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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.
They are thick timbers running continuously under and 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.
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.
Now, a series of hogging and sagging strains alternately compresses and opens every resisting join in every timber, with the inevitable result of loosening the whole. To meet these strains longitudinal strength must be supplied. Four means are therefore employed to hold the parts together lengthwise keelsons, shelf-pieces, fillings, and some form of truss.
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