Tunnelling through rock, like the Mont Cenis and St. Gothard mountains, is a comparatively simple engineering feat, as no lining is required; so also is the ordinary railway tunnel, carefully bratticed and propped inside, and securely cased with brick or stone. But it is, as the Great Western Railway knows to its cost, in dealing with water-bearing strata, _vide_ the Severn Tunnel, that a system is required, not only to protect the men as they bore with a gigantic centre-bit through clay, chalk, or gravel, but, pholas-like, to line the tunnel simultaneously. This is obtained by the use of the famous shield invented by the late Mr. J. H. Greathead, and employed by him in the construction of the City and South London and Waterloo and City Railways, though he did not live to witness the adoption of his principle in the Twopenny Tube. RAILWAY TUBES, HOW THEY ARE BORED A revolution in tunnelling has been brought about in constructing Tube railways. By the new process a great cylinder or shield at the bottom of a shaft is pushed forward by hydraulic power into the soil ahead of it. The navvies work inside, excavating the earth in front of them, and fit up iron segments at the rear of the tail end of the cylinder, or shield. Thus, on the one hand, the exact size and shape of the tunnel is ensured, and the workers are fully protected from the risk of the roof falling in. This arrangement of shield and iron tube resembles an old-fashioned single-drawn telescope; the outer case being the shield, and the inner tube the lining of the tunnel. These shields have fronts that bear a row of steel knives forming a true cutting edge, and are so arranged that they can, if required, bore a circle slightly larger than the iron segments of the tube. As the shield slides away from the inner tube, the space it occupied is filled in with what is called “grout,” a kind of porridge of water and lime, which soon sets as hard as stone. This is ingeniously blown in through apertures in the iron lining by means of compressed air, and effectually fills up cracks accidentally formed in the soil, which might otherwise extend to the surface and cause subsidence in the foundations of buildings. Theoretically, therefore, no disturbance of the ground below or above the tubular lining is possible. In the pioneer Tube railways, the City and South London for instance, the diameter of the tunnels was only 10 feet 6 inches, that of the Central 12 feet, but the Great Northern and City Company made a new departure by fixing the width at 16 feet. For the construction of this railway, the shield was designed by Mr. E. W. Moir, M. INST. C. E., and varies in some important respects from the Greathead shield. A remarkable photograph, which, by the courtesy of the _Tramway and Railway World_, I am able to present to the readers of this book, shows this shield at work in the construction [Illustration: FIG. 13. SHIELD AT WORK IN A TUBE RUNNING TUNNEL