202
DOCK ENGINEERING.
executed in the open air, by pumping the enclosed water from the interior
of a large, bottomless, metallic chamber, forming a cofferdam. The remainder
of the wall, to its full height of nearly 8 feet above datum, is constructed in
the ordinary way.
The walls are entirely constructed in ordinary rubble masonry, with the
exception of a dressed stone coping and a picked facing down to low water
level.
Five caissons are allocated to the execution of the work under compressed
air. These caissons are movable, and the work is carried out in such a way
as to obtain a continuous block, without any interposition of metal in its
inferior.
lig* 136. —Dock Wall at Marseilles—Section 11.
Four øf these caissons are identical in disposition (figs. 137 and 138).
The interior height of the working chamber is 6 feet 6 inches uniformly,
but the dimensions in plan vary somewhat. The size of the largest chamber
is 66 feet 3 inches by 21 feet 9 inches, the smallest 59 feet by 17 feet
9 inches. Above the working chamber is a compartment having the same
horizontal dimensions, in which is deposited the necessary ballast. This
ballast is formed partly by a layer of masonry, or of concrete, and partly by
iron kentledge. From the roof of the working chamber rise three vertical
shafts, situated on the longitudinal axis of the caisson, each surmounted by
an air lock above the water level. The middle shaft serves for the work-
men ; its diameter is, according to circumstances, 2 feet 3 inches or 3 feet
6 inches. The entrance lock is a cylindrical chamber 8 feet 3 inches dia-
meter. The other two shafts serve for the raising of excavations and the
lowering of materials ; they are 3 feet 6 inches diameter, as also are their
locks.
The working chamber is lighted, and the lifts are worked, by electricity