CHAPTER VI.
TUNNELLING.
Progress of the work—1881.As it is impossible to write an account of this description without continually using more or less technical terms, it will be as well here to state briefly the methods by which tunnels are constructed, and so to explain many of the terms which are unavoidably used.
Generally tunnels are adopted where it is necessary to construct a railway or a canal through a hill at a greater depth than 60 feet. The comparative cost of open cutting, or tunnelling, may be said to decide whether a tunnel should be made or not; but if a tunnel is to be adopted, an engineer of experience in these matters will select a line of route and a line of levels which will bring the tunnel into the most favourable strata for its construction.
Of course, in dealing with the Severn, a tunnel under the river, or a bridge over it, could have been constructed to complete the connection between Bristol and South Wales.
If a tunnel were adopted, the only open question