In [fig. 617.] this face is shewn inside, in order to explain how the mill wheel is placed and supported. The same letters point out the same objects, both in the preceding and the following figures.
In each of these summers a square hole is cut out, exactly opposite to the other; in which are adjusted by friction, a square piece of oak a a, [fig. 617.], whose extremities are perforated with a conical hole, which receives the two ends of the arbor H of the wheel I, and forms its socket. This square bar is adjusted at a convenient height, by a double wooden wedge b b.
The cross bar in the middle E supports the table c c, a strong plank of oak. It is pierced with two large holes whose centres coincide with the centre of the conical holes hollowed out at the end of the square pins. These holes, of about 6 inches diameter each, are intended to let the arbor pass freely through, bearing its respective wheel. (See one of these holes at I, in [fig. 621.] below.)
Each wheel is composed of an iron arbor H, [fig. 618.], of a grinding-wheel I, which differs in substance according to circumstances, as already stated, and of the pulley J, furnished with several grooves (see [fig. 619.]), which has a square fit upon the arbor. The arbor carries a collet d, on which are 4 iron pegs or pins that enter into the wheel to fasten it.
The wheel plate, of which the ground plan is shown at K, is hollowed out towards its centre to half its thickness; when it is in its position on the arbor, as indicated in [fig. 619.], a washer or ferrule of wrought iron is put over it, and secured in its place by a double wedge. In [fig. 619.] the wheel-plate is represented in section, that the connection of the whole parts may be seen.
A board g (see [fig. 616.] and [fig. 624.]), about 71⁄2 inches high, is fixed to the part of the frame opposite to the side at which the lapidary works, and it prevents the substances made use of in the cutting and polishing, from being thrown to a distance by the centrifugal force of the wheel-plate.
Behind this apparatus is mounted for each grinding-plate, a large wheel L (see [fig. 616.]), similar to a cutler’s, but placed horizontally. This wheel is grooved round its circumference to receive an endless cord or band, which passes round one of the grooves of the pulley J, fixed below the wheel-plate. Hence, on turning the fly-wheel L, the plate revolves with a velocity relative to the velocity communicated to the wheel L, and to the difference of diameter of the wheel L and the pulley J. Each wheel L, is mounted on an iron arbor, with a crank (see M, [fig. 620.])