Engineering Wonders of the World
Volume I

230 ENGINEERING WONDERS OF THE WORLD. Northern Railway and afford a new and direct means of access to the heart of the city, the tunnels between stations (the “ running ” tunnels) have a diameter of 16 feet. We may remark in passing that these tunnels were formed originally as complete iron tubes, and that afterwards brickwork in cement was substituted for the lower half of the metal- THE MATHEMATICS OF TUNNELLING. The uninitiated wonder how engineers man- age to drive their tunnels on the exact lines assigned to them, and how sections of the same tunnel, started from two or more points, and constructed simultaneously, can be made to join up with such precision that the errors STATION TUNNEL ON CENTRAL LONDON RAILWAY BEFORE PLATFORMS WERE COMPLETED, SHOWING TWO RUNNING TUNNELS. work. This composite form of construction was expected to minimize noise and vibration and give a more elastic roadbed, but the results have not justified the extra expense incurred. Of the other Tube systems generally, it may be said that their tunnels have a diameter varying within a few inches of 12 feet, and that the diameter is increased on sharp curves to give the extra clearance needed for a long bogie car. of alignment seldom exceed a small fraction of an inch. The mystery deepens when it is brought home to them that the tunnels curve here to the left, there to the right, and are constantly changing their level in accordance with the requirements of the gradient or rights of way. Under a broad street the two tunnels of a railway may run side by side, whereas under a narrow road it often becomes necessary to carry one vertically above the other, and this change of relative position