In either case the leading center over which the arch masonry terminates temporarily is left in position until the next section of the arch is built. =Masonry.=--The masonry of the roof arch, which is the first part built, is of necessity begun at the springing lines, and the first course rests on short lengths of heavy planks. These planks, besides giving an even surface upon which to begin the masonry, are essential in furnishing a bearing to the struts inserted to support the arch while the earth below them, part No. 5, Fig. 68, is being excavated. As the arch masonry progresses from the springing lines upward, the radial posts of the strutting are removed, and replaced by short struts resting on the lagging of the centers, which support the crown bars or longitudinals until the masonry is in place, when they and the poling-boards are removed, and the space between the arch masonry and walls of the excavation is filled with stone or well-rammed earth. Considering now the side wall masonry, it will be remembered that in excavating the part No. 5, Fig. 68, of the section, frequent side trenches were excavated, and struts inserted to take the weight of the masonry. These struts are inserted on a batter, with their feet near the center of the tunnel floor, so that the side wall masonry may be carried up behind them to a height as near as possible to the springing lines of the arch. When this is done the struts are removed, and the space remaining between the top of the partly finished side wall and the arch is filled in. This leaves the arch supported by alternate lengths or pillars of unexcavated earth and completed side wall. The next step is to remove the remaining sections of earth between the sections of side wall, and fill in the space with masonry. Fig. 71 is a cross-section, showing the masonry completed for one-half and the inclined props in position for the other half; and Fig. 72 is a longitudinal section showing the pillars of unexcavated earth between the consecutive sets of inclined struts and several other details of the lining, strutting, and excavating work. [Illustration: FIG. 71.--Sketch Showing Method of Underpinning Roof Arch with the Side Wall Masonry.] [Illustration: FIG. 72.--Longitudinal Section Showing Construction by the Belgian Method.]