A floating body, propelled from one bank to the other by the current of the stream, is termed a flying-bridge. The usual mode of establishing a ferry of this kind, is to attach the head of the boat by means of a cable and anchor to some point near the middle of the stream.  By steering obliquely to the current, the boat may be made to cross and recross at the same point.  A single passage may be made in the same way, by the action of the current without the cable and anchor, but the boat in this case will be carried some distance down the stream.  Rowboats are employed for crossing over infantry by successive debarkations; but this process is too slow for the passage of a large force; it may very well be resorted to as auxiliary to other means.

Steam craft are so common at the present day on all navigable streams, that an army in the field will frequently be able to avail itself of this means of passing the larger rivers.  But, in a hostile country, or in one already passed over by the enemy, it will not be safe to rely with confidence upon obtaining craft of this character.  A well-organized army will always carry in its train the means of effecting a certain and speedy passage of all water-courses that may intercept its line of march.

Flying-bridges or rowboats were employed in the passage of the Dwina, in 1701, by the Swedes; the passage of the Po, in 1701, by Prince Eugene; the passage of the Rhine, at Huninguen, in 1704; Jourdan’s passage of the Rhine in 1795; Moreau’s passage in 1796; the sieges of Kehl and Huninguen in 1797; Massena’s passage of the Limmat, and Soult’s passage of the Linth, in 1799; the passage of the Rhine, at Lucisteig in 1800; the passage of the Po, by the French, just before the battle of Marengo; and others in Italy, Germany, and Spain, in the subsequent campaigns of Napoleon.

Military bridges have sometimes been formed 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.