Floating supports of this character are often made in compartments, so as to prevent their sinking when injured by the enemy's projectiles.

Indian-rubber pontons may be folded up into a small space, and their slight weight renders them convenient for transportation.
On navigable streams a part of the bridge resting on one or two bateaux should be so arranged that it can be shipped out of its place, forming a _draw_ for the passage of river-craft.

Indeed, it would be well, even where the river is not navigable, to form a draw for the passage of trees, and other floating bodies, sent down by the enemy against the bridge.
An ordinary bridge-equipage of bateaux, or light pontons, for crossing a river of from three to four hundred yards in width, and of moderate current, will require a train of from sixty to eighty wagons.[46] Under favorable circumstances, and with a well-instructed corps of pontoniers, the bridge may be thrown across the river, and prepared for the passage of an army in a few hours at most.[47] After the troops have passed over, the bridge may be taken up, and replaced on the wagons in from a quarter to half an hour.
[Footnote 46: The number of wagons in a ponton train will be greatly diminished if it be found that Indian-rubber boats may be used as supports for the bridge.

The engineer department of our army are making experiments to determine this point.] [Footnote 47: In 1746, three bridges of bateaux were thrown across the Po, near Placentia, each fifteen hundred feet in length, and entirely completed in eight hours.

In 1757, two bridges of bateaux were thrown across the Rhine, at Wesel, in half an hour; again, in the same year, a third bridge was thrown across this river near Dusseldorf, in six hours.
In 1841, Col.