distributed shock of a simple undulation beating against a
vertical wall. Moreover, the recoil of broken waves down a sloping beach
or rubble mound produces a greater scour than the simple reflection of
an undulation from a vertical wall, especially where the depth is
sufficient to provide a cushion of water below the undulation,
protecting the toe of the wall from the wash of recoil.

_Types of Breakwaters_.--There are three distinct types of
breakwaters:--(1) A simple rubble or concrete-block mound; (2) a mound
for the bottom portion, surmounted on the top by a solid superstructure
of masonry or concrete; and (3) an upright-wall breakwater, built up
solid from the sea-bottom to the top. The second type forms a sort of
combination of the first and third types; and each type presents several
varieties. In a few harbours, two different types have been adopted for
different situations at the same place; but generally the choice of type
is determined by the materials available at the site for the
construction of the breakwater, the nature of the sea-bottom and the
depth into which the breakwater has to be carried.

Rubble mound.

1. _Rubble and Concrete-Block Mound Breakwaters._--A rubble mound
consists merely of a mass of rubble stone, just as it is obtained from
a neighbouring quarry, tipped into the sea along a predetermined line,
till the mound emerges out of water. The rubble stone is deposited,
either from barges, as adopted for the construction of the detached
breakwater sheltering Plymouth Bay, or from wagons, having hinged
opening flaps at the bottom for dropping their load, run out from the
shore along staging erected in the proposed line, according to the
method employed for the outer breakwater enclosing Portland Harbour,
and the north-east breakwater at Colombo Harbour. The mound thus
deposited is gradually consolidated under the action of the sea; and a
tolerably stable form is by degrees attained by continued deposits of
stone. This system of construction is very wasteful of materials, and
can only be resorted to where extensive quarries close at hand are
able to furnish readily and cheaply very large quantities of stone,
especially where, as at Portland and Table Bay, convict labour has
been advantageously utilized in quarrying. When the site is very
exposed, the large waves in storms, dashing over a rubble-mound
breakwater, carry the stones on the top, if unpro