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 ceiling itself and the outside planking diagonally. Sometimes the holds were strengthened with vertical and sometimes with diagonal riders, but none of these plans gave lasting satisfaction. The means adopted by Sir Robert Seppings were as follows:-- Firstly, the spaces between the frames were filled in solid with timber (Fig. 62). In this way the bottom of the ship was transformed into a solid mass of timber admirably adapted to resist working. At the same time the customary interior planking below the orlop beams was omitted. [Illustration: FIG. 62.--Sir Robert Seppings' system of construction.] Secondly, the beams were connected with the sides of the ship by means of thick longitudinal timbers below the knees running fore and aft, called shelf-pieces, _a_, _a_ (Fig. 63), and similar pieces above the beams, _b_, _b_ (Fig. 63), called waterways. These not only added to the longitudinal strength of the ship, but formed also very convenient features in the connection between the deck-beams and the ship's sides. [Illustration: FIG. 63.--Sir Robert Seppings' system of construction.] Thirdly, a trussed frame was laid on the inside of the transverse frames in the hold of the ship. This frame consisted of diagonal riders making an angle of about 45 deg. with the vertical, together with trusses crossing them, and longitudinal pieces, as shown in Fig. 62. This trussed frame was firmly bolted through the transverse frames and the planking of the ship.