The cistern D is taken up in a pair of tongs E, shewn separately in Fig. 16. They open and shut upon the joint e, and are retained by an arch x with pin holes. At one end is a cross handle f, and at the other end two handles g. These tongs are suspended by four chains, from two levers h, h, which, as shewn in Fig. 17, are fitted on the ends of a bar of iron i, and through the centre of this passes a cylindrical iron pin 1, which, at the upper end, has an eye to receive the hook of the crane rope, which takes up the whole, as shewn in Fig. 15. An iron plate k is fixed on the top of the pin, to preserve the rope from burning.
The operation of casting is performed before the mouth of the annealing oven, into which the glass plate is intro duced immediately after casting. These annealing furna ces are zu ranged round the building,' which has the great furnace in the centre, and the table is mounted upon wheels, for removing it from one annealing furnace to another. The crane is also moveable as well as a t•essel F, formed for the reception of the roller 13. when it has travelled over the whole length of the plate. The roller B, which weighs 500 weight, is transported by a carriage on wheels. By these means, the table and all its apparatus can be very quickly prepared for use before any one of the annealing furnaces.
The cistern being brought to the table, Fig. 15, is set down at the side, and the tongs (Fig. 16.) being opened, the cistern is included between the blades, which are shut up and fastened. During this time the surface of the glass is skimmed, to remove any scum which may float at the top. By means of the crane the cistern is now drawn up, and conveyed over the table in the situation of Fig. 15. The two workmen, who govern the cistern by its handles."' and g g, now incline it on the centres of the levers h, h, as shewn in Fig. 17, and pour out a stream of liquid glass up on the table AA. Just before the roller 13, an iron ruler 12, which is to fur m the end of the plate, retains the fluid mat ter from running off at the end of the table ; and the work man, who manages this by its long handle S, withdraws it as the matter flows forwards upon the table, so as to suffer it to extend itself over the whole table a moment after wards. When the glass has completely covered the table, the roller B is advanced with great regulatity and steadi ness. Its great weight presses down and reduces the up per surface of the glass to the level of the side rulers a, a. In this operation, some glass will often be forced over the edge of the table, in which case it is received into troughs K, placed on the ground for that purpose. The roller hav ing passed over the whole length of the table, is guided into the t•essel F at the end. The plate of glass thus cast, is not suffered to remain longer upon the table than to cool itself, so far that it will not warp by its own weight, and it is then shoved off at the end of the table into the annealing furnace, before which the table is placed. This is done by a tool, Fig. 18, which has a very long handle for two or three men to push it forwards, and the cross end is made to receive the end of the plate. The rest of the men guide
the plate sidewise with irons, and assist in placing it pro perly in the annealing oven, which does not require a parti cular description, as it so nearly resembles the oven used by bakers ; but its dimensions must be sufficient to contain the largest glasses, which contiue here baking in a mode rate heat for fully 14 days, the heat being at last suffered to die away as gradually as possible. When quite cool, the plate of glass is withdrawn and carried to the magazine, where it is examined and cut square by a large diamond, fixed in a wooden handle, and attached to a block of wood to hold it in the pi oper position to cut in the same manner as the cutter of a plane. This tool being drawn over the surface of the glass, cuts so far, that very slight blows with a sharp edged hammer on the opposite side of the glass will break it ; and if the fracture is very rough, the irre gularities are reduced by breaking them off with pincers.
The glass plate only requires to be ground and polished, which is performed by bedding the plate with plaster of Paris, upon a table covered with a large slate stone, and laying a smaller plate upon it, which is loaded with weights, and drawn backwards and forwards over the great plate : S:md, plentifully moistened with water, grinds away all the prominences of the glasses, until the surfaces of both plates become plane or even. The upper or moveable glass is defended by cementing it to a strong plank, and upon the back of this the weights are laid, which cause the pressure. To give the workmen hold of the plate, a large • light coach wheel is placed upon a pin, which projects up wards front the centre of the plank, and two or four work men take hold of the rim of the wheel on opposite sides, alternately pushing and pulling it in all directions. As this action is transferred by the wheel to the centre pin, the plate is at liberty to move in any direction ; and the work men must take care to vary this every instant, to prevent the glasses grinding each other into furrows or channels. The table upon which the grinding is performed, is sur rounded by a ledge of about two inches high, to retain the sand and water with which the lower plate is covered. When great nicety is required, the upper plate should he changed for another upper plate which has been ground on a different table ; because two plates may grind themselves to a portion of a sphere, one becoming concave and the other convex ; but by changing the grinding plates, so as to bring two convex or two concave surfaces together, they will correct each other. When the plates are small, the wheel is not used to move the upper one ; but the board to which the upper glass is cemented has four small handles projecting up from it, by which the workmen take hold. NVhen, by the grinding, a perfect surface is obtained, a finer sand is used to produce a smoother surface. A succession of emeries of different degrees of fineness, are used after the fine sand, and with these the operation of grinding is finished.