nd outer concrete blocks, were
the regular form of superstructure adopted. The breakwater for the
extension of the harbour at Naples provides an interesting example of
this change of design. A solid superstructure, formed of large
concrete blocks capped with masonry, about 50 ft. wide at the base, is
laid on a high rubble mound at a depth of 31 ft. below mean sea-level,
and provides a quay on the top, 24-1/2 ft. wide, protected on the sea
side by a promenade wall, 10 ft. high and 12-1/2 ft. wide at the top,
raised 19-2/3 ft. above sea-level (fig. 12). In view of the increased
depth at which superstructures are now founded upon rubble mounds,
causing the breakwaters to approximate more and more to the
upright-wall type, it might seem at first sight that the rubble base
might be dispensed with, and the superstructure founded directly on
the bed of the sea. Two circumstances, however, still render the
composite form of breakwater indispensable in certain cases: (1) the
great depth into which breakwaters have sometimes to extend, reaching
about 56 ft. below low water at Peterhead, and 102 ft. below mean
sea-level at Naples; and (2) the necessity, where the sea-bottom is
soft or liable: to be eroded by scour, of interposing a wide base
between the upright superstructure and the bed of the sea.

[Illustration: FIG. 12.--Naples Harbor Extension Breakwater.]

The injuries to which composite breakwaters appear to have been
specially subject must be attributed to the greater exposure and depth
of the sites in which they have been frequently constructed, as
compared with rubble mounds or upright walls. The latter types,
indeed, are not well suited for erection in deep water, in the first
case, on account of the very large quantity of materials required for
a high mound with flat slopes, and in the second, owing to the
increased pressure of air under which divers have to work in laying
blocks for an upright wall in deep water. The ample depth in which
superstructures are founded, the due protection afforded to their
outer toe, the adoption of the sloping-block system for their
construction, and the dispensing in most cases with a high sheltering
wall on the sea side of the superstructure, render modern
superstructures as stable as upright-wall breakwaters of similar
height. Nevertheless, superstructures require to be given a greater
thickness than similar upri