ncy in compactness. The open joints between the blocks
laid below low water enable the air to penetrate, on the recoil of the
waves at low tide, into any internal fissures resulting from
settlement; and the following wave, on striking the superstructure,
compresses the air inside, which, on its expansion when the wave
recedes, forces out any unconnected face stones. The hole thus formed
is rapidly enlarged by the sea if the storm continues; and a breach is
eventually formed. The sloping-block system was, accordingly devised
to provide against the dislocation of superstructures by the
inevitable irregular settlement, by forming them of a series of
sloping sections, composed of concrete blocks laid at an angle, free
to settle independently on the mound, as shown in fig. 10. In the
first superstructure thus constructed, in 1869-1874, at the entrance
to Karachi harbour, founded 15 ft. below low water on a rubble mound
and 24 ft. high, the blocks in each section, consisting of two rows of
three superposed blocks laid at an inclination of 76 deg. shorewards,
were entirely unconnected; and, consequently, though the
superstructure offered as little opposition as practicable to the
waves by having its top slightly below high water, the waves in a
storm forcing their way into the vertical joint between the two rows,
threw some of the top 27-ton blocks of the inner row down on the
harbour slope of the mound. This cause of damage was obviated in
effecting the repairs, by connecting the top blocks with the next ones
by stone dowels. The superstructures of the breakwaters forming Madras
harbour, commenced in 1876, were similarly constructed in sloping,
independent sections, 4-1/2 ft. thick, composed of two distinct rows
of four tiers of blocks founded upon a rubble mound 22 ft. below low
water (the rise of tide at springs being 3-1/3 ft.), and raised 3-1/2
ft. above high water. The blocks in each row were connected by a
tenon, projecting at the top of each block, fitting into a mortise in
the block above it. The retention of the vertical joint however,
between the two rows led to the overthrow of the greater part of the
superstructures of the outer arms at Madras, situated in a depth of 45
ft. and facing the Indian Ocean, during a cyclone of 1881. In the
reconstruction of these superstructures, bond was introduced in the
successive tiers of each sloping section; and