MINING. 245 
sluice should ‘be near the level of the ground, and as that may 
be stcep or level below the claim, the grade of the sluice must 
to some extent conform to it. There are thus a multitude of 
points to be taken into consideration in fixing the grade of a 
sluice ; but a fall of less than eight or more than twenty inches, 
in a box of twelve feet, would be considered as unsuitable for 
the board-sluice. Sometimes the upper part of the sluice is 
made steeper so as to dissolve the dirt, and the lower part has 
asmall grade to catch the gold. The clayey matter of ordi- 
nary pay-dirt is fully dissolved in a sluice two hundred feet 
long with a low grade, so the use of the boxes beyond that 
length is merely to catch the gold. There are claims however 
in which the clay is so extremely tough that it will roll in 
large balls more than a quarter of amile through a steep sluice 
with a large head of water, and come out at the lower end 
scarcely diminished in size. 
The gold is caught in the sluice-boxes by false bottoms of 
various kinds. It would not do to leave the smooth boards, 
for the water would sweep all the gold away, and the boards 
themselves would soon be worn through. The most common 
false bottom is the longitudinal riffle-bar, which is from two to 
four inches thick, from three to seven inches wide, and six feet 
long. Two sets of these riffle-bars go into each sluice-box, the 
box being twice as long as the bar. A set of riffle-bars is as 
many as fill one-half of a box. They are wedged in, from an inch 
to two inches apart; the wedging being used, because tlie bars 
can more readily be fastened in their places, and more easily 
taken up, than if nails were used. Before the work of sluicing 
commences, all the boxes are fitted with riffle-bars, and the 
bottom of the sluice is therefore full of holes from one to two 
inches wide, from three to seven inches deep, and six feet long. 
These are the places in which the gold, quicksilver, and amal- 
gam are caught. Quicksilver is used now in nearly all the sluices, 
and is the more necessary the smaller the particles of gold. 
The large pieces of the metal would all be caught by their 
specific gravity without the aid of amalgamation. 
