| Sluice. | Grade. | Water. |
| 6 ft. × 36 in. | 4 to 5 p.c. | 2,000 to 3,500 m. in. |
| 4 ft. × 30 in. | 4 p.c. | 1,800 to 2,000 m. in. |
| 3 ft. × 30 in. | 1½ p.c. | 600 to 1,000 m. in. |
"The longer the better," is the sluice-builder's motto. The best "riffles" are made of blocks of pine 8 to 13 inches deep, wedged into the bottom of the sluices. They are laid in rows separated by a space of an inch or an inch and a half. Riffle strips keep them in position, these latter being laid crosswise on the bottom. When worn down to five inches, the blocks should be replaced. This amount of wear will probably require six months. Stone and longitudinal riffles running lengthwise of the box are often preferred.
An undercurrent is a broad sluice set at a heavy grade below the level of the main sluice. The fine stuff drops through a grating, while the coarse gravel continues on down the sluice.
Refuse material from quartz, hydraulic or other mines is known as tailings. Tailings are deposited on a dump, which in the case of a hydraulic claim must be sufficiently spacious to receive the thousands of yards of debris deposited on it each day. When available a narrow, deep canyon, or a tunnel, may take the places of dumps.
Quicksilver is used in the sluices, 14 to 18 flasks being used every fortnight in a long sluice. It is not placed in the last 300 or 400 feet.
In working, keep the face of the bank "square." Washing should be carried on continuously. Watches must be set over the sluices, or gold is likely to be missed. As an extra precaution, the sluices should be run full of gravel before shutting off the water. There is no fixed custom regulating "clean ups." Some managers do so every 20 days, others run two or three months, others again clean up but once in a season. In large operations, the first 2,000 feet of sluice are cleaned up every fortnight; the remaining boxes once a year.
Sluices are cleaned from the head downward, the blocks being taken up for that purpose. The amalgam of gold and quicksilver is collected in sheet iron buckets. The final step is reached when the amalgam is retorted and melted in a graphite crucible.
The principle of which the hydraulic miner takes advantage is the great specific gravity of gold as compared with water and rock. To illustrate this quality it may be noted that on a smooth surface inclined at an angle of 1 in 48, subjected to a heavy stream of water, 95 per cent. of the fine gold in gravel does not travel three feet.
The loss of quicksilver fed into sluices will vary, even under good management, from 11 per cent. to 25 per cent. of the amount fed to the boxes.
Hydraulic mines under favorable conditions are very paying investments. Gravel yielding 10 cents a cubic yard has been worked for 6 cents a cubic yard, at the rate of a million cubic yards a year. On another large claim 600,000 cubic yards were worked for 6 cents a cubic yard, yielding 13 cents a cubic yard.