The most difficult point lies here in the passage through the narrow way formed by the river and its valley. It is not now only the volume of water in the river with which we are concerned, but the whole of the defile, and, as a rule, a deep rocky valley is a greater impediment to pass than a river of considerable breadth. The difficulty of the march of a large body of troops through a long defile is in reality much greater than appears at first consideration. The time required is very considerable; and the danger that the enemy during the march may make himself master of the surrounding heights must cause disquietude. If the troops in front advance too far, they encounter the enemy too soon, and are in danger of being overpowered; if they remain near the point of passage then they fight in the worst situation. The passage across such an obstacle of ground with a view to measure strength with the enemy on the opposite side is, therefore, a bold undertaking, or it implies very superior numbers and great confidence in the commander.
Such a defensive line cannot certainly be extended to such a length as in the direct defence of a great river, for it is intended to fight with the whole force united, and the passages,