The subsequent courses of finer abrasives are only required to remove the coarse pittings left in the surface by the action of the first rough grinding sand; the finer abrasive replaces the deep pits of the former grade by shallower pits, and this is carried on in a number of steps until a very smooth “grey” surface is attained and the smoothing process is complete. The revolving table or “platform” is now detached from the driving mechanism, and moved along suitably placed rails on wheels provided for that purpose, until it stands below the polishing mechanism. Here it is attached to a fresh driving mechanism, and it is then either raised so as to bring the glass into contact with the felt-covered polishing rubbers, or the latter are lowered down upon the glass. The polishing rubbers are large felt-covered slabs of wood or iron which are pressed against the glass with considerable force; their movement is very similar to that of the grinding rubbers, but in place of an abrasive they are supplied with a thin paste of rouge and water. The time required for the polishing process depends upon the perfection of the smoothing that has been attained; in favourable cases two or three hours are sufficient to convert the “grey” surface into a perfectly polished one; where, however, somewhat deeper pits have been left in the glass, the time required for polishing may be much longer, and the polish attained will not be so perfect. The mode of action of a polishing medium such as rouge is now recognised to be totally different in character from that of even the finest abrasive; the grains of the abrasive act by their hardness and the sharpness of their edges, chipping away tiny particles of the glass, so that the glass steadily loses weight during the grinding and smoothing processes. During the polishing process, however, there is little or no further loss of weight, the glass forming the hills or highest parts of the minutely pitted surface being dragged or smeared over the surface in such a way as to gradually fill up the pits and hollows. The part played by the polishing medium is probably partly chemical and partly physical, but it results, together with the pressure of the rubber, in giving to the surface molecules of the glass a certain amount of freedom of movement, similar to that of the molecules of a viscid liquid; the surface layers of glass are thus enabled to “flow” under the action of the polisher and to smooth out the surface to the beautiful level smoothness which is so characteristic of the surfaces of liquids at rest. This explanation of the polishing process enables us to understand why the proper consistency of the polishing paste, as well as the proper adjustment of the speed and pressure of the rubbers, plays such an important part in successful polishing; it also serves to explain the well-known fact that rapid polishing only takes place when the glass surface has begun to be perceptibly heated by the friction spent upon it.