The process of grinding and polishing is still regarded in many plate-glass works as consisting of three distinct processes, known as rough grinding, smoothing and polishing respectively. Formerly these three stages of the process were carried out separately; at first by hand, and later by three different machines. In the most modern practice, however, the rough and smooth grinding are done on the same machine, the only change required being the substitution of a finer grade of abrasive at each step for the coarser grade used in the previous stage. For the polishing process, however, the rubbing implements themselves must be of a different kind, for while the grinding and smoothing is generally done by means of cast-iron rubbers moving over the glass, the polishing is done with felt pads. The table of the machine, to which the glass under treatment is attached, is therefore made movable, and when the grinding and smoothing processes are complete, the table with its attached glass is moved so as to come beneath a superstructure carrying the polishing rubbers, and the whole is then elevated so as to allow the rubbers to bear on the glass. The earliest forms of grinding machines gave a reciprocal motion to the table which carries the glass, or the grinding rubbers were moved backward and forward over the stationary table. Rotary machines, however, were introduced and rapidly asserted their superiority, until, at the present time, practically all plate-glass is ground on rotating tables, some of these attaining a diameter of over 30 ft. The grinding “rubbers” consist of heavy iron slabs, or of wood boxes shod with iron, but of much smaller diameter than the grinding table. The rubbers themselves are rotary, being caused to rotate either by the frictional drive of the rotating table below them, or by the action of independent driving mechanism, but the design of the motions must be so arranged that the relative motion of rubber and glass shall be approximately the same at all parts of the glass sheets, otherwise curved instead of plane surfaces would be formed. This condition can be met by placing the axes of the rubbers at suitable points on the diameter of the table. The abrasive is fed on to the glass in the form of a thin paste, and when each grade or “course” has done the work required of it, the whole table is washed down thoroughly with water and then the next finer grade is applied. The function of the first or coarsest grade is simply to remove the surface irregularities and to form a rough but plane surface. The abrasive ordinarily employed is sharp sand, but only comparatively light pressure can be applied, especially at the beginning of this stage, since at that period the weight of the rubber is at times borne by relatively small areas of glass that project here and there above the general level of the slab. As these are ground away, the rubbers take a larger and more uniform bearing, and greater pressure can be applied.