ber of ships built for purposes of
war. The _Howe_, of 120 guns (Fig. 61), is given as an illustration of a
first-rate of this period.

During the earlier years of the present century great improvements were
introduced by Sir Robert Seppings and others into the structural
arrangements of ships. During the long wars abundant experience had been
gained as to the particular kinds of weakness which ships exhibited when
exposed to the strains produced by waves. It had been felt for many
years that the system of building was very defective, and the life of a
man-of-war was consequently short, only fifteen years for a ship built
of English oak in the Royal dockyards, and about twelve years for
similar vessels built in private yards. Amongst the greatest defects was
the absence of longitudinal strength to enable a ship to resist the
effects of hogging and sagging strains in a sea-way.

[Illustration: FIG. 61.--The _Howe_. 1815.]

When a ship at sea is so placed that the crest of a large wave is
passing about the midship section, the two ends may happen to be in the
hollows between the waves, and in this case are to a great extent
unsupported by the water, and consequently have a tendency to droop. The
result is that the ship tends to arch up in the centre like a hog's
back, and the upper decks are put into a state of tension, while the
bottom of the vessel, on the contrary, undergoes compression. The
strains set up in this way are called hogging strains. When the position
of the waves is exactly reversed so that the two ends are supported by
the crests, while the hollow between them passes under the middle, the
latter part of the ship has a tendency to droop or sag, and the bottom
is consequently extended, while the upper works are put into a state of
compression.

It will be noticed, on referring to the illustration of the _Royal
George_ (Fig. 55), that the framework of ships built on the old system
consisted of a series of transverse ribs which were connected together
in the longitudinal direction by the outside planking and by the
ceiling. As there was no filling between the ribs, the latter tended
alternately to come closer together, or recede further apart, according
as they experienced the influence of hogging or sagging stresses. The
French during the eighteenth century had at various times proposed
methods of overcoming this defect. One was to cross the ceiling with
oblique iron riders. Another was to lay the ceilin