d of ropes, cables stretched
across the stream, and firmly attached at each end to trees, or posts
let into the earth. If the shore is of rock, rings with staples let into
the stone form the best means for securing the ends of the main ropes.
Plank are laid on these cables to form the roadway. The ropes forming
the "side-rail" of the bridge are passed over trestles at each shore,
and then fastened as before. Short vertical ropes attach the main
supports to these side ropes, in order that they may sustain a part of
the weight passing over the bridge. 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