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SYSTEMATIC MINING
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329
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The
time of the journey through the shaft now varies only a little with
depth, being from thirty-five to forty seconds from the 1200 or
1520-foot levels. On reaching the surface, the blue ground is tipped
automatically from the skips into loading boxes. The "self-dumping"
skips in present use were introduced by me in 1888, and were made from
drawings supplied by the Union Iron Works of San Francisco, and are
similar to the skips used in the mining districts of the Pacific Coast.
(On page 327 are shown the plans for the skip and the manner of tipping
into the surface chutes.) From these chutes the blue ground is loaded
into side - tipping trucks holding 20 cubic feet each. The average
weight of the blue ground in a surface truck is 2000 pounds. The trucks
used underground hold 16 cubic feet, and are end-tipping in the
intermediate levels where the ground is dumped into passes, but have
solid ends on the main levels where revolving tippers are used. From
the depositing surface boxes at the winding shafts, the ground is taken
by means of an endless wire rope haulage to the " floors," where it is
treated as described in another chapter.
Record Hoisting
With
alert and orderly handling of the blue ground in the mines, the
rapidity of extraction has advanced to extraordinary record points.
During the month of July, 1889, 142,567 loads were hoisted through a
single shaft in No. 2 incline, De Beers mine. The best day's work of 24
hours was 6222
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