The
dirt as it enters the sluice has its lighter portion taken up and
carried in suspension by the current, whilst the coarse and heavy
material moves along on, and in part above, the riffles, but below the
surface of the water. Boulders and rocks move down the sluices with
varying velocities and in different directions as they advance, aiding
in stirring and disintegrating the cement gravel and earthy stuff,
which little by little fall to pieces and into particles that,
segregated as light material, rise towards the surface of the water.
The rocks and boulders travelling over the riffles assist in keeping
the material thoroughly agitated in the sluices, where it is
alternately changing position from the bottom to the top, until it is
finally discharged.
The
material, wearing down as it advances, is kept from packing by the
presence of the rolling rocks which still maintain their solidity.
Light, sandy gravel requires very wide and shallow sluices, as it
cannot be washed advantageously in deep sluices, unless by a proper
mixture of rocks, which permits the use of a greater quantity of water,
so that the capacity of the same sluice is increased.
A
heavy grade will compensate for a limited supply of water. With an
abundant supply of water and material, the capacity of sluices will
depend upon:
1 st. The character of the material washed ;
2d. The size and minimum grade of the sluices ;
3d. The character of the riffles used.
The
statement of some engineers that the transporting power (meaning
capacity) of a sluice increases with the third power of its grade is
not verified by the comparative tests which have been recorded.
However, these tests, which give the only reliable data extant, were
not made with the same material, so there is still a very important
factor undetermined.
The
empirical results thus far obtained demonstrate that the transporting
capacity of a sluice set on a 2.08 per cent, grade, and that of a
sluice on a 4^ per cent, grade,
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