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 two sections of every arch
approached each other until they met over the turbid water and were
permanently connected. With the completion of the three arches, built
entirely from the piers supporting them, the great stream was spanned.
The Eads Bridge was practically a double series of cantilevers balancing
on the towers. Three arches were built, the longest being 520 feet long
and the two shorter ones 502 feet each.

Every situation that confronts the bridge builder requires different
handling; at one time he may be called upon to construct a bridge
alongside of a narrow, rocky cleft over a rushing stream like the Royal
Gorge, Colorado, where the track is hung from two great beams stretched
across the chasm, or he may be required to design and construct a