a moderate depth
of water on a hard bottom of rock, chalk or boulders, by erecting
timber framing in suitable lengths, lining it inside with jute cloth,
and then depositing concrete below low water in closed hopper skips
lowered to the bottom before releasing the concrete, which must be
effected with great care to avoid allowing the concrete to fall
through the water. The portion of the breakwater above low water is
then raised by tide-work with mass concrete within frames, in which
large blocks of stone may be bedded, provided they do not touch one
another and are kept away from the face, which should be formed with
concrete containing a larger proportion of cement. As long continuous
lengths of concrete crack across under variations in temperature, it
is advisable to form fine straight divisions across the upper part of
a concrete breakwater in construction, as substitutes for irregular
cracks.

[Illustration: FIG. 16.--Zeebrugge Harbour Breakwater with Quay.]

Upright-wall breakwaters should not be formed with two narrow walls
and intermediate filling, as the safety of such a breakwater depends
entirely on the sea-wall being maintained intact. A warning of the
danger of this system of construction, combined with a high parapet,
was furnished by the south breakwater of Newcastle harbour in Dundrum
Bay, Ireland, which was breached by a storm in 1868, and eventually
almost wholly destroyed; whilst its ruins for many years filled up the
harbour which it had been erected to protect. In designing its
reconstruction in 1897, it was found possible to provide a solid
upright wall of suitable strength with the materials scattered over
the harbour, together with an extension needed for providing proper
protection at the entrance. This work was completed in 1906.

Upright-wall breakwaters and superstructures are generally made of the
same thickness throughout, irrespective of the differences in depth
and exposure which are often met with in different parts of the same
breakwater. This may be accounted for by the general custom of
regarding the top of an upright wall or superstructure as a quay,
which should naturally be given a uniform width; and this view has
also led to the very general practice of sheltering the top of these
structures with a parapet. Generally the width is proportioned to the
most exposed part, so that the only result is an excess of