Under such conditions a cast iron column open at the bottom
and constructed of complete rings (or, if large, of rings made in
segments) with watertight joints, is let down upon the ground,
while the top of it is kept above water by constant additions to
its height as it sinks. This process is carried on until the
edge at the bottom reaches a stratum of sufficient firmness to
serve as a foundation. The inside of the column is then



32 Works in Iron.
cleared out, and may be filled with concrete to stiffen the pier.
These cylinders are generally made of cast iron, and their di-
mensions and shape depend on the firmness of the foundation
ground, on the weight to be carried, and on the height above
the river bed. A column, for instance, to support a load
of 400 tons (including its own weight), resting on a stratum of






stiff clay, and about 30 feet above the bed of the river, might
have a diameter of 10 feet at the base, but would properly be
contracted above the river bed, in the manner shown in Fig. i.
But if the height is much greater than 30 feet, more stability
would be required in the shaft of the column, and the proper
shape would be a column of 10 feet diameter (Fig. 2), without
the conical reducing piece shown in the sketch. A parallel
column would be suitable also if the foundation ground were
superior to stiff clay, for instance, rock; an enlarged base
would not in such a case be necessary (see Fig. 3, also
Bridge Example No. 30). Where the superstructure rests on
the iron pier, proper flanges or caps should be given to the
cylinder, to afford sufficient bearing surface for the girders.
