Tunnel, a way hollowed underground, whether for water passage, as in the case of canals, drains, sewers, and the like, or land passage, as in the case of railways and roads. Among ancient tunnels for water may be noted the Cloaca Maxima of Rome, Claudius's tunnel at Lake Fucino. and the tunnel by which the Bridgewater Canal communicates with the coal-pits at Worsley. But the chief use of tunnels has been in the preservation of, as far as possible, a uniform gradient upon railways when passing through hilly country or beneath a town or river. There comes a time when to make a safe cutting would cost more than tunnelling, and then the latter is resorted to. The usual method is to sink shafts of not less than nine feet in diameter, and to work from each of these, and the difficulty and expense of the work depends upon the nature of the soil to be cut through, both of these being sometimes enormously increased by the occasional occurrence of quicksands or underground watercourses. Often the shafts are kept permanently open for the purpose of ventilation. In cutting through a mountain shafts become impossible, and here ventilation must be artificial. The tunnel of Mont Cenis was begun simultaneously at either end, and the calculations were so true that the two lines of excavation met accurately in the centre. Generally a tunnel is lined with bricks, but in a very solid soil this becomes unnecessary. In the case of the South London Electric Railway, which is tunnel throughout, the two lines pass through iron tubing, and at one point one tube passes over the other. The tunnels and subways beneath the Thames were considered wonderful feats of engineering till eclipsed by that beneath the Mersey and the wonderful tunnel, more than four miles long, beneath the Severn.