It consists of a strong wooden axle A (fig. 320) moving on a spindle in a beam B above it, and resting on an iron pivot beneath, turning in an iron bearing, which is inserted into a post of wood C, which rises about a foot above the ground in the centre of the arrastre. The shaft Ais crossed at right angles by two strong spars D D, which form four arms, each about 5 feet long, one excepted, which is 9 feet long, to admit of two mules being attached to it; by this arm the machine is worked. The grinding is performed by four large porphyritic or basaltic stones, two of which are shown, E E. These are loosely attached by thongs of leather, or small-sized rope, to the four arms, and are dragged round over the ore, which is put in with water, until it is ground to a very fine slime or mud, called the lama. One of these machines, wlien in good working condition, will grind from 600 to 800 pounds weight of ore in twenty-four hours. In Guanaxuato, where the best and finest grinding is obtained in the arrastres, the lining or foundation and the grinding stones arc, of course, grained porphyry and form a rough surface. The cost of this apparatus in Mexico, including the paving of the bottom, and the four metapiles or stones, is on an average £7. The original weight of a metapile is about 700 pounds, its dimensions are 2 feet 8 inches long, 18 inches broad, and 18 inches deep. Notwithstanding the hardness of the stones employed, they are so worn as to become unserviceable in the course of ten or twelve weeks ; the bottom, however, is only replaced once in twelve months.