=General Description.=--The method of tunneling through hard rock by drifts is preferred by European engineers. All the great Alpine tunnels, from the Mont Cenis tunnel to the Simplon, are examples of tunneling by drifts. In this method the sequence of excavation is shown diagrammatically by Fig. 55. The work begins by excavating a drift close to the floor of the proposed tunnel (as shown in the center of the figure) and far in advance of the excavation of any other part. The section marked 2 is next removed and still later the portions marked 3. Then with the removal of the parts marked 4 the whole section of the tunnel will be open. The drift is usually strutted by means of side posts carrying a cap-piece placed at intervals, and having a ceiling of longitudinal planks resting on the successive caps. In hard rock the roof of the section does not, as a rule, require regular strutting, occasional supports being placed at intervals to prevent the fall of isolated fragments: When the rock is disintegrated or full of seams, a regular strutting may be necessary, and this may be either longitudinal or polygonal in type. When longitudinal strutting is employed, a sill is laid across the roof of the drift, and upon this are set up two struts converging toward the top and supporting a cap-piece close to the roof. On this cap-piece are placed the first longitudinal crown bars carrying transverse poling-boards. Additional props standing on the sill and radiating outward are inserted as parts No. 3 are excavated. These radial props carry longitudinal bars which in turn support transverse poling-boards. When polygonal strutting is used, it may take the form of three or five segment arches of heavy timbers. In hard rock tunnels, as a rule, there is no danger of caving in because of heavy pressures, and the whole section is left open for some time before it is lined. The lining may be of concrete masonry, but in many long tunnels, excavated through hard rock, the side walls are lined with rubble masonry and the arch with brick, and, in some instances, even the arch has been lined with rubble masonry. With skilful laborers at hand the rubble masonry lining has proved most efficient and economical, because the rock is utilized as it is excavated without any further operation. Concrete, however, is more extensively employed for lining tunnels than any other material. Tunnels excavated by drifts enable simple means of hauling to be employed, and this is one of the reasons why the method finds so much favor with European engineers. The tracks are laid along the floor of the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well as from the front of the drift itself. As fast as the full section is completed, this single track in the drift is replaced by two tracks running close to the sides of the tunnel, or by a broad-gauge track with a third rail. THE SIMPLON TUNNEL.[8]