In making crown-glass, which is used for windows, a slight alteration in the process is observed. When the globe is prepared as before at the end of the tube, it is flattened at its extremity by pressure against a plain surface; the new material at the end of the pontil is then attached to the flattened side, and the whole mass detached from the tube, leaving a circular hole at the point of separation. The mass is now twirled round and round, at first slowly, then more quickly, till its diameter, obeying the centrifugal force, becomes wider and wider, the hole expanding in proportion. At last, as the motion increases in velocity, the double portion suddenly bursts open, the whole forming a plain disc of uniform density throughout, except at the spot in the centre where the pontil is attached to it, and where there is accumulated that small lump which is vulgarly called a _bull's eye_. The most surprising incident in this process is the bursting open of the flattened globe, a circumstance which would shiver the entire mass if it were not kept up at a certain heat. The mode of casting plate-glass presents a remarkable illustration of the skillful adaptation of means to ends. When the glass is melted in the crucible, a portion of it is transferred to a smaller crucible, called a cuvette, which contains the exact quantity requisite for the size of the plate about to be formed. The cuvette is then raised by means of a crane, and lifted over the casting table. These tables have smooth metallic surfaces which are carefully ground and polished, and wiped perfectly clean, and heated before they are used. Formerly they were made of copper, but the British Plate Glass Company have found that iron slabs answer the purpose better. The table used by them is fifteen feet long, nine feet wide, and six inches thick, and weighs fourteen tons. For the convenience of moving it to the annealing ovens it is placed upon castors. The cuvette being swung over the casting table, is gradually turned over, and a flood of molten glass is poured out upon the surface, and prevented from running off by ribs of metal. As soon as it is entirely discharged, a large hollow copper cylinder is rolled over the fluid, spreading it into a sheet of equal breadth and thickness. When the glass is sufficiently cool to bear removal it is slipped into the annealing oven, where it is placed in a horizontal position,[14] great care having been taken to exclude the external air, it being indispensable to the beauty of these plates that the process of cooling should be regular and gradual. [Footnote 14: In this respect plate-glass is treated differently from crown and broad glass, which is always placed on its edge in the annealing furnace.]