Page:Popular Science Monthly Volume 34.djvu/628

 is accomplished is one of the triumphs of modern glass-making. In this department particularly, the merits of natural gas are each day more apparent. The hearth of the laying-in furnace is circular, and is divided into a number of sectors by fire-clay bridges. It is made movable about a vertical axis. As it rotates, the different sectors of the hearth pass through as many separate compartments of the furnace, the temperature of which may be varied at pleasure. The first compartment, the laying-in oven, is only moderately warm, and permits the glass cylinder to become gradually heated. It is then carried by a partial rotation of the hearth into the next compartment, the laying-out oven, where the heat is sufficient to make the glass plastic. A large, flat stone occupies each hearth-section and forms the ironing-board upon which the cylinders of glass are to be smoothed. In the layingout oven, the crack in the cylinder is brought uppermost. Under the influence of the heat the glass gradually unfolds, until it lies open on the stone like a sheet of rumpled paper. In the succeeding compartment—the flattening oven—the plastic sheet is made perfectly smooth and flat by means of a moist block of wood on the end of a long iron rod. The cylinder has now disappeared, and in its place there is a pane of red-hot glass. One more turn of the hearth carries the glass into the compartment known as the dumb oven, where it gradually cools. It is then brought, by a final movement of the hearth, to the entrance of the annealing leer. One whole rotation has now been accomplished, and the circuit is complete. Meanwhile other cylinders have been put into the furnace and are in different stages of the flattening process.

The operation of the furnace is continuous, and speedily transforms the cylinder into a smooth sheet. But still it is not ready for use. Were the glass removed to the air immediately, it would be much too brittle for service. It must first go through the process of annealing, or gradual cooling, before it can possess any durability. In the improved "rod leer" the hot glass is received at one end of a long brick chamber, and in thirty or forty minutes it is automatically discharged at the other end, nearly or quite cold. Where gas is used, the glass, just as it comes from the leer, is beautifully clear and brilliant. It could scarcely be more so had it been washed with hot water and dried with linen—the process, we believe, by which madam, our hostess, secures such a glittering display on her table. The sheets are at once cut into proper sizes and stored away in suitable wooden frames. The process of manufacture is completed, and only the service of the glazier is needed to put the pane in place, and so inaugurate its luminous mission.

Strange fancies attend the visitor as he wanders through the silent warehouse. He loses himself amid the possible pictures