h the face, THE RIVER SURVEY. so that the charge lifts eut the rock by its explosion. A cavity being made in the middle of the heading, holes are drilled around it and the surrounding rock blown into it. Only one blast is exploded at a time, as great care has to be taken not to shake the structure… … feet of tunnels had competition with it. been opened,and 4,648 The cost of drilling, cubic yards of rock after a long trial with removed. The total the Burleigh drill, length of tunnels and is found to be be- galleries then amoun- tween 36 and 37 cents ted to 6,780°67 feet. per foot,including re-…
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