18 GLASS above his head, still keeping it well filled with air. This operation is skilfully continued until a cylinder is produced about 11 in. in diameter and about 50 in. long, closed at one end and attached to the blowpipe at the other. The next step is to open the end of the cylinder, which the workman does by filling it with air and, after closing the aperture of the pipe with his thumb, exposing the end to the heat of the furnace. The heat expands the air in the cyl- inder, which bursts open at the end where the glass is the softest. The aperture thus made is widened to the required diameter by rapid- ly revolving the cylinder at the furnace hole, the pipe resting on an iron support, and subse- quently holding it in a vertical position with the open end downward until the glass is cooled sufficiently to retain its shape. The cylinder is now laid upon a wooden rest, or trestle, and detached from the pipe by touching with a piece of cold iron the pear-shaped neck near the nose of the pipe, and gently striking the pipe ; an opening about three inches in diam- eter is thus formed. This end, the cap of the FIG. 14.- The Cylinder in Different Stages of Manufacture. cylinder, is now taken off by winding around it a thread of hot glass, and after removing it applying a piece of cold iron to any point which the thread covered. After trimming the other end by cutting off about two inches in length with a diamond, the cylinder is split open longi- tudinally by drawing along its inside surface a diamond attached to a long handle and guided by a wooden rule. Formerly this splitting was done with a red-hot iron, which is still some- times used. The cylinder is now taken to the flattening oven, where it is placed, with the slit uppermost, upon the flattening stone, from the irregularities of whose surface it is protect- ed by a sheet of glass. The cylinder soon be- comes heated and opens out into a wavy sheet, the movement being accelerated by the iron rod of the workman. The surface of the sheet is next rubbed with a piece of .wood attached to the end of an iron rod for the purpose of re- moving the irregularities of the surface. The flattening stone is now moved on wheels to the adjoining annealing oven, where the sheets are placed for annealing, which usually requires from 24 to 36 hours. From the annealing oven the sheets are taken to the warehouse, where they are smoothed, polished, assorted, and cut into panes of the required dimensions. The former method of grinding and polishing sheet glass by imbedding the sheets in plaster of Paris proved inadequate to remove the de- fects in the glass consequent upon the mode of manufacture. The chief of these was the undulating or wavy appearance of the surface, called cockles, which was attributed to the dif- ference of diameter between the inner and out- er surfaces of the cylinder, and which caused objects seen through the glass to be distorted. Notwithstanding the glass was made very thick after the superficial roughness was removed, the result was a thin sheet much inferior to plate glass. The ingenious process devised by Mr. James Chance for producing patent plate glass, which is now used in England and most factories on the continent, is one of the most important improvements in the manufacture. By removing the thin outer surface of the glass by this method, an evenness and a polish are secured, even on the thinnest sheet, which make it in many respects equal to plate glass, and far superior to the sheet glass produced by the old process. The improved method consists in placing the sheet to be ground and polished upon a flat surface covered with a piece of damp soft leather or cotton cloth. A slight pressure applied to the glass causes it to adhere to the surface of cotton or leather, and by thus producing a vacuum the entire sheet is firmly maintained in a flat position by atmos- pheric pressure. The exposed surfaces of two sheets fixed in this manner are rubbed against each other in a horizontal position by machine- ry, emery and water being constantly supplied to keep up the friction. Both sides of the sheet are polished in this manner, with only a slight diminution of the thickness of the glass. After the removal of the sheets from these surfaces, they resume by their own elasticity their original shape, which is often more or less curved. The final polish is given to the sheets by a process similar to that used in polishing plate glass. In each process through which the glass has passed, it was exposed to some imperfection, and some of the sheets bear the peculiar defects of them all and are of little value ; others are suitable for inferior uses, and but few are perfect. The wide difference be- tween the quality of the best nnd the worst sheets is indicated by the fact that the former are valued at three times more than the latter. The same kind of material is used in the pro- duction of both crown and sheet glass. The remarkable brilliancy of surface of the former gives to it a certain advantage over sheet glass; but the larger size easily attained in making the latter gives it the supremacy in commerce. Of crown glass it is difficult to obtain panes of 34 x 22 in., while the usual size of the sheets of cylinder glass is 47x32 in., and
