From these remarks it will be clear that a new method of construction which, by substituting the triangle for the rectangle, prevented the distortion of a ship’s hull under the stresses of hogging and sagging, would constitute an important innovation: even more important if, in addition, the new method resulted in a large economy of material. Such a system Sir Robert Seppings introduced. Treating the hull as a girder liable to bend, he disposed the timbers to the best advantage to resist deformation. The rectangular system, wherein frames and riders formed rectangular cells with no other power of resisting distortion into rhomboids than that derived from the rigidity of the joints, had been proved inefficient; just as a common field gate would be inefficient, and would easily distort, if built up solely of vertical and horizontal timbers without any diagonal brace to make it a rigid figure. He solved the problem with the triangle. By bracing each quadrilateral cell with a diagonal timber he thereby divided it into two rigid and immovable triangles, and thus made the whole ship rigid. The quadrilateral, when braced, was known as a _trussed frame_. All the chief frames in the ships he trussed; and since all bending took place from the centre of the ship downwards to its ends, he made the trussed frames symmetrical about the centre: the diagonals sloped forward in the after body, and aft in the fore body, so as to resist the arching by extension. The truss frame was embodied, not only in the lower part of the vessel (where its effect in resisting longitudinal bending was comparatively small), but in the more nearly vertical planes, and even in the topsides between the gun-ports (where it was most effective). Its use was estimated to result in the saving of nearly two hundred oak trees in the building of a 74-gun ship. [Illustration: DIAGRAM REPRESENTING A SHIP WITH TRUSSED FRAMES] This was one element of Seppings’ system. The others were: the filling in of the spaces between the ground frames of the ship, so as to oppose with a continuous mass of timber the tendency of the lower parts to compress longitudinally, and to form a thick and solid bottom; the omission of the interior planking below the orlop clamps; the connection of the beams with the frames by means of shelf-pieces, waterways, and side binding-strakes to the deck; and the laying of the decks diagonally. In two other important respects Seppings improved on previous construction.