=Centers.=--Two standard forms of centers are employed in the English method of tunneling, as shown by Figs. 86 and 87. Both consist of an outer portion, constructed much like a typical plank center, which is strengthened against distortion by an interior truss framework. The elemental members of this truss framework take the form of a queen-post truss, as is shown more particularly by Fig. 86. In Fig. 87 the queen-post truss construction is less easily distinguished, owing to the cutting of the bottom tie-beam and other modifications, but it can still be observed. The possibility of cutting the tie-beam as shown in Fig. 87, without danger, is due to the fact that the lateral pressures on the haunches of the center counteract the tendency of the center to flatten under load, which is usually counteracted by the tie-beam alone. The object of cutting the tie-beam is to afford room for the props running from the completed masonry to the transverse framework of the strutting as shown by Fig. 85. [Illustration: FIGS. 86 and 87.--Sketches of Typical Timber Roof-Arch Centers, English Method.] Generally four or five centers are used for each length of arch built. They are set up so that the tie-beams rest on double opposite wedges carried by a transverse beam below. This transverse beam in turn rests on another transverse beam which is supported by posts carried on blocking on the invert masonry. It is usually made with a butted joint at the middle to permit its removal, since it is so long that the masonry has to be built around its extreme ends. The lagging is of the usual form, and rests on the exterior edges of the curved upper member of the centers.