Referring to the transverse and longitudinal sections shown by Fig. 166, it will be seen that two side trestles were built to carry an adjustable centering for the roof arch. Two sections of these trestles and centerings were used alternately, one being carried ahead and set up to remove the timbering while the masons were at work on the other. The manner of setting up and adjusting the trestles and centerings is shown by Fig. 166 and also by Fig. 167, which is an enlarged detail drawing of the set screw and rollers for the centering ribs. The following is the bill of material required for one set of trestles and one center: Trestles: Caps and sills 8 pieces 8 × 8 ins. × 20 ft. Posts 18 „ 8 × 8 „  × 11 „ Braces 16 „ 6 × 4 „  × 7 „ Centerings: Ribs 27 „ 2 × 18 „  × 7 „ Bracing 12 „ 2 × 8 „  × 7 „ Support to crown lagging 2 „ 6 × 6 „  × 10 „ Crown lagging 20 „ 3 × 6 „  × 2 „ Side lagging 30 „ 3 × 6 „  × 10 „ Side strips 2 „ 2 × 12 „  × 9 „ Blocking for rollers 1 „ 5 × 8 „  × 12 „ 6 screw and roller castings complete with bolts and lever; 114 bolts ³⁄₄-ins. in diameter; 7¹⁄₂ U. H. hexagonal nut and 2 cast washers each. With this arrangement the progress made per day varied from 2 lin. ft. to 3 lin. ft. of lining complete. By work complete is meant the entire lining, including stone packing between the brickwork and the rock. On Feb. 23, 1900, 363 ft. of lining had been completed, at a cost of $33.50 per lin. ft. This cost includes the cost of removing the old timber, the loose rock above it, and all other work whatsoever. CHAPTER XXIV. THE VENTILATION AND LIGHTING OF TUNNELS DURING CONSTRUCTION. VENTILATION. In long tunnels, especially when excavated in hard rock, proper ventilation is of great importance, because the air cannot be easily renewed, and the amount of oxygen consumed by miners horses and lamps during construction is very large. The gases produced by blasting also tend to fill the head of excavation with foul air. Pure atmospheric air contains about 21% of oxygen and only 0.04% of carbonic acid; when the latter gas reaches 0.1% the fact is indicated by the bad odor; at 0.3% the air is considered foul, and when it reaches 0.5% it is dangerous. It is generally admitted that the standard of purity of the air is when it contains 0.08% of carbonic acid.