Then Rome invented the “corvus” or great hinged gangplank with its heavy barbed end. This gangplank was swung at the forward end of the ship and was loosely hinged to the deck, being kept upright by a tackle holding it to the mast. When an enemy’s ship was approached the Romans did not attempt to ram, but ran alongside, let go the tackle, and the heavy corvus fell to the enemy’s deck, where its metal barb fastened itself in the deck planks. Thereupon, the soldiers, with whom the Romans crowded the decks of their ships, rushed across and the sea battle became a mêlée. [Illustration: THE _MONITOR_ _The first armoured ship to mount a turret. This is the ship that fought with the_ Merrimac _the first battle between armoured ships._] For nearly two hundred years these heavy ships were the “battleships” of the Roman fleet. But at the Battle of Actium, in 31 B. C., Mark Antony’s ships, which were of this type, were soundly beaten by light, swift two-banked ships called the Liburnian biremes. Thereupon these Liburnians became the most important ships of war, and later grew into the great galleys of the Middle Ages. The later development, however, tended to the use of one bank, while the oars grew longer and longer until they reached such size that several men were used on each—sometimes as many as seven men being employed on a single oar. This form of rowed war vessel was in more or less common use, principally in the Mediterranean, until the beginning of the 17th Century. In the north of Europe the Viking influence was felt plainly for many years, but finally it was outgrown, or practically outgrown, largely, perhaps, because of the introduction of the raised forecastles and sterncastles, and the introduction of more highly developed rigging. During the Crusades most of the fleets consisted largely of merchant ships, which were more or less converted into war vessels by the addition of raised castles. These castles were, perhaps, of Roman origin, for the old Roman ships sometimes had somewhat similar contrivances at bow and stern. The invention of gunpowder brought about many changes in ship design. At first the guns were small and were pivoted in the rails, as they were on Columbus’s ships, but later, as larger cannon came into use, a new arrangement of them became necessary. Galleys found it difficult to use many cannon, for they could not be mounted amidships, that part of these ships being crowded with rowers, who, by the way, were now seldom below deck. Guns, consequently, had to be mounted at bow and stern, where only a few could be installed. This, then, was one reason for the decline of galleys, for ships driven exclusively by sail were able to mount cannon on deck, where many of them could be carried and fired over the sides. As ships increased in size it became possible to mount cannon below deck and to cut portholes through which they could fire.