THE DUTY OF THE MINER'S INCH.
The quantity
of material that is washed by an inch of water in twenty-four hours is
called its " duty." Estimates of the average duty have of necessity
differed greatly, since the inch itself denotes a varying discharge of
1.20 to 1.76 cubic feet per minute in different parts of the State.
Therefore the determination of the "duty " is good only for the
specific condition under which it is made.
The
circumstances by which it is affected are, the quantity of water,
character of the material washed, height of banks, use of explosives,
size and grade of sluices, and class of riffles. The sluice affects the
duty of the inch in so far as its capacity regulates the quantity
washed.
The
banks of the mines which discharge their tailings into the American
River consist principally of small, fine sediment, disintegrated rock,
and materials which are easily moved. The duty of the inch in this
locality is assumed by the State Engineer to be 4-1/2 cubic yards ;
while at Dutch Flat, in the deep washings, he found it to average only
from 1.4 to 2 cubic yards.
The
duty of the inch in the mines which "tail" into the Yuba River is
estimated by the same authority to be 3.5 cubic yards. The gravel
deposits here are composed of all grades of material.
The following table from Lieutenant-Colonel Men-dell's report shows the State Engineer's estimates of the
duty of the inch in various localities:
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