We see from what precedes that the port of Boulogne agrees with the text of the "Commentaries." But the peremptory reason why, in our opinion, the port where Cæsar embarked is certainly that of Boulogne, is, that it would have been impossible to prepare elsewhere an expedition against England, Boulogne being the only place which united the conditions indispensable for collecting the fleet and embarking the troops. In fact, it required a port capable of containing either eighty transport ships and galleys, as in the first expedition, or 800 ships, as in the second; and extensive enough to allow the ships to approach the banks and embark the troops in a single tide. Now these conditions could only be fulfilled where a river sufficiently deep, flowing into the sea, formed a natural port; and, on the part of the coasts nearest to England, we find only at Boulogne a river, the Liane, which presents all these advantages. Moreover, it must not be forgotten, all the coast has been buried in sand. It appears that it is not more than a century and a half that the natural basin of Boulogne has been partly filled; and, according to tradition and geological observations, the coast advanced more than two kilomètres, forming two jetties, between which the high tide filled the valley of the Liane to a distance of four kilomètres inland.
None of the ports situated to the north of Boulogne could serve as the basis of Cæsar's expedition, for none could receive so great a number of vessels, and we cannot suppose that Cæsar would have left them on the open coast, during more than a month, exposed to the tempests of the ocean, which were so fatal to him on the coasts of Britain.
Boulogne was the only point of the coast where Cæsar could place in safety his depôts, his supplies, and his spare stores. The heights which command the port offered advantageous positions for establishing his camps,[379] and the little river Liane allowed him to bring with ease the timber and provisions he required. At Calais he would have found nothing but flats and marshes, at Wissant nothing but sands, as indicated by etymology of the word (_white sand_).
It is worthy of remark, that the reasons which determined Cæsar to start from Boulogne were the same which decided the choice of Napoleon I. in 1804. In spite of the difference in the times and in the armies, the nautical and practical conditions had undergone no change. "The Emperor chose Boulogne," says M. Thiers, "because that port had long been pointed out as the best point of departure of an expedition directed against England; he chose Boulogne, because its port is formed by the little river Liane, which allowed him, with some labour, to place in safety from 1,200 to 1,300 vessels."