Page:Walker (1888) The Severn Tunnel.djvu/295

182 The top and bottom nozzles are connected by two vertical polished pipes, one of which is used as a steam-pipe, and the other conducts the steam—on the opening of the equilibrium valve—from top to bottom of piston. This pipe is fitted with a throttle-valve (under the control of the driver) which regulates the speed of the engine on the return or outdoor stroke.

The number of strokes performed by the engine is controlled by the cataract, which automatically regulates it from one to about fifteen strokes per minute.

By an ingenious contrivance the speed of the six engines is regulated by two cataracts, which, although allowing each engine to perform the same number of strokes per minute, so regulates them that no two engines make their stroke at the same moment, this insures uniform pressure of steam, and an even discharge of water into the culverts. Provision is made in case either of the engines remains idle.

The engine-beam is of wrought-iron throughout, being built up of plates and angles having a web of strong section, and made of double faggoted iron secured to, and entirely surrounding, the outer edge. This web alone is sufficiently strong to resist the entire load. The centres are all accurately bored to receive the respective pins. The main pin, or gudgeon, is secured by two keys placed at the quarters, and rests in two massive gun-metal bearings, supported by heavy cast-iron stools and wall-plate. The wall-plate is secured to the main wall of building.

A parallel motion of polished wrought-iron is fitted to each end of the beam, which parallelizes the path of piston and pump-rods, the anchorage of same being attached to strong wrought-iron spring beams and girders extending across the building, and secured to the wall.

The piston and pump-rods are attached to these motions by strong wrought-iron caps.

The condenser is of the surface-condensing type, consisting of about 400 galvanized wrought-iron tubes, secured by screwed ends into wrought-iron plates above and below.