low water, and formed of iron caissons
partially filled with concrete and floated out, sunk in position, and
filled up with concrete blocks and concrete. It thus consists of a
continuous row of concrete blocks, each of them being 42-2/3 ft. in
width across the breakwater, 23 ft. in length along the line of the
breakwater, 23 ft. high, and weighing 1400 tons. These caisson blocks,
raised 6-3/4 ft. above low water, form the base of the superstructure,
upon which the upper part was built of concrete blocks on each face
with mass concrete filling between them, forming a continuous quay, 24
ft. wide, raised 8 ft. above high tide, and slightly sheltered by a
curved parapet block only 5 ft. high. The outer toe of the caisson
blocks is protected from being undermined by two tiers of large
concrete blocks laid flat on the rubble mound. This superstructure has
successfully resisted the attacks of the Atlantic waves rolling into
the bay. At this breakwater and at Tynemouth advantage has been taken
of the protection unintentionally provided by previous failures, by
which the waves are broken before reaching the superstructure and pier
respectively; but instead of introducing a wave-breaker of concrete
blocks, for a protection to the superstructure, as arranged at
Marmagao (fig. 11) and the outer arms at Madras, it would appear
preferable to increase the width of the solid superstructure, if
necessary, as carried out at Naples (fig. 12). and to dispense with a
parapet and keep the superstructure low, as being unsuitable for a
quay in exposed situations, according to the plan adopted at Colombo
(fig. 9).

3. _Upright-Wall Breakwaters._--The third type of breakwater consists
of a solid structure founded directly on the sea-bottom, in the form
of an upright wall, with only a moderate batter on each face. This
form of breakwater is strictly limited to sites where the bed of the
sea consists of rock, chalk, boulders, or other hard bottom not
subject to erosion by scour, and where the depth does not exceed about
40 to 50 ft. If a solid breakwater were erected on a soft yielding
bottom, it would be exposed to dislocation from irregular settlement;
and such a structure, by obstructing or diverting the existing
currents, tends to create a scour along its base; whilst the waves in
recoiling from its sea face are very liable to produce erosion of the
sea-bottom along its