The slow progress of naval architecture up to the end of the eighteenth century, an advance the rate of which may be gauged from the fact that, except for sheathing and pumps, no important improvement was patented between the years 1618 and 1800, has been characterized as consisting mainly of approximations to the successive forms and arrangements of Italian, Portuguese, Spanish, and French ships, all of which had been in their turn superior to ours. Until the end of the eighteenth century the “bigotry of old practice” had effectually opposed any radical improvement, even though such improvement had been operating for years in foreign navies and were brought continually before the eyes of our professionals, embodied in captured prizes. In his _Naval Development of the Century_ Sir Nathaniel Barnaby has drawn attention to the remarkable similarity which existed between the _Caledonia_ of the early nineteenth, and the old _Sovereign_ of the seventeenth century: “Almost the only things of note were the reduction in height above water, forward and aft, and a slight increase in dimensions. The proportion between length and breadth had undergone but little change. There was almost the same arrangement of decks and ports; the same thin boarding in front of the forecastle; the same mode of framing the stern, the same disposition of the outside planking in lines crossing the sheer of the ports; nearly the same rig; the same external rudder-head, with a hole in the stern to admit the tiller; and probably the same mode of framing the hull. For the ships of 1810 had no diagonal framing of wood or iron, but the old massive vertical riders; no shelf or waterway to connect the beams with the sides; no fillings above the floor-head; and no dowels in the frames. Ships were still moored by hempen cables, and still carried immense stores of water in wooden casks.” To Sir Robert Seppings was due the series of innovations in constructional method which placed shipbuilding on a relatively scientific basis and thereby rendered it capable of meeting the increasing demands involved in the growing size and force of warships. His scheme, some elements of which had already been tested in H.M. ships, was described in a paper read before the Royal Society in 1814. In the briefest language we will attempt to explain it. In the theory of structures, a jointed figure formed of four straight sides is known as a _deficient_ frame, since it has not a sufficient number of members to keep it in stable equilibrium under any system of loading. A triangle, on the other hand, is a _perfect_ frame, since it has enough, and not more than enough, members to keep it in equilibrium however it may be loaded.