for conveying intelligence included, we can march four miles, therefore every eight miles 20,000 men would be required, which would make 60,000 for the defence of a length of twenty-four miles of river. These would be sufficient for the appearance of 20,000 men at any point, even if the enemy attempted the passage at two points at the same time; if at only one point twice 20,000 could be brought to oppose him at that single point.
Here, then, there are three circumstances exercising a decisive influence: (1) the breadth of the river; (2) the means of passage, for the two determine both the time required to construct the bridge, and the number of troops that can cross during the time the bridge is being built; (3) the strength of the defender’s army. The strength of the enemy’s force itself does not as yet come into consideration. According to this theory we may say that there is a point at which the possibility of crossing completely stops, and that no numerical superiority on the part of the enemy would enable him to force a passage.
This is the simple theory of the direct defence of a river, that is, of a defence intended to prevent the enemy from finishing his bridge and from making the passage itself; in this there is as yet no notice taken of the effect of demonstrations which the enemy may use. We shall now bring into consideration particulars in detail, and measures requisite for such a defence.
Setting aside, in the first place, geographical peculiarities, we have only to say that the corps as proposed by the present theory, must be posted close to the river, and each corps in itself concentrated. It must be close to the river, because every position further back lengthens unnecessarily and uselessly the distance to be gone over to any point menaced; for as the waters of the river give security against any important movement on the part of the enemy, a reserve in rear is not required, as it is for an ordinary line of defence, where there is no river in front. Besides, the