The bottom, which would soon wear away, is revetted with rough
planks and paved with hard stones, weighing ten to twenty pounds, the
grain being placed vertically. With a full head of water 400 cubic yards a
day can easily be washed. The gold, as usual, gravitates through the
chinks to the bottom, and finally is cradled or panned out. It is most
efficiently treated when the sluice is long; it demands six times more
water than the artificial article, but it wants less manual labour. This
last property should recommend it to the Gold Coast. Here, I repeat,
machinery must be used as much and manual labour as little as possible.
The artificial or portable box-sluice is a series of troughs each about
twelve feet long, like the upper compartment of 'Long Tom.' They are made
of half-inch boards, rough from the saw, the lower end being smaller to
fit into its prolongation. Each compartment is provided with a loose metal
bottom pierced with holes to admit the dust; the true bottom below it has
cross-riffles, and above it are bars or gratings to catch the coarser
stones. These sluices are mounted on trestles, and the latter are disposed
upon a slope determined by the quantity of water: the average fall or
grade may be 1 to 50.
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