A ship is not built for mere passive resistance, like a house, or even for resistance only to pressures and vibrations, like a bridge. She is built to resist every imaginable strain of pitching and rolling, and so requires architectural skill of a far higher kind than is required (in the constructional, not the aesthetic, sense) for any structure on the land. When a ship is on the top of a single wave she tends to hog, because there is much less support for her ends than for her centre, and so her ends dip down, racking her upper and compressing her lower parts amidships. When the seas are shorter she often has her ends much more waterborne than her centre, and this in spite of the fact that the extreme ends are not naturally waterborne themselves. Then she sags, and the strains of racking and compressing are reversed, because her centre tends to sink and her ends to rise. Now, a series of hogging and sagging strains alternately compresses and opens every resisting join in every {85} timber, with the inevitable result of loosening the whole. To meet these strains longitudinal strength must be supplied. The keel supplies much of it, so does the planking (or skin) to a lesser degree; but not enough; and the ribs, by themselves, are for transverse stiffening only. Four means are 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.