VIEW OF MACHINE-SHOP.
Before any practical work upon the construction of a bridge is begun the data and specifications are made, and a plan of the structure is drawn, whether it is for a railroad or for ordinary travel, whether for a double or single track, whether the train is to pass on top or below, and so on. The calculations and plans are then made for the use of such dimensions of iron that the strain upon any part of the structure shall not exceed a certain maximum, usually fixed at ten thousand pounds to the square inch. As the weight of the iron is known, and its tensile strength is estimated at sixty thousand pounds per square inch, this estimate, which is technically called "a factor of safety" of six, is a very safe one. In other words, the bridge is planned and so constructed that in supporting its own weight, together with any load of locomotives or cars which can be placed upon it, it shall not be subjected to a strain over one-sixth of its estimated strength.
NEW RIVER BRIDGE ON ITS STAGING.
After the plan is made, working drawings are prepared and the process of manufacture commences. The eye-bars, when made, are tested in a testing-machine at double the strain which by any possibility they can be put to in the bridge itself. The elasticity of the iron is such that after being submitted to a tension of about thirty thousand pounds to the square inch it will return to its original dimensions; while it is so tough that the bars, as large as two inches in diameter, can be bent double, when cold, without showing any signs of fracture. Having stood these tests, the parts of the bridge are considered fit to be used.
BRIDGE AT ALBANY.
When completed the parts are put together—or "assembled," as the technical phrase is—in order to see that they are right in length, etc. Then they are marked with letters or numbers, according to the working plan, and shipped to the spot where the bridge is to be permanently erected. Before the erection can be begun, however, a staging or scaffolding of wood, strong enough to support the iron structure until it is finished, has to be raised on the spot. When the bridge is a large one this staging is of necessity an important and costly structure. An illustration on another page shows the staging erected for the support of the New River bridge in West Virginia, on the line of the Chesapeake and Ohio Railway, near a romantic spot known as Hawksnest. About two hundred yards below this bridge is a waterfall, and while the staging was still in use for its construction, the river, which is very treacherous, suddenly rose about twenty feet in a few hours, and became a roaring torrent.