From the moment you are master of a position which commands the opposite bank, facilities are acquired for effecting the passage of the river; above all, if this position is sufficiently extensive to place upon it artillery in force. This advantage is diminished, if the river is more than three hundred toises (or six hundred yards) in breadth, because the distance being out of the range of grape, it is easy for the troops which defend the passage to line the bank and get under cover.
Hence it follows that if the grenadiers, ordered to pass the river for the protection of the bridge, should reach the other side, they would be destroyed by the fire of the enemy; because his batteries, placed at the distance of two hundred toises from the landing, are capable of a most destructive effect, although removed above five hundred toises from the batteries of the crossing force. Thus the advantage of the artillery would be exclusively his.
For the same reason, the passage is impracticable, unless you succeed in surprising the enemy, and are protected by an intermediate island, or, unless you are able to take advantage of an angle in the river, to establish a crossfire upon his works. In this case, the island or angle forms a natural tête de pont, and gives the advantage in artillery to the attacking army.
When a river is less than sixty toises (or one hundred and twenty yards) in breadth, and you have a post upon the other side, the troops which are thrown across derive such advantages from the protection of your artillery, that, however small the angle may be, it is impossible for the enemy to prevent the establishment of a bridge. In this case, the most skilful generals, when they have discovered the project of their adversary, and brought their own army to the point of crossing, usually content