While it is evident that careful design of the rolling table will avoid all tendency to the formation of sheets of such undesirable form, it is a much more difficult matter to avoid all distortion of the sheet during the annealing process and while the sheet is being moved from the rolling table to the annealing kiln. Owing to the great size of the slabs of glass to be dealt with, and still more to the stringent requirement of flatness, the continuous annealing kiln, in which the glass travels slowly down a tunnel from the hot to the cold end, has not been adopted for the annealing of plate-glass, and a form of annealing kiln is still used for that glass which is similar in its mode of operation to the old-fashioned kilns that were used for other kinds of glass before the continuous kiln was introduced. These kilns simply consist of chambers in which the hot glass is sealed up and allowed to cool slowly and uniformly during a more or less protracted period. In the case of plate-glass, the slabs are laid flat on the stone bed of the kiln. This stone bed is built up of carefully dressed stone, or blocks of fire-brick bedded in sand in such a way that they can expand freely laterally without causing any tendency for the floor to buckle upwards as it would do if the blocks were set firmly against one another. The whole chamber is previously heated to the requisite temperature at which the glass still shows a very slight plasticity. The hot glass slabs from the rolling table are laid upon the bed of this kiln, several being usually placed side by side in the one chamber, and the slabs in the course of the first few hours settle down to the contour of the bed of the kiln, from which shape and position they are never disturbed until they are removed when quite cold. In modern practice the cooling of a kiln is allowed to occupy from four to five days; even this rate of cooling is only permissible if care is taken to provide for the even cooling of all parts of the kiln, and for this purpose special air-passages are built into the walls of the chamber and beneath the bed upon which the glass rests, and air circulation is admitted to these in such a way as to allow the whole of the kiln to cool down at the same rate; in the absence of such special arrangements, the upper parts of the kiln would probably cool much more rapidly than the base, so that the glass would be much warmer on its under than on its upper surface. When the slabs of plate-glass are removed from the annealing kilns they very closely resemble sheets of rolled plate in appearance, and they are quite sufficiently transparent to allow of examination and the rejection of the more grossly defective portions; the more minute defects, of course, can only be detected after the sheets have been polished, but this preliminary examination saves the laborious polishing of much useless glass.