252 EESOURCES OF CALIFORNIA. 



into tlie cylinder, and pressing it into tlie 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 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 w^ater 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, wdth an additional supply of clean w^ater, 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 w^ould 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, w^hich 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 ^ften 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 

 ihe 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 



