The other form of the valve, shown in Fig. 5, is similarly placed in the centre of the bottom of the tub, and for the same purpose, though rather more simple in its construction. It consists merely of a conical brass or other metal stopper, turned and ground, or packed so as to fit tightly into the hole of the metal plate t t, which is let into the bottom of the tub: this stopper is moved, as in the former valve, by the iron or metal lever ef and attached to the plug or stopper by the iron rod g, so that the valve may be opened or shut at pleasure, by applying the hand to the end of the lever. It will be observed in all the above figures, and particularly in the perspective view of the agitator at Fig. 4, that there are no stirrers I I I in the centre of the agitator, but that a certain space, fully equal to the size of the central valve, is left free for them, not only for the purpose of permitting the valve to rise between the stirrers, but also to prevent the same degree of motion being given to the central part of the contents of the tub, that is given to the sides of it.
Having so far described the general form and construction of the apparatus, we shall next proceed to describe the manner of using it, for the purpose of extracting the gold, silver, or other metals or materials, from the sand, earth, or other matrices with which they may happen to be mixed. For this purpose the tub A A must be about half filled with water, or, what is better, may communicate by a pipe, shoot, or trough, with water, which can at pleasure be permitted to run into the tub, or may be stopped; the cock S and central valve being of course closed at this time. The ore and matrice, or other material to be operated upon, reduced to a state of powder, must now be thrown in, in such quantity that it will not exceed in weight more than about half the weight of the water in the tub at any one time; but a greater or less quantity may be added, according to its density, which will be easily ascertained by practice. The agitator is then to be put into motion, beginning slowly at first, but quickening it until the whole quantity of water, and the materials that have been thrown into it, are put into rapid motion, and the whole of the ore, or other material, however heavy, has become completely incorporated with, and floats in, the water.
It will soon be found, that the water, by its centrifugal force, will rise against the sides of the tub, and leave a hollow space in the middle of it, in the form of an inverted cone, as shown by the dotted lines k k k k, in the section of Fig. 3. This effect takes place to such an extent (if the height of the tub and the size of the agitator are properly proportioned to one another, and the motion is sufficiently rapid), that the central valve at the bottom of the tub can be distinctly seen from above, and may even be opened without danger of discharging much of the water; and if, after continuing this rapid motion for two or three minutes, it is gradually abated, and the agitator is brought to a state of rest, it will be found that all the gold or silver, or other metals, so mixed with the water, will be deposited in a heap in the centre of the tub, immediately over the central fig. 5. valve, with very little admixture of the sand or earth that was previously mixed with it; and, consequently, if the piston d of the tub-valve in Fig. 3 is lowered, so as to form the chamber or cavity, at the same time that the motion of the agitator is slackened, such heavy material will be deposited in the said chamber or cavity, and may be drawn off with a little of the sand, earth, and water accompanying it, into the receiving tube E, by lowering the said piston below the holes described atfffin the figure; but if the discharge should be followed by too much sand, earth, and water, it may instantly be stopped by raising the piston above the holes.
Should the ore or other material not be sufficiently heavy to deposit itself in the centre of the tub, then the stopper valve, shown in Fig. 5, is to be used in preference, which is not to be opened until the fluid in the tub has been moved for a minute, and the central hollow cone is formed in the middle, when the stopper may be raised, and the speed of the agitator diminished, until the water begins to flow gently from the valve, when, in running, it will bring the ore, or other heavy materials with it, and must be permitted to run so long as this is the case; the valve is then to be closed, and the agitator again put into rapid motion; after which the valve is to be again opened, and so in succession, until the whole of the ore, or other heavy material, is obtained, which will be known by its ceasing to run from the lower central valve, when the remaining refuse is to be drawn off by opening the valve and spigot S, having previously placed another tub, called the waste-tub, under the machine for the purpose of receiving it; and while so running off, the agitator must be kept in motion to stir it up and wash out the contents of the tub.
When empty, the waste-tub, with its contents, must be removed, and the tub A A must be supplied with a fresh quantity of water and ore, or other heavy material, to resume the operation.
From the foregoing description of this machine in its most simple state, it will appear, that a much less proportionate quantity of motion takes place near the centre of the agitator, than near its outside, particularly when the machine is made on a large scale, on which account it is necessary, in large machines, to construct a double agitator, that is to say, one in which the central part turns or moves with greater velocity than the external part, as shown in section at Fig. 5, where I I I I F F show the agitator constructed as before, except that its arms and stirrers are more extended from the centre, so as to make room for the smaller central agitator iii i which may be constructed in the same way as before described, or may have its stirrers fixed into the circular block of wood or metal jj, and the iron axes, instead of being fixed into the central block G, now passes through it, and is fixed to the small or internal agitator. For this purpose, the central block G of the agitator should be lined with a brass box, or have proper bearings upon its ends, so that it may revolve freely upon the iron spindle H; it has also a bearing at n, in the lower part of the cross frame D D, to assist in supporting it; and on account of the greater weight that now hangs on the said iron spindle H, two friction wheels are fixed to its upper end, as at o o, which run upon the top of the brass bearing p, and materially diminish the friction.