Constructions of this character are fully described in Douglas's Essay on Military Bridges.

For example, see the passage of the Po, near Casal, in 1515, by the Swiss; the bridge thrown over the Clain by Admiral Coligni, at the siege of Poitiers, in 1569; the operations of the Prince of Orange against Ghent and Bruges, in 1631; the passage of the Tagus, at Alcantara, in 1810, by the English; the bridge constructed across the Zezere, by the French, in 1810; the bridge thrown across the Scarpe, near Douai, in 1820; the experiments made at Fere in 1823, &c.
The passage of a river in the presence of an enemy, whether acting offensively or in retreat, is an operation of great delicacy and danger.
In either case the army is called upon to show the coolest and most determined courage, for its success will depend on its maintaining the strictest discipline and good order.
In the case of a retreat the bridge should be covered by field intrenchments, called a _tete de pont_, and defended by a strong guard.
If the river be of moderate width, the enemy may be kept at a distance by heavy batteries on the opposite shore.

As soon as the passage is effected by the main body, the bridge, if permanent, will be blown up, or otherwise destroyed by the miners, and if floating, will be swung round to the other shore.

The rear-guard will pass over in rowboats, or the end pontons detached for that purpose.

An army retreating in the face of an enemy should never rely upon one single bridge, no matter what may be its character: for the slightest accident happening to it might expose the whole army to inevitable destruction.
The passage of a river by main force, against an enterprising and active enemy on the opposite shore, is always an operation of the greatest difficulty, and not unfrequently accompanied with the most bloody results.
The most effectual method of accomplishing this object is by stratagem.
Demonstrations are made at several points at the same time: bodies of troops are thrown across, after nightfall, in rowboats or by flying-bridges, to get possession of the opposite bank.