Figs. 202 and 203 give end view and longitudinal section of timber framework frequently adopted for average tunnel work. The positions of the different pieces will explain themselves and the duty they have to perform. The main struts, or raking pieces, which have to sustain great pressure, may be shored against the finished lengths of masonry or brickwork. The timbering of the sides can be removed as the lining proceeds, but in many cases the round logs and boards near the crown cannot be withdrawn, and have to be left in the work, the space between the top of the arching and under side of the boards being firmly packed with brickwork, masonry, or dry rubble stonework. As the tunnel lining is generally carried forward in short lengths, following up the main excavations, the centering for the arching should be of such description that it can be readily transferred or moved forward as the work proceeds. The form of the centering, and the spacing of its upright supports, must admit of sufficient width for one or more lines of rails for the waggons required to remove the excavated _débris_ and convey the building materials used in the lining. Picks, bars, and shovels are the tools used in the excavation of the softer material and loose disintegrated rock, but for the hard rock, blasting will be necessary. The tunnel opening being comparatively small, only moderate blasting charges can be used with safety, and these must be placed so as to break up the rock-bed in a suitable manner for working, and without shaking or damaging the already completed excavation. Ordinary hand-drills, or _jumpers_, may be used for forming the charge holes, a number of them being at work at the same time, and the charges fired very closely one after the other. As the blasting operations necessitate the retiring of the miners to a considerable distance, out of the way of flying fragments, and the remaining away until the foul air has been dispelled, it is advisable to fire off several charges about the same time, and thus minimize as much as possible the stoppage to the drilling and clearing away the loosened material. [Illustration: Fig. 204, 205, 206, 207, 208, 209, 210, 211] Mechanical drills, worked by compressed air or other motive-power, are now very extensively used where the rock is solid and continuous. They are much more expeditious than the hand drills, but they are costly in 174 their installation, and also in their working and maintenance. In some tunnels, where the material has been firm and dry, the upper portion of the excavation has been first removed, and the masonry and brickwork lining built in position down to about the springing of the arch, the remainder of the excavation being afterwards taken out, and the side walls built by means of shoring and underpinning.