LAPIDARY 41 



other parallel face. These two pieces are called summers (lintels) ; the ona placed 

 at D is the upper ; the one at F the lower. 



In fig. 1318 this face is shown inside, in order to explain how the mill wheel ia 

 placed and supported. The same letters point out the same objects, both in the pre- 

 ceding 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. 1318, 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 centres 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. 1320 below). 



Each wheel is composed of an iron arbor H, fig. 1318, 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. 1320), which has a square fit upon the arbor. 

 The arbor carries a collet d, on which are four 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. 1320, a washer or ferrule of wrought iron is put over it, and secured in its place 

 by a double wedge. In fig. 1320 the wheel-plate is represented in section, that the 

 connection of the whole parts may be seen. 



1319 



1320 



1321 



A board g (see fig. 1316 an&fiff. 1324) about 7 inches high, is fixed to the part of 

 the frame opposite to the side at which the lapidary works, and it prevents the sub- 

 stances made use of in the cutting and polishing from being thrown to a distance by the 

 centrifugal force of the wheel-plate. 



Behind the apparatus is mounted for each grinding-plate, a large wheel i. (see 

 fig. 1316), similar to a cutler's, but placed horizontally. The 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 lly-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 i, and the pulley J. Each whee. L, 

 is mounted on an iron arbor, with a crank (see M,fig. 1317). 



The lower pivot of the arbor h is conical, and turns in a socket fixed in the floor. 

 The great wheel t rests on the collet i, furnished with its 4 iron pins, for securing the 

 connection. Above the wheel an iron washer is laid, and the whole is fixed by a double 

 wedge, which enters into the mortise I, fig. 1317. 



Fig. 1321 exhibits a ground-plan view of all this assemblage of parts, to explain the 

 structure of the machine. Everything that stands above the upper summer-bar has 

 been suppressed in this representation. Here we see the table c c ; the upper summer 

 m ; the one-wheel plate I, the other having been removed to show that the endless cord 

 does not cross ; the two large wheels L L, present in each machine, the crank bar N, 



