LAPIDARY 



43 



1328 



On making it slope a little; 5 degrees, for example, all the facets "will now lie in the 

 same zone provided that the inclination be not allowed to vary. On turning round tho 

 cement-rod and the index g marks the divisions so that by operating on the circle with 

 16 divisions, stopping for sometime at each, 16 facets will have been formed, of perfect 

 equality, and at equal distances, as soon as the revolution is completed. 



In cutting the stones, they are mounted on the cement-rod s, fig. 1326, whose stem 

 is set upright in a socket placed in a middle of a sole piece at A, which receives the 

 stem of the cement-rod. The head of the rod fills the cup of A. A melted alloy of 

 tin and lead is poured into the head of the cement-rod, in the middle of which the 

 stone is immediately plunged ; and wherever the solder has become solid, a portion 

 of it is pared off from the top of the diamond, to give the pyramidal form shown in the 

 figure at u. 



There is an instrument employed by the steel polishers for pieces of clock-work, and 

 by the manufacturers of watch-glasses for polishing their edges. It consists of a 

 solid oaken table, fig. 1327. The top is perforated with two holes, one for passing 

 through the pulley and the arbor of the wheel -plate B, made either of lead or of hard 

 wood, according to circumstances ; and the other c for receiving the upper part of the 

 arbor of the large pulley D. The upper pulley of the wheel-plate is supported by an 

 iron prop B, fixed to the table by two wooden screws. The inferior pivots of the two 

 pieces are supported by screw sockets, working in an iron "screw-nut sunk into the 

 summer-bar F. The legs of the table are made longer or shorter, according as the 

 workman chooses to stand or sit at his employment. Emery with oil is used for 

 grinding down, and tin-putty or colcothar for polishing. The workman lays the piece 

 on the flat of the wheel-plate with one hand, and presses it down with a lump of cork, 

 while he turns round the handle with the other hand. 



A very convenient form of apparatus has been devised by Mr. James B. Jordan, 

 and manufactured by Messrs. Cotton and Johnson, of Grafton Street, Soho, for the 

 purpose of preparing thin sections of 

 minerals, rocks, and other hard sub- 

 stance for microscopical observation. 

 This machine is represented in figs. 

 1328, 1329. and 1330. It consists of 

 a wooden frame-work, a a, support- 

 ing a crank-axle and driving-wheel, 

 two feet diameter ; the top part of 

 this frame consists of two cross- 

 pieces a', fixed about an inch apart, 

 as in the bed of an ordinary turning- 

 lathe ; into the slot between them is 

 placed a casting B, carrying the 

 bracket for the angle-pulleys c ; this 

 casting is bored to receive the spindle 

 D, which, by means of the treadle, 

 is made to revolve at the rate of 400 

 or 500 revolutions per minute. It 

 is also bored to receive another 

 spindle E, to the top of which is 

 fixed a metal plate r, for carrying 

 the small cup H, to which the speci- 

 men is attached by means of pre- 

 pared wax. This means of mecha- 

 nically applying the work to, the 

 slicer is far preferable to holding it 

 in the hand in the ordinary way ; 

 the requisite pressure against the 

 cutting disc is regulated by the 

 weight o, and the thickness of the 

 slice by the thumb-screw K, on which 

 the spindle rests. By this means it 

 is possible to cut tolerably thin and 

 parallel slices, of from ^th to th 

 of an inch in thickness ; the thin- 

 ness of course varying according to 

 the strength of the rock which is 

 being operated upon. The slitting 

 disc is made of soft iron, eight inches diameter, and about ^gth of an inch in thickness, 

 and it is fixed on the spindle D, between two brass plates 4 inches diameter, charged 

 with diamond-powder in the usual way. 



