Tunnel, an arched passage driven through an elevation, or mountain, or under a river. They may be divided into land or subaqueous tunnels, and according to their purpose into railway or canal tunnels, tunnels for sewage, water-supply, &c. Tunnels have been in use from an early period in history, notable ancient ones being those under the Pyramids (q.v.), that for draining Lake Fucino (q.v.), and the Cloaca Maxima (q.v.). But it is since the 16th century that they have been most largely used. From the difficulties and uncertainty connected with their construction, they have proved to be the most costly part of the work in making canals and railways, averaging from £45 to £60 per lineal yard. In making a tunnel, the nature of the material to be excavated is first ascertained by vertical shafts at different points in its length. Some of these are temporary, others are permanent, and serve for ventilation when the tunnel is completed. These shafts must not be less than 9 feet in diameter, so as to admit of lowering and raising men and the material excavated, fixing pumps, &c. When the workmen have reached the top of the tunnel they begin by excavating in opposite directions, and thus, independent of the two ends, many faces can be worked at the different shafts, until a small 'adit' or hole is driven through its entire length. This first passage when completed through rock is 6 or 7 feet high and along the top of the tunnel; the centre line is then marked off, and the adit or passage enlarged at the sides and bottom to the required dimensions. Sometimes in hard rock no brick lining is required. If, however, the material excavated is clay, loose soil, or friable rock, the full size of the tunnel is carried forward from the beginning, the brick lining following close up to the faces of the excavations. Tunnels through lofty mountains, or below rivers or arms of the sea, can only be worked from the two ends, vertical shafts being inadmissible, and from the experience gained in recent years in tunnels of great length, and with improved boring and drilling-machinery driven by compressed air, combined with the use of more powerful blasting materials, it is probable that the older methods of constructing tunnels from vertical shafts will be superseded in the future, the comparative progress under equal conditions being in favour of the newer machinery by as much as three and four to one. Important departments of the work of tunnel-making have been already discussed in the articles Blasting (q.v.) and Boring (q.v.); see also MINING. There are sixty-eight tunnels on canals and nearly 87 miles on the railways of Great Britain. The Channel Tunnel (q.v.) remains a project. See also AQUEDUCT, PNEUMATIC DESPATCH, RAILWAYS.
The longest in the world are the Alpine mountain tunnels under the St Gotthard (9½ miles; 1872-80), the Mont Cenis, and the Arlberg, noticed under their several heads, as are also the other more important tunnels (see HOOSAC MOUNTAIN, ST CLAIR, &c.). The 'Underground Railway' in London is practically a series of tunnels. The old Thames Tunnel of Brunel (1825-43) is now used for a railway. In 1891-97 the London County Council constructed a new tunnel under the Thames at Blackwall at a cost of £1,500,000. The longest canal tunnels in England are the Marsden, on the Huddersfield Canal (5280 yards; opened 1798), and the Sapperton, on the Thames and Liverpool (4300 yards; 1790). The longest railway tunnels are the Severn tunnel (7665 yards; 1886), the Dore and Padley (6200 yards; 1892), the Stanbridge (5342 yards; 1839), and the Woodhead (5297 yards; 1867).