company sold out to the
Saint Louis company. Had the truss bridge been built, there is no
knowing how long it might have stood, for the engineer who designed it
did not arrange to base the foundations on the bed-rock of the river.
Afterwards it was shown how necessary it was to do this; but at the
time many people thought it quite superfluous, and on that, as well as
on many other points, Eads met with opposition.

In every case it turned out that he had been right. No one else knew so
well as he the immense power and the waywardness of the Mississippi.
Good engineers supposed that the greatest imaginable scour at the river
bottom in extreme high water would not remove over twenty-two feet of
sand, and it was believed that there were perhaps one hundred feet of
it along the east shore. But Eads had been sixty-five feet below the
river's surface at Cairo, and there he had found the river bottom to be
a moving mass at least three feet deep; and in cutting through the
frozen river to liberate his diving-bell boats, he had found that the
floating ice which goes underneath solid ice, as well as the rising or
"backing-up" of the water above ice-gorges, forces the undercurrents
lower than even a flood does; and he had found on cutting a wreck out
of the ice that she had been held up by the gorged ice underneath her,
which must therefore have been packed to the bottom. Knowing all this
and much more about what goes on under the turbid surface of the river,
he did not doubt that even beneath 100 feet of sand the bed-rock might
at times be laid bare, and he was absolutely convinced that his bridge
must be founded on it.

Moreover, he saw that on account of the exceptional force of the
current in its rather narrow bed at Saint Louis, the masonry piers of
his bridge must be made unusually big and strong to withstand it. Since
they must be so big and sunk so very deep, it was evident that they
would be so costly that the fewer there need be of them the better. The
central span was required to be 500 feet; with three spans about that
length the river could be crossed, and three spans would require only
four piers. Steel trusses 500 feet long would have to be made extremely
heavy; but Eads showed that a steel arch the same length, while quite
as strong, would be lighter and consequently much cheaper. When his
opponents objected that there was no engineering precedent for such
spans, while he pointed out their mistake, at the same tim