the river bottom, are perhaps the most common in use. The piles are
sawed off below the surface of the water and a platform built upon them,
which in turn serves as the foundation for the masonry.

The great Eads Bridge, which was built across the Mississippi at St.
Louis, is supported by towers the foundations of which are sunk 107 feet
below the ordinary level of the water; at this depth the men working in
the caissons were subjected to a pressure of nearly fifty pounds to the
square inch, almost equal to that used to run some steam-engines.

The bridge across the Hudson at Poughkeepsie was built on a crib or
caisson open at the top and sunk by means of a dredge operated from
above taking out the material from the inside. The wonder of this is
hard to realise unless it is remembered that the steel hands of the
dredge were worked entirely from above, and the steel rope sinews
reached down below the surface more than one hundred feet sometimes;
yet so cleverly was the work managed that the excavation was perfect all
around, and the crib sank absolutely straight and square.

It is the fourth department of bridge-building that requires the
greatest amount not only of knowledge but of resourcefulness. In the
final process of erection conditions are likely to arise that were not
considered when the plans were drawn.

The chief engineer in charge of the erection of a bridge far from
civilisation is a little king, for it is necessary for him to have the
power of an absolute monarch over his army of workmen, which is often
composed of many different races.

With so many thousand tons of steel and stone dumped on the ground at