=Side Walls.=--The construction of the side walls above the foundation courses is carried out as any similar piece of masonry elsewhere would be built. To direct the work and insure that the inner faces of the walls follow accurately the curve of the chosen profile, leading frames previously described are employed. =Roof Arch.=--For the construction of the roof arch, the centers previously described are employed. Beginning at the edges of the center on each side, the masonry is carried up a course at a time, care being taken to have it progress at the same rate on both sides, so that the load brought onto the centering is symmetrical. As soon as the centers are erected, the roof strutting is removed, and replaced by short props which rest on the lagging of the centers and support the poling-boards. These props are removed in succession as the arch masonry rises along the curve of the center, and the space between the top of the arch masonry and the ceiling of the excavation is filled with small stones packed closely. The keystone section of the arch is built last, by inserting the stones or bricks from the front edge of the arch ring, there being no room to set them in from the top, as is the practice in ordinary open-arch construction. The keying of the arch is an especially difficult operation, and only experienced men skilled in the work should be employed to perform it. The task becomes one of unusual difficulty when it becomes necessary to join the arches coming from opposite directions. =Invert.=--In all but one or two methods of tunneling, the invert is the last portion of the lining to be built. In the English method of tunneling, the invert is the first portion of the lining to be built, and the same practice is sometimes necessary in soft soils where there is danger of the bottoms of the side walls being squeezed together by the lateral pressures unless the invert masonry is in place to hold them apart. The ground molds previously described are employed to direct the construction of the invert masonry. =General Observations.=--In describing the construction of the roof arch, mention was made of the stone filling employed between the back of the masonry ring and the ceiling of the excavation. The spaces behind the side walls are filled in a similar manner. The object of this stone filling, which should be closely packed, is to distribute the vertical and lateral pressures in the walls of the excavation uniformly over the lining masonry. As the masonry work progresses, the strutting employed previously to support the walls of the excavation has to be removed. This work requires care to prevent accident, and should be placed in charge of experienced mechanics who are familiar with its construction, and can remove it with the least damage to the timbers, so that they may be used again, and without causing the fall of the roof or the caving of the sides by removing too great a portion of the timbers at one time.