are nailed down.
In all sluices, men must keep watch to see that the boxes do not choke;
that is, that the dirt and stones do not collect in one place, so as to
make a dam, and cause the water to run over the sides, and thus waste
the gold.

There are small sluices, from which all stones as large as a doubled
fist are thrown out. For this purpose the miner uses a sluice-fork,
which is like a large manure-fork or garden-fork, but has tines which
are blunt and of equal width all the way down; the bluntness being
intended to prevent the tines from catching in the wood, and the
equality of width to prevent the stones from getting fast in the fork.

In some sluices, the "block riffle-bars"--that is, bars cut across the
grain of the tree--are set transversely in the boxes, and about two
inches apart.

Another device is, to fill the pores of such riffle-bars with
quicksilver. This is done by driving an iron cylinder with a sharp edge
into the surface of the bar, then putting mercury into the cylinder,
and pressing it into the wood. The quicksilver, thus fastened in the
wood, catches particles of gold, which must be scraped off when the
time for "cleaning up" comes.

_Double Sluices._--Sluices are sometimes made double--that is, with a
longitudinal division through the middle, so that there are two
distinct sluice-boxes side by side. Two companies may be working side
by side, so that it will be cheaper for them to build their sluices
jointly. In some places the amount of water varies greatly; so that in
the winter there is enough to run two sluices, and in the summer only
one. And there are companies which wish to continue washing without
interruption; so they wash first on one side and then on the other, and
clean up without any interruption to the process of washing.

Another device for saving gold in sluices is the "under-current box."
There is a grating of iron bars in the bottom of a box, near the lower
end of a sluice; and under this grating is another sluice, with an
additional supply of clean water, and with a lower grade. The grating
allows only the fine material to fall through; and the current of water
being moderate, many particles of gold, that would otherwise be lost,
are saved. Sometimes the matter from the under-current box is led back
to the main sluice.

_Rock-Sluices._--Large sluices are frequently paved with stone, which
makes a more durable false bottom than wood, and catches fine gold
better than