MINING. 309 



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 liattish. 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 fast- 

 ened 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. 



231. Hydraulic Washing. Most of the gold of the 

 placer mines of California is obtained by hydraulic washing 

 that is, throwing water under a strong pressure against the 

 banks of auriferous gravel, which is then carried by the water 

 into a sluice. The hydraulic process is applied only in claims 

 where the dirt is deep and where the water is abundant. II' 

 the dirt were shallow in the claim and its vicinity, the neces- 

 sary 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 five 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. From the top of the 

 bank, a hose or iron pipe extends down to the bottom of the 

 claim. The hose is of heavy duck, sometimes double sewn, by 

 machine. When full, it is from four to ten inches in diameter, 

 and will bear a perpendicular column of water fifty feet high ; 

 but a greater height will burst it. Now, as the force of the 

 stream increases with the height of the water, it is a matter of 

 great importance to have the hose as strong as possible ; and 



