The most powerful and commercial, however, of all the tribes of Gaul, that inhabited the coasts near the ocean, in the time of Cæsar, were the Vanni. These people carried on an extensive and lucrative trade with Britain, which was interrupted by the success of Cæsar, (who obliged them, as well as the other tribes of Gaul, to give him hostages,) and which they apprehended was likely to be still further injured by his threatened invasion of Britain; in order to prevent this, as well as to liberate themselves, they revolted against the Romans. As Cæsar was sensible that it would be imprudent and unsafe to attempt the invasion of Britain, so long as the Vanni were unsubdued and powerful at sea, he directed his thoughts and his endeavours to build and equip such a fleet as would enable him successfully to cope with them on their own element. In building his ships, he followed the model of those of his enemies, which were large, flat-bottomed, and high in the head and stern: they were strong-built, and had leathern sails, and anchors with iron chains. They had a numerous squadron of such vessels, which they employed chiefly in their trade with Britain: they seem also to have derived considerable revenue from the tribute which they levied on all who navigated the adjacent seas, and to have possessed many ports on the coast. Besides their own fleet, the Britons, who were their allies, sent ships to their assistance; so that their united force amounted to 220 sail, well equipped, and manned by bold and expert seamen. To oppose this formidable fleet, Caesar ordered ships to be built on the Loire, and the rivers running into it, exactly, as we have just stated, after the model of the ships of the Vanni; for he was informed, or learnt by experience, that the vessels which were used in the Mediterranean were not fit for navigating and fighting on the ocean, but that such as were employed on the latter must be built, not only stronger, but flat-bottomed, and high at the head and stern, in order to withstand the fury of the waves and winds, which was greater in the ocean than in the Mediterranean, and at the same time to sail up the rivers, or in very shallow water, and to take the ground, without injury or danger. Not being able, however, to build in time a sufficient number of ships in Gaul, after the model of those of the Vanni, he was under the necessity of bringing some from the south coast of Gaul, and other parts of the Mediterranean Sea; he also collected all the experienced pilots he could meet with, who were acquainted with the coasts, and with the management of such ships, and exercised a sufficient number of men at the oar, to navigate them.