The art of constructing temporary bridges for the passage, by 
troops, of large rivers and narrow arms of the sea, was well known 
to the ancients, whose works in this respect are sometimes of 
surprising magnitude. Darius passed the Bosporus and Danube, 
and Xerxes the Hellespont, by bridges of boats, the description of 
which we find in Herodotus.* The army of Xerxes constructed 2 
bridges across the Dardanelles, the first of 360 vessels, anchored 
head and stern alongside each other, their keels in the direction of 
the current, the vessels connected with each other by strong cables, 
over which planks were laid, fastened by a rail on either side, and 
covered in by a bed of earth. The 2d bridge had 314 vessels, and 
was similarly constructed. According to Arrian, Alexander had a 
regular pontoon-train of light boats attached to his army.” The 
Romans had _ wicker-work vessels, covered with the skins of 
animals, destined to support the timber platform of a bridge; 
these formed a part of the train of their armies until the end of 
the empire. They, however, also knew how to construct a more 
solid kind of military bridge, whenever a rapid river had to be 
crossed; witness the famous bridges on piles, on which Caesar 
passed the Rhine.?” 

During the middle ages we find no notice of bridge equipages, 
but during the 30 years’ war” the various armies engaged carried 
materials with them to form bridges across the large rivers of 
Germany. The boats used were very heavy, and generally made of 
oak. The platform of the bridge was laid on trestles standing in 

the bottoms of these boats. The Dutch first adopted a smaller kind 
of vessel, flat-bottomed, with nearly vertical sides, pointed head 
and stern, and both ends projecting, in an inclined plane, above 
the surface of the water. They consisted of a framework of wood, 
covered with sheets of tin, and were called pontoons. The French, 
too, according to Folard,* claim the invention of pontoons made of 
copper, and are said to have had, about 1672, a complete pontoon 
train. By the beginning of the 18th century all European armies 
had provided themselves with this kind of vessels, mostly wooden 
frames, covered in with tin, copper, leather, or tarred canvas. The 
latter material was used by the Russians. The boats were small, 
and had to be placed close together, with not more than 4 or 5 
feet clear space between them, if the bridge was to have any 
buoyancy; the current of the water was thereby greatly obstructed, 
the safety of the bridge endangered, and a chance given to the 
enemy to destroy it by sending floating bodies against it. 

The pontoons now employed by the continental armies of 
Europe are of a larger kind, but similar in principle to those 100 
years ago. The French have used, since 1829, a flat-bottomed 
vessel with nearly vertical sides, diminishing in breadth toward the 
stem, and also, but a little less, toward the stern; the 2 ends rise 
above the gunwales and are curved like those of a.canoe. The 
dimensions are: length, 31 ft.; breadth, at top, 5 ft. 7 in.; at 
bottom, 4 ft. 4 in. The framework is of oak, covered with fir 
planking. Every pontoon weighs 1,658 Ibs. and has a buoyancy 
(weight of cargo which would sink the vessel to the top of the 
gunwales) of 18,675 Ibs. When formed into a bridge, they are 
placed at intervals of 14 ft. clear space from gunwale to gunwale, 
and the road of the bridge is 11 ft. wide. For the advanced guard 
of an army a smaller kind of pontoon is used, for bridging over 
rivers of less importance. The Austrian pontoons are similar to the 
larger French pontoon, but divided transversely in the middle, for 
more convenient carriage, and put together in the water. Two 
vessels placed close alongside each other, and connected by short 
timbers, a longitudinal timber supporting the balks of the 
platform, constitute a floating pier of a bridge. These pontoons, 
invented by Birago, were introduced in 1825. The Russians have a 
framework of wood for their pontoons, so constructed that the 
centre pieces, or thwarts, may be unshipped; over this frame is 
stretched sail-cloth, covered with tar or a solution of India rubber. 

They are in length, 21 ft. 9 in.; breadth, 4 ft. 11 in.; depth, 2 ft. 4 

in., and weigh 718 Ibs. each. Breadth of road of bridge, 10 ft.; 
distance from pontoon to pontoon, 8 ft. The Russians also have 
pontoons with a similar framework, covered over with leather. 
The Prussians are said to have been the first to divide their 
pontoons transversely into compartments, so as to prevent one 
leak from sinking them. Their pontoons are of wood and 
flat-bottomed. The span or clear distance between the pontoons, 
in their bridges, varies from 8 to 16 ft., according to cir- 
cumstances. The Dutch, since 1832, and the Piedmontese, have 
pontoon trains similar to those in the Austrian service. The 
Belgian pontoon has a pointed head, but is not contracted at the 
stern. In all continental armies small boats to carry out the anchors 
accompany the pontoon train. 

The British and the U.S. armies have entirely abandoned the 
use of boats for the formation of their pontoon trains, and 
adopted hollow cylinders of light material, closed on all sides, to 
support their bridges. In England the cylindrical pontoons, with 
conical, hemispherical or paraboloidal ends, as constructed in 1828 
by Col. Blanchard, were adopted in 1836 to the exclusion of all 
other kinds. The larger British pontoon is 24'/, ft. long and 2 ft. 8 
in. in diameter. It is formed of sheet tin, framed round a series of 
wheels constructed of tin, having hollow cylinders of tin for their 
spokes; a larger tin cylinder, 1°/, in. in diameter, forms their 
common axis, and runs through the entire length of the pontoon. 

