Concentration of grains from 10 to 30 mm. is effected by hydraulic jigs with two compartments, and in the case of the smaller grains down to 2 mm. by jigs with five compartments. The construction of the jigs is the same in both cases. Fig. 3 gives the details of a jig with two compartments; it is formed of three cast-iron plates which support the bearings of the eccentric shaft, joined by a wooden casing or wooden walls so as to form two communicating chambers for pistons and screens. This construction has no special advantage beyond facilitating the transportation and mounting of the jigs. But in some details the Monteponi jig differs greatly from those in general use.
The eccentrics have a variable stroke. A first eccentric fixed to the shaft is surrounded by a moving eccentric; the first has a flange which partly covers the second at the side, and both have holes through which the bolt is passed to hold them together. The holes being at a different distance in the two eccentrics, the combination forms a kind of vernier caliper, which allows variation in the eccentricity.
Eccentrics of three sizes are used: one for strokes up to 20 mm.; a