Page:Popular Science Monthly Volume 30.djvu/491

Rh by surface drainage after heavy rains, and sometimes covers fifty or sixty acres, when good boating and wild-duck shooting may be had upon it, but in times of drought it is entirely dry. The water of the pan, and of another body, Blankenberg's Vley, is leased by the diggers for mining purposes. Except that it is semicircular instead of being circular or oval, the description of the Dutoitspan is very similar to that of the other mines.

The gems were extracted from the earth, at the beginning, when everything was crude and done in haste, and when water was scarce, by the method of "dry-sorting," which consisted in sifting the excavated ground through hand-sieves, and then passing the finer portions over a sorting-table. By this method as many diamonds were missed as were found, and frequently the yield from the rewashing of the earth over which the process had been performed gave better returns than were gained in the first instance. A modification of the "cradle" was then introduced, but it eventually gave way to the "Rotary Washing-Machine," which is still generally employed. It consists of an annular-shaped pan, from eight to fourteen feet in diameter, which is closed by an outer and an inner rim, of which the inner rim is about four feet in diameter, and is not so high as the outer rim. A vertical shaft rotates in the center of the open space, carrying ten arms radiating around it, each of which has half a dozen vertical knives, or teeth, set within half an inch of scraping the bottom of the pan. The diamondiferous ground, mixed with water, enters through an orifice in the outer rim of the pan, and is stirred up into a ripple by the revolving knives, whereby the lighter stuff comes to the surface and continually floats away through an orifice in the inner rim, while the heavier gravel falls to the bottom of the pan. The mud, or "tailings," which flows to waste over the inner rim, is led by a shoot to a pit, whence it is lifted by a chain and bucket elevator some twenty or thirty feet high. At the top of the elevator the buckets deliver the tailings onto suitable screens, over which the solid mud runs to waste, while the muddy water is led back by an overhead shoot to the machine to assist in forming a puddle of sufficient consistency to float the lighter stones in the pan, and allow only the heaviest ground to accumulate at the bottom. For the better mixing of this puddle, an inclined cylindrical screen is fixed above the level of the pan. The dry ground from the mine is tipped into the upper end of the screen, where it is met by the muddy water from the elevator and a certain amount of clear water. The large stones, of a size unlikely to include diamonds, roll out at the lower end of the cylinder, but the puddle, carrying all the smaller stones with it, passes through the wire netting of the screen and down a shoot into the pan, as above described. At the end of the day's work the machine is stopped, and the contents of the pan, after they have undergone an intermediate cleaning in a cradle, or in a small gravitating machine, called a "pulsator," are emptied upon the