THE KANSAS CITY BRIDGE. 37

section was 13 feet high, and as soon as this was completed it was lowered
between the boats almost to the surface of the water, and a second section, 11.
feet high, added to it. The lower edge was shaped to fit the irregularities of
the rock, and the caisson was surrounded by a sheet piling of planks sharpened
to a feather edge and secured by a double set of guides. The caisson was.
thoroughly caulked, braced internally, aud fitted with a false bottom; this
bottom, which was put in to facilitate handling, was built in sections, placed a
little above the lower edge, and held in position by inclined braces bearing
against the caisson timbers and adjusted with folding wedges. To aid in placing
the caisson upon the rock, eight posts were provided, each 60 feet long and
16 inches square, built of 8 by 16 oak timber of shorter lengths, with a central
hole three inches in diameter extending from end to end, through which a two-
inch steel drill, welded to an iron rod 65 feet long, might be worked.

When complete, the caisson, still carried by the boats, was carefully floated |
into position, and four of the 60-foot posts with the drills, raised on either side
of it. The posts sank by their own weight through the thin layer of sand, and
were at once made fast by working the drills two feet into the rock ; they were
then well braced together and secured to the caisson by sets of rollers and shoes,
the two sets of posts being placed about an inch nearer together at the top than
bottom, to secure clearance in lowering the caisson. Four pairs of cross
timbers, attached to the posts, were placed above the caisson, each of them
carrying two suspension screws 20 feet long and two inches in diameter,
with a thread cut from end to end. The total weight of the caisson being
72 tons, each screw was required to carry nine tons. On the 6th of Sep-
tember every thing was in readiness, the caisson had been attached to the
screws and the lowering was begun. Three men at each screw were required
to handle the weight. Hase and regularity of descent were secured by
admitting water above the false bottom; when one half of the caisson had
become submerged this was found to be no longer necessary, and the bottom
was set free by striking the folding wedges which held the braces, and taken
out in parts. On the 11th of the same month the caisson came to a bearing
upon the sand, and the screws were removed. The use of long posts secured by
drills rendered the matter of false works exceedingly simple ; this device, which