of the material penetrated, and the method of construction employed. For the purpose of detailed discussion the construction of masonry lining may be divided into four parts,--the side-wall foundations, the side walls themselves, the roof arch, and the invert. Concrete and reinforced concrete linings are now extensively used on account of cheapness and facility of handling, but they have the great disadvantage of resisting pressure after they become hard, which is some time after being placed. The strutting should, therefore, be left to support the roof so as to prevent direct pressure on the fresh material. The roof, as a rule, is supported by longitudinal planks held in position by five or seven segments of arched frames placed across the tunnel. A large quantity of timber and carpenter work is thus entirely wasted and these costly items, in many cases, make the concrete lining of a tunnel more expensive than the one built of brick and stone. To avoid these inconveniences tunnels have been successfully lined with concrete on the side walls and concrete blocks in the arches. These blocks have been built by hand and molded in the shape of the arch voussoirs. =Foundations.=--In tunnels through rock of a hard and durable character the foundations for the side walls are usually laid directly on the rock. In loose rock, or rock liable to disintegration, this method of construction is not generally a safe one, and the foundation excavation should be sunk to a depth at which the atmospheric influences cannot affect the foundation bed. In either case the foundation masonry is made thicker than that of the side walls proper, so as to distribute the pressure over a greater area, and to afford more room for adjusting the side-wall masonry to the proper profile. In yielding soils a special foundation bed has to be prepared for the foundation masonry. In some instances it is found sufficient to lay a course of planks upon which the masonry is constructed, but a more solid construction is usually preferred. This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep all along the bottom of the foundation trench, or in some cases by sinking wells at intervals along the trench and filling them with concrete, so as to form a series of supporting pillars. [Illustration: FIG. 45.--Diagram Showing Forms Adopted for Side-Wall Foundations.] The form given to the foundation courses and lower portions of the side walls varies. Where a large bearing area is required, the back of the wall is carried up vertically as shown by the line _AB_, Fig. 45, otherwise the rear face of the wall follows the line of excavation _AC_. For similar reasons the front face of the wall may be made vertical, as at _FG_, or inclined, as at _FH_. The line _FE_ indicates the shelf construction designed to support the feet of the posts used to carry the arch centers during the construction of the roof arch.