The Japanese idea of sea-fighting in those times was to use open boats propelled chiefly by oars. They closed as quickly as possible with the enemy, and then fell on with the trenchant swords which they used so skilfully. Now during the 15th century and part of the 16th the Chinese had been so harassed by Japanese piratical raids that their inventive genius, quickened by suffering, suggested a device for coping with these formidable adversaries. Once allow the Japanese swordsman to come to close quarters and he carried all before him. To keep him at a distance, then, was the great desideratum, and the Chinese compassed this in maritime warfare by completely covering their boats with roofs of solid timber, so that those within were protected against missiles, while loop-holes and ports enabled them to pour bullets and arrows on a foe. The Koreans learned this device from the Chinese and were the first to employ it in actual warfare. Their own history alleges that they improved upon the Chinese model by nailing sheet iron over the roofs and sides of the "turtle-shell" craft and studding the whole surface with _chevaux de frise_, but Japanese annals indicate that in the great majority of cases solid timber alone was used. It seems strange that the Japanese should have been without any clear perception of the immense fighting superiority possessed by such protected war-vessels over small open boats. But certainly they were either ignorant or indifferent. The fleet which they provided to hold the command of Korean waters did not include one vessel of any magnitude; it consisted simply of some hundreds of row-boats manned by 7000 men. Hideyoshi himself was perhaps not without misgivings. Six years previously he had endeavoured to obtain two war-galleons from the Portuguese, and had he succeeded, the history of the Far East might have been radically different. Evidently, however, he committed a blunder which his countrymen in modern times have conspicuously avoided; he drew the sword without having fully investigated his adversary's resources. Just about the time when the van of the Japanese army was entering Seoul, the Korean admiral, Yi Sun-sin, at the head of a fleet of 80 vessels, attacked the Japanese squadron which lay at anchor near the entrance to Fusan harbour, set 26 of the vessels on fire and dispersed the rest. Four other engagements ensued in rapid succession. The last and most important took place shortly after the Japanese troops had seized Phyong-yang. It resulted in the sinking of over 70 Japanese vessels, transports and fighting ships combined, which formed the main part of a flotilla carrying reinforcements by sea to the van of the invading army. This despatch of troops and supplies by water had been a leading feature of Hideyoshi's plan of campaign, and the destruction of the flotilla to which the duty was entrusted may be said to have sealed the fate of the war by isolating the army in Korea from its home base.