It was the first time that a regular army attempted to cross the Rhine. The bridge was constructed in the following manner. (_See Plate 15._) Two trees (probably in their rough state), a foot and a half in thickness, cut to a point at one of their extremities, and of a length proportionate to the depth of the river, were bound together with cross-beams at intervals of two feet from each other; let down into the water, and stuck into the ground by means of machines placed in boats coupled together, they were driven in by blows of a rammer, not vertically, like ordinary piles, but obliquely, giving them an inclination in the direction of the current. Opposite them, and at a distance of forty feet below, another couple of piles were placed, arranged in the same manner, but inclined in a contrary direction, in order to resist the violence of the river. In the interval left between the two piles of each couple, a great beam was lodged, called the _head-piece_, of two feet square; these two couples (_hæc utraque_) were bound together on each side, beginning from the upper extremity, by two wooden ties (_fibulæ_), so that they could neither draw from nor towards each other, and presented, according to the “Commentaries,” a whole of a solidity so great, that the force of the water, so far from injuring it, bound all its parts tighter together.[309] This system formed one row of piles of the bridge; and as many of them were established as were required by the breadth of the river. The Rhine at Bonn being about 430 mètres wide, the bridge must have been composed of fifty-six arches, supposing each of these to have been twenty-six Roman feet in length (7·70 mètres). Consequently, there were fifty-four rows of piles. The floor was formed of planks reaching from one head-piece to the other, on which were placed transversely smaller planks, which were covered with hurdles. Besides this, they drove in obliquely, below each row of piles, a pile which, placed in form of a buttress (_quæ pro ariete subjectæ_), and bound in with it, increased the resistance to the current. Other piles were similarly driven in at a little distance above the rows of piles, so as to form stockades, intended to stop trunks of trees and boats which the barbarians might have thrown down in order to break the bridge. These works were completed in ten days, including the time employed for the transport of the materials. Cæsar crossed the river with his army, left a strong guard at each extremity of the bridge, and marched towards the territory of the Sicambri, proceeding, no doubt, up the valley of the Sieg and the Agger, (_See Plate 14._) During his march, deputies from different peoples came to solicit his alliance. He gave them a friendly reception, and exacted hostages. As to the Sicambri, at the beginning of the erection of the bridge, they had fled to the deserts and forests, terrified by the reports of the Usipetes and Tencteri, who had taken refuge among them.