grade
this was the greatest possible curve that could be successfully given
to a 4-foot sluice. The curve, however, could be increased in
proportion to the grade.
At
the turn-in and turn-out it is necessary to place a board diagonally
across the main sluice. This concentrates the discharge and prevents
the forming of bars.
Undercurrents.—In
order to relieve the sluices of the finer material, and thereby aid in
saving the gold, undercurrents are introduced into the sluice line.
These may be described as broad sluices set on a heavy grade at the
side of and below the main sluice.
Where
a drop off can be made in the main line, parallel steel or iron bars,
1 by 4 inches, with intervals of 1 inch between them, and 10 to 20 in
number, according to the size of the undercurrent, are placed edgewise
across the sluice. A set of such bars is called a " grizzly." It is set
1 inch below the sluice pavement, which is raised as it wears down. If
too low, the grizzly clogs with gravel.
The
coarse material passes over the grizzly, and, if the topography
permits, is dropped and picked up again in sluices at a lower level.
The
finer gravel drops through the bars into a box about 20 inches deep,
lined with blocks and set at right angles to the main line. This box
carries the material to the chute at the upper end of the undercurrent.
This
chute is lined with cobbles and provided with " dividers" of wood to
evenly distribute the material over the surface of the undercurrent. It
has a 2 or 3 per cent, grade and gradually narrows towards the lower
end.
The
undercurrent proper is a shallow wooden box, 20 to 50 feet wide, 40 to
50 feet long, with sides about 16 inches high. It should have, if
possible, 8 to 10 times the width of the main sluice. The bottom is
made of 1-1/2-inch plank tongued and grooved, and set on a grade of 8
to 10 per cent., according to the smoothness of the riffles employed.
It is paved with cobbles, wooden rails shod with strap iron, or small
wooden blocks. With the smooth
