Fortunately, Frankish and old English chronicles, which are quite independent of those of the North, help to corroborate the general truthfulness of the Eddas and Sagas, and from them we have several accounts of the number of vessels which sailed up the Seine, the Rhine, the Elbe, and the Weser, or went to England. The largest fleet that ever met in the North was that which assembled for the battle of Bravöll; though the number of vessels is not mentioned, we read that the Sound was covered with vessels. This fleet reached from Kjöge to Skanör, so that, if the account is trustworthy, people could walk as on a bridge from Zeeland over the Sound, a distance of some twenty miles. Sigurd Hring had 2,500 (3,000) ships to oppose him.[147] The maritime expeditions of the Northmen to distant lands were undertaken with a great deal of care and foresight; the men were under strict discipline, and were attired with the greatest splendour. It would be a mistaken idea to think that the Northmen started on these voyages without any previous knowledge of the country they were to invade, or of the shores where they were going to land, or that they sailed with no definite object. Their previous knowledge of these far-off lands was no doubt gained in trading, and it was only after being thoroughly well acquainted with the geography of the part to be attacked that they ventured on their invasion. Many of the places in foreign countries mentioned in the Sagas where the Norse fleets were safely moored and sheltered against storm, are to this day good harbours, and if others are no longer so it is because the shores of those coasts have been subjected to changes which are still taking place. The geographical positions of the rivers they ascended were well chosen; they knew what size of vessel to take there, and though their operations seemed to be detached, we find that their fleets were in communication with each other, and that their armies could assist one another in case of need, crush the enemy between the rivers they had ascended, or between them and the sea. In a word, their tactics showed considerable boldness and strategical skill, which generally left them a way of retreat, if necessary, to their vessels or to some island. Though the Sagas give us a good and vivid idea of the Norse mode of warfare at sea, they are very incomplete in regard to the description and details of the land expeditions, and we have to go to the Frankish chronicles in order to see the manner in which they attacked or besieged a city. From these we learn that the ships ascended the rivers as far as possible; if anything stopped the navigation, a canal was made or the vessels were drawn along the shore, and the obstacle thus passed. The Norsemen took possession of all the large islands, fortified them, and wintered there; and there they kept their spoils of war or plunder. They also brought cavalry in their ships, a fact proved by the Bayeux tapestry.