RIVETING A COLUMN.
These rolls, when they are driven by steam, obtain this generally from a boiler placed over the heating-or puddling-furnace, and heated by the waste gases from the furnace. This arrangement was first made by John Griffin, the superintendent of the Phoenix Iron-works, under whose direction the first rolled iron beams over nine inches thick that were ever made were produced at these works. The process of rolling toughens the iron, seeming to draw out its fibres; and iron that has been twice rolled is considered fit for ordinary uses. For the various parts of a bridge, however, where great toughness and tensile strength are necessary, as well as uniformity of texture, the iron is rolled a third time. The bars are therefore cut again into pieces, piled, re-heated and rolled again. A bar of iron which has been rolled twice is formed from a pile of fourteen separate pieces of iron that have been rolled only once, or "muck bar," as it is called; while the thrice-rolled bar is made from a pile of eight separate pieces of double-rolled iron. If, therefore, one of the original pieces of iron has any flaw or defect, it will form only a hundred and twelfth part of the thrice-rolled bar. The uniformity of texture and the toughness of the bars which have been thrice rolled are so great that they may be twisted, cold, into a knot without showing any signs of fracture. The bars of iron, whether hot or cold, are sawn to the various required lengths by the hot or cold saws shown in the illustrations, which revolve with great rapidity.
FURNACE AND HYDRAULIC DIE.
For the columns intended to sustain the compressive thrust of heavy weights a form is used in this establishment of their own design, and to which the name of the "Phoenix column" has been given. They are tubes made from four or from eight sections rolled in the usual way and riveted together at their flanges. When necessary, such columns are joined together by cast-iron joint-blocks, with circular tenons which fit into the hollows of each tube.
To join two bars to resist a strain of tension, links or eye-bars are used from three to six inches wide, and as long as may be needed. At each end is an enlargement with a hole to receive a pin. In this way any number of bars can be joined together, and the result of numerous experiments made at this establishment has shown that under sufficient strain they will part as often in the body of the bar as at the joint. The heads upon these bars are made by a process known as die-forging. The bar is heated to a white heat, and under a die worked by hydraulic pressure the head is shaped and the hole struck at one operation. This method of joining by pins is much more reliable than welding. The pins are made of cold-rolled shafting, and fit to a nicety.
The general view of the machine-shop, which covers more than an acre of ground, shows the various machines and tools by which iron is planed, turned, drilled and handled as though it were one of the softest of materials. Such a machine-shop is one of the wonders of this century. Most of the operations performed there, and all of the tools with which they are done, are due entirely to modern invention, many of them within the last ten years. By means of this application of machines great accuracy of work is obtained, and each part of an iron bridge can be exactly duplicated if necessary. This method of construction is entirely American, the English still building their iron bridges mostly with hand-labor. In consequence also of this method of working, American iron bridges, despite the higher price of our iron, can successfully compete in Canada with bridges of English or Belgian construction. The American iron bridges are lighter than those of other nations, but their absolute strength is as great, since the weight which is saved is all dead weight, and not necessary to the solidity of the structure. The same difference is displayed here that is seen in our carriages with their slender wheels, compared with the lumbering, heavy wagons of European construction.
