therefore employed to hold
the parts together lengthwise--keelsons, shelf-pieces, fillings, and
some form of truss.

The keelson is an inverted keel inside the vessel. The floors, which
are the timbers uniting the two sides of the frame (or ribs), are given
a middle seating on the keel. The keelson is then placed over them,
exactly in line with the keel, when bolts as long as the thickness of
all three are used to unite the whole in one solid backbone, and this
backbone with the ribs. Side or 'sister' keelsons were used in the
Navy on either side of the mainmast for a distance equal to about a
third of the length of the keelson. But they were little used in
merchant vessels, and their longitudinal resistance was only partial
and incidental. Shelf-pieces and waterways were adapted from French
models by Sir Robert Seppings, who became chief constructor to the Navy
some years after Trafalgar. They are thick timbers running
continuously under and {86} over the junctions of the deck beams with
the ship's sides, to both of which they are securely fastened.

The keelson was an old invention and shelf-pieces and waterways were
soon in vogue. But fillings and trusses, both expensive improvements,
were not much favoured in any mercantile marine. The truss is even
older than the keelson, having been used by the ancient Egyptians at
least thirty-five centuries ago, and probably earlier. Four to eight
pillars rose in crutches from the bottom amidships to about six feet
above the gunwale. The Egyptians ran a rope over the crutches and
round the mast, and then used its ends to brace up the stem and stern.
The moderns discarded the rope, took the strains on connecting timbers,
and modified the truss, sometimes out of recognition. But many
Canadian and American river steamers of the twentieth century A.D.
employ the same principle for the same object as the Egyptians of the
seventeenth century B.C. Fillings came from the French, like
shelf-pieces and waterways. Seppings put them between the ribs, in the
form of thick timbers. The whole frame thus became almost solid
against any tendency of the ribs to close together, and quite strong
{87} enough against their other tendency to draw apart.

All means that strengthen a well-built hull longitudinally have also
been made to add their quota to its transverse strength. The ribs
spring from the solid mass of their own floors bolted in between the
keelson and the keel; and th