Over the Niagara gorge also was built one of the first cantilever bridges ever constructed.  To uphold it, two towers were built close to the water’s edge on either side, and then from the towers to the shores, on a level with the upper plateau, the steel fabric, composed of slender rods and beams braced to stand the great weight it would have to carry, was built on false work and secured to solid anchorages on shore.  Then on this, over tracks laid for the purpose, a crane was run (the same process being carried out on both sides of the river simultaneously), and so the span was built over the water 239 feet above the seething stream, the shore ends balancing the outer sections until the two arms met and were joined exactly in the middle.  This bridge required but eight months to build, and was finished in 1883.  From the car windows hardly any part of the slender structure can be seen, and the train seems to be held over the foaming torrent by some invisible support, yet hundreds of trains have passed over it, the winds of many storms have torn at its members, heat and cold have tried by expansion and contraction to rend it apart, yet the bridge is as strong as ever.

Sometimes bridges are built a span or section at a time and placed on great barges, raised to just their proper height, and floated down to the piers and there secured.

A railroad bridge across the Schuylkill at Philadelphia was judged inadequate for the work it had to do, and it was deemed necessary to replace it with a new one.  The towers it rested upon, therefore, were widened, and another, stronger bridge was built alongside, the new one put upon rollers as was the old, and then between trains the old structure was pushed to one side, still resting on the widened piers, and the new bridge was pushed into its place, the whole operation occupying less than three minutes.  The new replaced the old between the passing of trains that run at four or five-minute intervals.  The Eads Bridge, which crosses the Mississippi at St. Louis, was built on a novel plan.  Its deep foundations have already been mentioned.  The great “Father of Waters” is notoriously fickle; its channel is continually changing, the current is swift, and the frequent floods fill up and scour out new channels constantly.  It was necessary, therefore, in order to span the great stream, to place as few towers as possible and build entirely from above or from the towers themselves.  It was a bold idea, and many predicted its failure, but Captain Eads, the great engineer, had the courage of his convictions and carried out his plans successfully.  From each tower a steel arch was started on each side, built of steel tubes braced securely; the building on each side of every tower was carried on simultaneously, one side of every arch balancing the weight on the other side.  Each section was like a gigantic seesaw, the tower acting as the centre support; the ends, of course, not swinging up and down.  Gradually the