The structure of the hulls of ancient ships was not dissimilar in its main features to that of modern wooden vessels. The very earliest types were probably without external keels. As the practice of naval architecture advanced, keels were introduced, and served the double purpose of a foundation for the framing of the hull and of preventing the vessel from making leeway in a wind. Below the keel proper was a false keel, which was useful when vessels were hauled up on shore, and above the keelson was an upper false keel, into which the masts were stepped. The stem formed an angle of about 70 deg. with the water-line, and its junction with the keel was strengthened by a stout knee-piece. The design of the stem above water was often highly ornate. The stern generally rose in a graceful curve, and was also lavishly ornamented. Fig. 18 gives some illustrations of the highly ornamented extremities of the stern and prow of Roman galleys. These show what considerable pains the ancients bestowed on the decoration of their vessels. There was no rudder-post, the steering having been effected by means of special oars, as in the early Egyptian vessels. Into the keel were notched the floor timbers, and the heads of these latter were bound together by the keelson, or inner keel. Beams connected the top timbers of the opposite branches of the ribs and formed the support for the deck. The planking was put on at right angles to the frames, the butting ends of the planks being connected by dovetails. The skin of the ship was strengthened, in the Athenian galleys, by means of stout planks, or waling-pieces, carried horizontally round the ship, each pair meeting together in front of the stem, where they formed the foundations for the beaks, or rams. The hulls were further strengthened by means of girding-cables, also carried horizontally round the hull, in the angles formed by the projection of the waling-pieces beyond the skin. These cables passed through an eye-hole at the stem, and were tightened up at the stern by means of levers. It is supposed that they were of use in holding the ship together under the shock of ramming. The hull was made water-tight by caulking the seams of the planking. Originally this was accomplished with a paste formed of ground sea-shells and water. This paste, however, not having much cohesion, was liable to crack and fall out when the vessel strained. A slight improvement was made when the shells were calcined and turned into lime. Pitch and wax were also employed, but were eventually superseded by the use of flax, which was driven in between the seams. Flax was certainly used for caulking in the time of Alexander the Great, and a similar material has continued to be employed for this purpose down to the present day. In addition to caulking the seams, it was also customary to coat over the bottom with pitch, and the Romans, at any rate, used sometimes to sheath their galleys with sheet lead fastened to the planking with copper nails.