ENGINEERING WORKS , ETC . 1057 arms fitted with teeth and making two or three revolutions per minute . At each turn about of an inch was removed from the chalk . The machine was worked with compressed air , and the excavated material removed by a moving bücket apparatus some - thing like the bücket… … Fig . 737 . — Section of Proposed Twin Tunnels with Brickwork Lining . a a , Ventilating pipes . b b , Ventilating pipes carried up at the middle of the tunnel . c , Drainage subway . In ordinary cases the double line of rails is contained in a single tunnel ( Fig . 736 ) ; but we consider that…
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