Page:Walker (1888) The Severn Tunnel.djvu/294

Rh steam from this case is conducted through the drain-pipes back again to the boilers.

The cylinder and case are mounted on a strong cast-iron bottom, and attached thereto by 36-1⅜ inch bolts.

The cylinder bottom has cast on it the aperture to which the exhaust nozzle is attached, and is secured by five long hold-down bolts to a massive foundation of masonry 15 feet in depth.

The piston is of the ordinary metallic kind, having a deep spring ring, packed at the back with rubber or gasket, and secured by a joint ring ground in true on piston and ring.

The rod, of wrought-iron, is connected to the piston by a deep cone, and secured by a wrought-iron cotter and fore-lock.

The cylinder-cover is of cast-iron, fitted with a deep stuffing-box, in which provision is made for a circulation of steam at boiler pressure, which prevents the possibility of air leaking through into the cylinder.

The cover is also fitted with an outer case of polished cast-iron, and with suitable arrangements for lubricating the piston and cylinder.

The steam, in its passage through the cylinder, is regulated by four valves, three of which—the governor, steam, and equilibrium—are fixed in the top nozzle, and the fourth—the exhaust valve—is fixed in the bottom nozzle.

All these valves are of the Cornish double-beat pattern, and are cast of gun-metal.

The governor valve is entirely under the control of the engine-driver, who can regulate the flow of steam according to any distinct variation of the pressure in the boiler.

The steam equilibrium and exhaust valves are worked by the Cornish valve-gear.

The engine load being constant, no variation is required in the expansion, and the point of cut-off is therefore definitely fixed at about one-fourth of the length of stroke. Provision is made for cutting off the steam at from one-eighth to three-quarters of the stroke if required.