And, lastly, the sluice begat the jet, or hydraulicking proper, which is at present the highest effort of placer-mining. We thus reverse the primitive process which carried the wash-dirt to the water; we now carry the water to the wash-dirt. In California I found the miners washing down loose sandstones and hillocks of clay, passing the stuff through sluices, and making money when the gold averaged only 9_d_. and even 4_d_. to the ton. A man could work under favourable circumstances twenty to thirty tons a day. An Australian company, mentioned by Mr. R. Brough Smyth, with 200 inches of water, directed by ten hands, 'hydraulicked' in six days 224,000 cubic feet of dirt. The results greatly vary; in some places a man will remove 200 cubic yards a day, and in others only 50.
Hydraulic mining on the Gold Coast, owing to the conformation of the country, will be a far simpler and less expensive process than in California or Australia. In the latter water has first to be bought, and then to be brought in pipes, flumes, leats, or races from a considerable distance, sometimes extending over forty miles. It is necessary to make a reservoir for a fall. The water then rushes through the flexible hose, and is directed by a nozzle against the face of the excavation. The action is that of a fireman playing upon a burning house. Most works on mining insist upon those reservoirs, and never seem to think of washing from below by the force-pump.
I have shown that the surface of the lands adjoining the Ancobra is a series of hummocks, rises, and falls, sometimes, though rarely, reaching 200 feet; that water abounds, and that it is to be had gratis. In every bottom there is a drain, sometimes perennial, but more often a blind gully or creek, [Footnote: The gully feeds a 'creek,' the creek a river.] which runs only during the Rains, and in the Dries carries at most a succession of pools. Here Norton's Abyssinian tubes, sunk in the bed after it has been carefully worked by the steam-navvy for the rich alluvium underlying the surface, would act like pumps, and dams would form huge tanks. Nor would there be any difficulty in making reservoirs upon the ridge-tops, with launders, or gutters, to collect the rain. Thus work would continue throughout the year, and not be confined, as at present, to the dry season. A pressure of 100 to 200 lbs. per square foot can easily be obtained, and the force of the jet is so great that it will kill a man on the spot. The hose should be of heavy duck, double if necessary, rivetted and strengthened by metal bands or rings--in fact, the crinoline-hose of Australia. Leather would be better, but hard to repair in case of accidents by rats; guttapercha would be expensive, and perhaps thin metal tubes with flexible joints may serve best. The largest hose carried by iron-clads measures 19 to 20 inches in diameter, and is worked by 30 to 40 horse-power. Other vessels have a 15-inch hose worked by manual labour, fifty men changed every ten minutes, and will throw the jet over the royal yards of a first-class man-of-war. The floating power-engines attached to the Dockyard reserves would represent the articles required.