The machine was the first practical boring apparatus for rock, and was used first in making the Mont Cenis Tunnel. With explosives, as gun-cotton, dynamite, etc., the time occupied in cutting tunnels has been much reduced. Thus the Mont Cenis Tunnel occupied about thirteen years, and cost three millions of pounds. The St. Gotthard—another Alpine subway—occupied eight years, though it is 9¼ miles in length; and the Arlberg—yet another Alpine tunnel—a little over 6 miles long, occupied something more than three years. Further, the railway of which the St. Gotthard Tunnel forms part, has been commercially very successful. This tunnel was commenced in 1872 and completed in 1880, the same year that saw the beginning of the Arlberg. Tunnels through hard rock do not always need a lining of brickwork; but if the soil be clay, or loose earth of any kind, the lining of brick or stone must be brought up close to the scene of actual excavation. The Mont Cenis is lined with stone or brick almost entirely, about 900 feet, however, being without such lining. And now, how was the actual work of tunnelling carried on? It will be seen at once that the problem was quite different from that of boring fifteen feet under the Thames, and sometimes through watery mud. In boring through mountains the quickest way of cutting and carting away rock is one of the chief points to be considered. At the Mont Cenis Tunnel the blasting took place by driving a series of shot holes into the soil, all over the surface to be cut, filling them with explosives, and firing them simultaneously in rings. Such explosives may be fired by a time-fuse or by electricity, giving the workmen ample time to escape out of reach. The shaken and shattered soil can then be cleared away. The blast holes in this small-shot system are about 1 to 1½ inch in diameter, and from 1½ to 7 or 9 feet in the rock. The explosive is forced to the end of each, and the hole is then tamped—that is, closed with clay or sand—and fired in due time. [Illustration: BORING MACHINE USED FOR THE MONT CENIS TUNNEL.] The cutters for boring in rock are often diamond drills, the cutting edges being furnished with a kind of diamond found in Brazil, of a black colour and of great hardness. These are placed round the edge of a cylinder of steel, to which iron pipes can be screwed as the edge cuts its way deeper in the rock. The stuff cut out as the drill revolves finds its way through the cylinder and the piping. There are, however, a great number of boring machines of different kinds, hard steel sometimes taking the place of the opaque diamonds for cutting purposes. The compressed air with which many of the machines are worked assisted in the St. Gotthard in the ventilation of the tunnel, frequently a great consideration, as the space is so small and the gas from explosions often so great.