Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/509

Rh MINT 487 Hollinp melted and chlorine gas is passed through the molten mass in the manner described in GOLD, vol. x. p. 750. The engine-room (shown in fig. 1) contains three 60-horse- povcr vertical condensing engines, which are provided with Corlis valves, and are specially devised for meeting the constantly vary ing strain to which they are subjected by the machinery, the whole of which they are capable of driving. The central engine acts directly on either or both of the rolling rooms placed on each side of the engine-house. There is, however, an additional 20-horse- power compound beam engine usually employed, in connexion with the pumps of a deep artesian well. Into one or other of these rooms the bars which have been cast in the melting-house are brought, and are rolled into strips the thickness of which depends on the kind of coins to be produced. Gold is rolled in one room and silver or bronze in the other. The details of manipulation involved in the conversion of gold, silver, or bronze bars into coin, however, do not differ materially, and the coinage of sovereigns will therefore be taken as typical. Each room contains six pairs of rolls, the diameter of the rolls varying from 10 to 14 inches. Smaller diameters are employed in most European mints, but on the other hand the use of very narrow rolls of far larger diameter has often been suggested, and there appears to be good ground for the belief that the rigidity of such rolls would enable strips or fillets of more uniform thickness to be FIG. 2. Furnace Apparatus. produced than is the case at present. The iron frame CO (fig. 3) is firmly bolted to the stone D, which rests on a solid foundation EE. This frame supports the two rolls A, B, the lower of which B revolves, but is not, like the upper, capable of adjustment in aver- FIQ. 3. Rolls. tical plane. The upper roll is centred in bearings, and may be raised or lowered by means of screws connected with toothed wheels F, F, which are turned by a handle G, both wheels being moved simulta neously by worms on the rod H. The bearings of the upper roll are -connected by vertical rods with weights below the level of the floor ; and, as it rises with the screws, it can thus be readily adjusted in a line exactly parallel with the lower roll, at a sufficient distance from it to admit the bar which is to be reduced to a strip or fillet. The rolls are turned by the shaft NN, the main wheel M, and the gearing K, L, 0, P. The sockets r by which the upper roll is connected with the gearing by the shaft I arc not rigid, as is the case with the shaft Q of the lower roll, but admit of the adjust ment of the roll. The portion of the roll used is determined by a guide a little wider than the bar. 1 The rolls throughout this department are driven at the rate of about 32 revolutions in a minute. The iron frame CO is braced by rods a, s ; and blocks bearing the driving shafts are shown at k, k, p, p. The initial thickness of a sovereign bar is |ths of an inch. The bars are weighed out to the workmen in batches of about sixty bars, an entire batch being passed through the rolls under precisely the same conditions of adjustment. The bars are only slightly reduced in width by repeated passages through the rolls, but arc successively reduced in thickness in the first stages of the rolling by ^th of an 1 In the second rolling room, shown in the plan on the right of the engine- house, the frames and gearing of the rolls are of newer pattern than those in the first room. In some of the six pairs the bottom rolls revolve and drive the upper ones. In the pair of &quot;breaking-down&quot; rolls in this room, that is, the roll by which the fillets are first treated, the upper roll is stationary, the lower roll alone revolving. The necessary &quot;bite&quot; is given to the fillet, when its end is introduced, by slightly turning the upper roll by means of a ratchet-wheel and lever.