The above description of the practical operations of tunnelling is founded upon the methods used by railway engineers ; but there is no difference between the execution of a railway or of a canal tunnel. A list of some of the most important tunnels is added, and their cost per yard lineal is affixed wherever it is possible so to do. It may, however, be added that all the works enumerated seem likely to be surpassed in magnitude by the tunnel in course of execution tinder the Alps of the Mont C6uis range : it ie proposed to be about 13,787 yards in length, under a mountain nearly 9000 feet above the level of the rails.
Railway tunnels are usually made about 24 feet wide, and from 23 to 25 feet high, on the narrow gauge ; those on the Great Western line are made 30 feet wide by 35 feet' high ; canal tunnels are rarely executed of larger dimensions than 16 feet 6 inches in breadth by 13 feet in height.
There had been two or three schemes for forming a tunnel under the Thames, prior to that brought forward in 1823 by Mr. (afterwards Sir) Isambard Mark Brunel i—one proposing to connect Gravesend with Tilbury, another to commence at Rotherhithe, a little below the site of the present tunnel ; and works had been actually begun at both these places but soon abandoned. Brunel proposed to effect his object
by means of a framework or shield, which should support the face of the excavation and allow the earth to be removed on many points simultaneously ; the frames or divisions of the shield being then moved slowly forward, and closely followed by a solid mass of brick work enclosing two arched passages 16 feet 4 inches in height from the invert of the arch, and 13 feet 9 inches span at the springing of the arch. Great difficulties were experienced in the course of the work, and some serious accidents happened from the river breaking through the moist sand and clay through which the tunnel had to bo carried. But ultimately all obstacles were overcome, and the work was in 1842 brought to a successful termination, about seventeen years from the commencement of the excavations.