introduced, and the vessels were built higher. Fore and after castles began to be erected. The rudder gradually displaced the steering oar. Two masts, and finally three, replaced the one of the early ships. The introduction of cannon, which dates from the fourteenth century, compelled changes in form. In order to support the weight of the guns, and the shock of firing them, it was necessary to build ships higher and stronger. The height could have been obtained by merely continuing the curve of the bottom farther; but if this had been done, the vessel would have been weak, and the leverage of the weight of the guns would have tended to tear her to pieces. To obviate this risk, the sides were curved in above the water-line in what was called "a tumble home." The guns were at first fired over the top of the bulwarks. A French builder, Descharges of Brest, has the credit of first constructing a ship with portholes through which the cannon could be pointed. In one respect the mediæval ship was curiously like the modern war vessel. She carried a crow's nest on her masts, a military top, in fact, from which archers and crossbowmen could fire, or stones be thrown, on to an enemy's deck. It must not be supposed that these improvements were all strictly successive. Old and new types would be found existing side by side. The rudder and the steering oar, for instance, are found in use together, but gradually the better drove the less good out of use. The long low galleys of the Mediterranean, or at least craft of that description, are heard of as employed in the Middle Ages, but our seas are not friendly to that class of vessel. It appears, from the account of the battle with Eustace the Monk, that the practice of lowering masts and sails on going into action had fallen into disuse by the thirteenth century. This implies at least a greater weight of spars and solidity of rigging than had obtained earlier. It will be easily understood that then, as at all times, there were wide differences in the sizes of ships. They ranged from mere row-boats to the vessel of 250 or 300 tons, known as "cog," or by other names of which we only dimly appreciate the significance.