dified in
1902, by commencing the stepped courses of large stones at 12 ft.
below mean low water on each slope, instead of at low water raising
this kind of superstructure to 22 ft. above low water in place of 18
ft., and capping the stepped courses at the top by large blocks of
stone, 20 ft. long and 5 ft. deep, laid across the breakwater, which
thus presented a marked resemblance to the upper section of the mound
at Civita Vecchia.

Superstructure below low-water level.

The breakwater at Sandy Bay just referred to, and the one at Civita
Vecchia, which it somewhat resembles, approximate to that class of
breakwater which has a superstructure founded below low-water level,
so far as stepped courses of blocks can be regarded as forming part of
a superstructure; but as the protection afforded by these courses
differs only in the arrangement of the blocks from that obtained by
blocks deposited at random, it appears expedient to restrict this
class to the more solid structures, resembling upright-wall
breakwaters, founded on a mound at some depth below low water As the
main object of this class of breakwater is to keep the mound below the
zone of disturbance by waves in severe storms, it is evident that the
depth at which the superstructure is founded should vary directly with
the exposure of the site, and inversely with the size of the materials
forming the mound.

[Illustration: FIG. 8.--Havre Breakwater.]

The depth at which waves striking against a superstructure may affect
a rubble mound near its toe by the recoil, has been only very
gradually realized. Thus, in 1847, the Alderney breakwater, though
fully exposed to the Atlantic Ocean, was begun with a superstructure
founded at low water of spring tides upon a rubble mound; but within
two years the foundations had to be carried down 12 it. below low
water, and this was adhered to till close to the head, though the
breakwater, completed in 1864, extended 4700 ft. from the shore into a
depth of 130 ft. at low tide, the rise of springs being 17 ft. The
great recoil of the waves in storms from the promenade wall on the sea
side of the superstructure, raised 33 ft. above low water, disturbed
the sea slope of the mound along the outer portion, situated in depths
of 80 to 130 ft. at low water, out to a distance of 90 ft. from the
superstructure and to a depth of 20 ft.; whilst the outer toe of t