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TUNNELS AND SLUICES.
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rails
a grade of 12 inches in 12 feet is sufficient; but with blocks the
grade should be increased to 14 inches in 12 feet, and with cobbles to
16 inches in 12 feet.
The gravel escaping from the undercurrent is led back to the main sluice.
The
chief cost of maintenance is occasioned, not by the undercurrent
itself, but by the repairs on the main sluice and grizzly, caused by
the introduction of the latter into the sluice line. The running
expense of a wide undercurrent is no more than that of a narrow one,
excepting in the slight matter of pavement and cleaning up.
At
French Corral, with a tail sluice 5 feet wide, the yield of the first
undercurrent, which was 20 feet wide, was 20 per cent, of the yield of
all the undercurrents. An addition of 10 feet to the width increased
its yield to 27 per cent, of the total, and the grizzly in the main
sluice was not changed.
TABLE XXV.
Lengths and Grades of the principal Tunnels in the Mining District of Smartsville, Yuba County, California.
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