ater
protecting the outer harbour at Leghorn, and in the central breakwater
in deep water sheltering the harbour of St Jean de Luz, and directly
facing the Bay of Biscay. These large concrete blocks are deposited by
cranes from staging, tipped into the sea from a sloping platform on
barges, or floated out between pontoons, or slung out from floating
derricks. This last method proved so expeditious for the upper blocks
at Alexandria, that, in conjunction with the tipping of the lower
blocks from the inclined planes on the decks of barges and the deposit
of the rubble from hopper barges, provided also with side flaps for
the higher portions, the detached breakwater, nearly 2 m. long,
sheltering a very spacious harbour, was constructed in two years
(1870-1872). Sometimes, when a mound breakwater has been raised out of
water, advantage is taken of a calm period of the year and a low tide
to form large blocks of concrete within timber framing on the top of
the mound, so as to provide a very efficient protection.

The large masses composing mound breakwaters give them great stability
against the attacks of the sea; and, moreover, the wide base of the
mounds enables them to be deposited on a sandy or silty sea-bottom,
without any fear of settlement or undermining. A mound breakwater,
however, has the disadvantages of requiring a large amount of
material, and of occupying a wide space on the bed of the sea, more
especially where the mound consists of rubble stone and is in deep
water, so that the system, though simple, is costly, and is unsuited
for harbours where the available space to be sheltered is limited.
Nevertheless, a mound breakwater can be rapidly constructed by the
employment of a large number of barges; and by the adoption of large
concrete blocks, the quantity of materials and the space occupied by
the mound can be considerably reduced. This form of breakwater, with
its long outer slope exposed to breaking waves, particularly where the
tidal range is considerable, is, indeed, more subject to frequent
small injuries than the other types, but they are readily repaired;
and a mound is not generally liable to the serious breaches which
occasionally are formed in solid superstructures and upright walls in
exceptional storms.

2. _Breakwaters formed of a Mound surmounted by a Superstructure._--The
second type of breakwater consists of a mound, comp