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TUNNELS AND SLUICES.
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sluice.
When wedges are used the sides of the blocks should be square where
they adjoin one another. A side lining is required in all sluices. In
cement claims blocks 3 inches thick, and covering 18 to 20 inches (in
depth) of the side, are used for side lining.
Advantage of Block Riffles.—The advantages afforded by blocks, which should always be used at the heads of sluices, are :
1st. The cross riffle which they make is not excelled by any other form.
2d. Their cheapness under ordinary conditions of timber supply.
3d. The convenience of cleaning up, which can be quickly and cheaply done.
This
last circumstance is of especial importance, because it is often
desirable to collect the gold at frequent intervals, as it is
injudicious to expose amalgam collected in the riffles to wear by the
gravel running over it for long periods.
Experience
shows square block riffles to be the best for saving gold. The
objection to their use is the cost of wear and tear. Rocks are the most
economical substitute, but sluices set with them require steeper
grades and more water.
Life of Blocks.—The
life of a block depends on the quality of the wood, the grade, the
character and quantity of the gravel, and the amount of water. The
larger the amount of water (on the same grade) in proportion to that of
gravel, the less the wear of the blocks. The quality of the wood varies
greatly in different localities. The best and most desirable timber
comes from the higher sierra. Wood which is long-grained and " brooms
up" makes the best riffle. Hard timber which wears smooth (as oak) is
not desirable. Nut pine is the best, but it is difficult to obtain.
Pitch pine answers all requirements. As a rule the price of lumber
governs the selection.
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