The bridge problem is always different, it constantly has the fascination of variety. That variety will come into play at unexpected turns. Once, down in a deep Colorado cañon, whose walls rose precipitously for a thousand-odd feet, and which was all but filled by a deep and rapid river, the engineers of the Rio Grande & Western found absolutely no ledge whatsoever upon which they might rest their rails. They puzzled upon the problem for a little while, and then they swung a girder bridge parallel with the river. The bridge was supported by braced girders, that fastened their feet in the walls of the cañon, hardly wider there than a narrow city house. The railroad has been running over that construction for more than thirty years; it is one of the scenic wonders of the land, and a triumph for the engineer that built it. In constructing the expensive West Shore Railroad up the Hudson River, similar difficulties were experienced south of West Point, and truss bridges were built parallel with the steep river banks to carry the tracks from ledge to ledge. It is not an unusual matter for the construction engineer to spend a quarter of a million dollars to span some deep, waterless gully in the mountains, which could not be filled for more than twice that sum. Many times, in these days of increasing weight of equipment, it becomes necessary to replace a bridge, without interrupting the traffic. The construction engineer never fails to meet the problem. Years ago, he took Roebling's famous suspension bridge at Niagara Falls, removed the stone towers and replaced them with towers of steel, without delaying a single train; and a little later he took that bridge itself, and substituted a heavy cantilever for it, while all the time a heavy traffic poured itself over the structure. The rebuilder of bridges works like the original builder--with plentiful falsework. He timbers in and around his structure, and then step by step and with exceeding caution removes the old and substitutes the new. An old girder is taken out between trains; before another train of cars shall roll over the structure a new one is ready, temporarily bolted until the riveters can make it fast. It sounds complicated, but it is remarkably simple, under the careful plans of a patient engineer, who has that infinite thing that we call genius.