riffle-bars. The stone bottoms have another advantage--that
it is not so easy for thieves to come and clean up at night, as is
often done in riffle-bar sluices. But, on the other hand, cleaning up
is more difficult and tedious in a rock-sluice, and so is the putting
down of the false bottom after cleaning up. The stones used are
cobbles, six or eight inches through at the greatest diameter, and
usually flattish. A good workman will pave eight hundred square feet of
sluice-box with them in a day; and after the water and dirt have run
over them for an hour, they are fastened very tightly by the sand
collected between them. In large sluices, wooden riffle-bars are worn
away very rapidly--the expense amounting sometimes, in very large and
long sluices, to twenty or thirty dollars a day; and in this point
there is an important saving by using the stone bottoms. They are used
only in large sluices, and they generally have a grade of twelve or
fourteen inches to the box of twelve feet.

_Hydraulic Mining._--After the board-sluice, with its various adjuncts
of riffle-bars, stone bottoms, copper plates, and so forth, the next
instrument of importance in the gold-mining of California, is the
hydraulic hose, used to let water down from a considerable height, and
throw it under the pressure of its own weight against the pay-dirt,
which is thus torn down, broken up, dissolved and carried into the
sluice below. The sluice is a necessary part of hydraulic mining. The
hose is used, not to wash the dirt, but to save digging with shovels,
and to carry it to the sluice.

The hydraulic process is applied only in claims where the dirt is deep
and where the water is abundant. If the dirt were shallow in the claim
and its vicinity, the necessary head of water could not be obtained.
Hydraulic claims are usually in hills. The water is led along on the
hill at a height varying from fifty to two hundred feet above the
bed-rock, to the claim at the end or side of the hill, where the water,
playing against the dirt, soon cuts a large hole, with perpendicular or
at least steep banks. At the top of the bank is a little reservoir,
containing perhaps not more than a hundred gallons, into which the
water runs constantly, and from which the hose extends down to the
bottom of the claim. The hose is of heavy duck, sometimes double, sewn
by machine. This hose when full is from four to ten inches in diameter,
and will bear a perpendicular column of water fi