping out, and it used to be said by the engineers that a good engine could work at ten strokes a minute or one stroke in ten minutes. To the miners in the gloomy, candlelit galleries far below the ground, the sound of the pump echoed for miles, sounding like a giant snore and giving them constant assurance that all was well.