252 RESOURCES OF CALIFORNIA. 
into the cylinder, and pressing it into the wood. The quick- 
silver, thus fastened in the wood, catches particles of gold, 
which must be scraped off when the time for “cleaning up” 
comes. 
§ 183. Double Slwices.—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-cur- 
rent 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 mate- 
rial 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. 
§ 184. Rock-Sluices.—Large sluices are frequently paved 
with stone, which makes a more durable false bottom than 
wood, and catches fine gold better than 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 diffi- 
cult and tedious in a rock-sluice, and so is the putting down of 
the false bottom after cleaning up. The stones used are cob- 
bles, 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 wa- 
ter and dirt have run over them for an hour, they are fastened 
