One or more shafts are then sunk from the surface through the gravel to the tunnel, and washing operations are begun by ground- sluicing, letting the water and gravel fall down the shaft and run through the tunnel, in which the sluices are sometimes laid. The surface near the shaft is thus gradually lowered, or it may be terraced by hand labour, until an excavation is made of sufficient size to enable the ground to be attacked with the hose. Washing then proceeds regularly in this manner, the bank being broken down by jets of water, and the products being allowed to fall down the shaft and pass through the tunnel.

The tunnels are often of great length, that at the North Bloomfield Mine, Nevada Co., Cal., for example, being 7,874 feet or 1½ miles long. It is necessary in such cases to run a tunnel from the nearest canon in the bed-rock to the lowest point in the gravel, this point being found or guessed at by prospecting operations. Later, some of the deposits occurring in those channels which are not intersected at favourable points by the present system of drainage were operated on. In California, the naturally occurring banks or cliffs in the gravels in the sides of the gulches were first selected for attack. Sufficient fall in the ground so that (a) the water may be delivered under the pressure of a head of from 100 to 300 feet, (b) the tailings can be easily carried away to a large dumping ground, which is most conveniently, either the sea or a large and rapid river.A plentiful and uninterrupted supply of water throughout considerable portions of the year.The gravel treated need not be rich, a mean yield of less than 1 grain of gold per cubic yard being often enough to furnish profits if the operations are on a sufficiently large scale. Large quantities of auriferous gravel, not less than 30 feet in thickness, and not overlaid by any appreciable thickness of barren material, which would necessarily be passed through the sluices with the pay-dirt.The chief requisites for the successful application of hydraulic mining are: The method of working by Hydraulic Mining consists, as has been already stated, in breaking down banks of gravel by the impact of powerful jets of water, and passing the disintegrated material through a line of sluices, without the agency of hand labour.

Mining water flume