When an engineer has to construct a railway up a hill having a still steeper slope, he must secure practicable gradients by laying out the line in ascending spirals, if necessary tunnelling into the hill, as on the St Gothard railway, or in a series of zigzags, or he must resort to a rack or a cable railway.
Where the depth to rail-level was too great for cut-and-cover methods, ordinary tunnelling processes were used; and where the trench was too shallow for the arched roof, heavy girders, sometimes of cast iron, bridged it between the side walls, longitudinal.
Underground railways are of three general types: the one of extreme depth, built by tunnelling methods, usually with the shield and without regard to the surface topography, where the stations are put at such depth as to require lifts to carry the passengers from the station platform to the street level.
The termites, or " white ants," are exceptionally destructive because of their habit of tunnelling through the softer woods of habitations and furniture, while some species of ants, like the sadba, are equally destructive to plantations because of the rapidity with which they strip a tree of its foliage.
Scanty traces of fortifications of the Roman period seem to have come to light in recent tunnelling operations.
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