XXV. THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR WORK 336 LIST OF ILLUSTRATIONS FIGURE PAGE 1. Diagram Showing Manner of Lining in Rectilinear Tunnels 10 2. B. R. Value’s Device for Locating the Center Line Inside of a Tunnel 11 3. Triangulation System for Establishing the Center Line of the St. Gothard Tunnel 12 4. Method of Transferring the Center Line down Center Shafts 13 5. Method of Transferring the Center Line down the Side Shafts 14 6. Method of Laying out the Center Line of Curvilinear Tunnels 15 7. Diagram of Polycentric Sectional Profile 19 8, 9 and 10. Typical Sectional Profiles for Tunnel 20 11. Soft Ground Bucket Excavating Machine; Central London Underground Railway 22 12. Column Mounting for Percussion Drill; Ingersoll Sargent Drill Co. 26 13. Sketch of Diamond Drill Bit 27 14. Diagram Showing Sequence of Excavation for St. Gothard Tunnel 36 15. Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile 38 16. Polar Protractor for Determining Profile of Excavated Cross- Section 39 17. Joining Tunnel Struts by Halving 48 18. Round Timber Post and Cap Bearing 48 19. Ceiling Strutting for Tunnel Roofs 49 20. Ceiling Strutting with Side Post Supports 49 21. Sill, Side Post and Cap Cross Frame Strutting 49 22. Reinforced Cross Frame Strutting for Treacherous Materials 49 23. Longitudinal Poling-Board System of Roof Strutting 50 24. Transverse Poling-Board System of Roof Strutting 50 25. Shaft with Single Transverse Strutting 52 26. Rectangular Frame Strutting for Shafts 53 27. Reinforced Rectangular Frame Strutting for Shafts in Treacherous Materials 53 28. Strutting of Timber Posts and Railway Rail Caps 56 29. Strutting Made Entirely of Railway Rails 56 30. Rziha’s Combined Strutting and Centering of Cast Iron 57 31. Cast-Iron Segment of Rziha’s Strutting and Centering 57 32. Cast-Iron Segmental Strutting for Shafts 58