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 themselves with opposing the passage of the bridge, by
forming a semicircle round its extremity, as round the opening of a
defile, and removing to the distance of three or four hundred toises
from the fire of the opposite side.