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 the planking, or skin, is let into the rabbets, or side grooves, of the keel and firmly fastened to the ribs throughout by hardwood pegs called treenails. The decks are, in themselves, a source of weakness. The beams supporting them are like the rafters of a house, which, of course, work the walls apart under pressure from the floors--and here, as in every other detail, the stability required for a house is nothing to what is required for a ship. The way to overcome this difficulty is to make the decks and beams so many bridges holding the sides together. At the point of junction of every beam-end with a shelf-piece, waterway, and rib there is an arrangement of bolts and dowellings (or dovetailings) which makes the whole as solid as possible. An extra bolt through the waterway, rib, and outside planking adds to the strength; and a knee, or angular piece of wood or iron connecting the shelf with the under side of the beam, almost completes the {88} beam-end connection. The final touches are the clamps below the shelves and the spirketing above the waterways, with short-stuff between the clamps of one deck and the spirketing of the next below.