Be a TS ha il le el lll 
the South and Middle Yuba, Cali ornia. 13 
third cubic feet of water, wee 1632 pounds, aete 
discharged against the fac the bank under a ure of 
ninety to two hundred ona mg to the square inch, vecying gs 
the height of the column. Under the continuous action of this 
. enormous mechanical force, pont Lae the softening power of the 
water, large sections of the gravelly mass come crashing down 
with great violence. The eben speedily dissolving and disap- 
pearing under the resistless force of the torrent of water, is hur- 
ried forward to the mouth of the shaft, down which it is precipi- 
tated with the whole volume of turbid water. Boulders of one 
hundred to two hundred pounds in weight are shot forward by 
this impetuous stream, accompanied by masses of the harder 
cement, which meet in the fall down the shaft and in the con- 
aeente of ve ee boulders, the crushing agencies required to 
inte, 
stratum on the ‘bed rock’ = strongly cemen opi sul. 
phuret of iron and great pressure, resists even the full force of 
the water stream until it has been loosened by gunpowder. For 
this purpose adits are driven in on the ‘bed rock’ forty or fifty 
feet from the face of the bank, and a tunnel extended at right 
angles therefrom to some distance each side of the adit. In 
this tunnel a large quantity of gunpowder is placed, from fifty 
to two hundred kegs, and fired as one blast by a train laid from 
without. In this manner, the compact conglomerate is broken 
up, and the water then rapidly completes the work. 
Sometimes the system of tunnels on the bed rock is extended 
much as ina coal mine, by cross alleys leaving blocks, which 
are then washed away, when the whole mass settles and disin- 
tegrates easily under the influence of the water. 
e tunnels in the bed rock already described are made 
dounle, for the convenience ie ‘cleaning up’ one of them while 
other is in action. The process of cleaning up is performed 
every ten or twenty days, ace to the size and richness of 
the work, and consists in removing the entire pavement of 
blocks from the bed of the sluice, and removin ng all the amal- 
gam of gold and ‘rich dirt’ collected in the ‘rifles,’ and re- 
placing the blocks in the same way as at first; advantage is 
taken of this occasion to reverse the position of the blocks when 
they are worn irregularly, and to substitute new ones for pees 
which are worn through. The mechanical action of the 
process on the blocks is of course very rapid and severe, so as 
to command a complete renewal of them once in eight or ten” 
weeks. Some miners prefer a pavement of egg-shaped stones 