Experiments have been made in the United States with India 
rubber cylindrical pontoons. In 1836 Capt. (afterward Col.) Lane 
constructed bridges over a deep and rapid river in Alabama with 
such pontoons, and in 1839 Mr. Armstrong submitted similar 
floats, 18 ft. long, 18 in. in diameter when inflated, and weighing 
39 Ibs. each, 3 to form 1 link of the bridge. Pontoons of inflated 
India rubber were, in 1846, introduced in the U.S. army, and used 
in the war against Mexico.”'° They are very easily carried, from 
their lightness and the small space they take up when folded; but, 
beside being liable to be damaged and rendered useless by friction 
on gravel, &c., they partake the common faults of all cylindrical 
pontoons. These are, that when once sunk in the water to '/y of 
their depth, their immersion becomes greater and greater with 
every equal addition of load, the reverse of what should be; their 
ends, moreover, easily catch and lodge floating matter; and finally, 
2 of them must be joined to a raft by a platform before they can 
be moved in the water, whereas boat pontoons are as capable of 
independent motion in the water as common boats, and may serve 
for rowing rapidly across the river a detachment of troops. To 

compare the buoyant power of the cylindrical pontoon with that of 
the boat pontoon, the following may suffice: The French pontoon 
supports about 20 ft. of bridge, and has a buoyancy (the weight of 
the superstructure deducted) of more than 150 cwt. A British raft 
of 2 pontoons, supporting about the same length of bridge, has a 
buoyancy, superstructure deducted, of only 77 cwt., '/> of which is 
a safe load. 

A pontoon train contains, beside the pontoons, the oars, 
boat-hooks, anchors, cables, &c., necessary to move them about in 
the water, and to fix them in their position, and the balks and 
planks (chesses) to form the platform of the bridge. With boat 
pontoons, every pontoon is generally secured in its place, and then 
the balks and chesses stretched across; with cylindrical pontoons, 2 
are connected to a raft, which is anchored at the proper distance 
from the end of the bridge, and connected with it by balks and 
chesses. Where circumstances admit of it, whole links, consisting of 
3, 4, or 5 pontoons bridged over, are constructed in sheltered 
situations above the site fixed on for the bridge, and floated down 
successively into their positions. In some cases, with very experi- 
enced pontoniers, the whole bridge has been constructed on one 
bank of the river and swung round by the current when the 
passage was attempted. This was done by Napoleon when cross- 
ing the Danube, the day before the battle of Wagram.*'’ The 
whole of this campaign is highly instructive with regard to the 
passing of large rivers in the face of the enemy by military 
bridges. 

Pontoon trains are, however, not always at hand, and the 
military engineer must be prepared to bridge over a river, in case 
of need, without them. For this purpose a variety of materials and 
modes of construction are employed. The larger kind of boats 
generally found on navigable rivers are made use of for bridges of 
boats. If no boats are to be found, and the depth or configuration 
of bottom of the river renders the use of floating supports 
necessary, rafts of timber, floats of casks, and other buoyant 
bodies may be used. If the river is shallow, and has a hard and 
tolerably level bottom, standing supports are constructed, consist- 
ing either of piles, which form the most durable and the safest 
kind of bridge, but require a great deal of time and labor, or of 
trestles, which may be easily and quickly constructed. Sometimes 
wagons loaded with fascines, &c., and sunk in the deeper places of 
the river, will form convenient supports for the platform of a 
bridge. Inundations, marshes, &c., are bridged over by means of 
gabions. For narrow rivers and ravines, where infantry only have 

to pass, various kinds of suspension bridges are adopted; they are 
generally suspended by strong cables. 

The construction of a military bridge under the actual fire of 
the enemy is now a matter of but rare occurrence; yet the 
possibility of resistance must always be provided for. On_ this 
account the bridge is generally constructed in a reentering bend of 
the river, so that the artillery placed right and left sweeps the 
ground on the opposite bank close to where the bridge is to land, 
and thus protects its construction. The concave bank, moreover, is 
generally higher than the convex one, and thus, in most cases, the 
advantage of command is added to that of a cross fire. Infantry 
are rowed across in boats or pontoons, and established immediate- 
ly in front of the bridge. A floating bridge may be constructed to 
carry some cavalry and a few light guns across. The division of the 
river into several branches by islands, or a spot immediately below 
the junction of some smaller river, also offers advantages. In the 
latter, and sometimes in the former case, the several links of the 
bridge may be composed in sheltered water, and then floated 
down. The attacking party, having commonly to choose between 
many favorable points on a long line of river, mav easily mislead 
his opponent by false attacks, and then effect the real passage at a 
distant point; and the danger of scattering the defending forces 
over that long line is so great, that it is nowadays preferred to 
keep them concentrated at some distance from the river, and 
march them in a body against the real point of passage as soon as 
it has once been ascertained, and before the enemy can have 
brought over all his army. It is from these causes that in none of 
the wars since the French revolution has the construction of a 
bridge on any of the large rivers of Europe been seriously 
contested.