In the movement of armies, especially on long marches in the enemy's
country, one of the greatest difficulties to be overcome is the
crossing of streams, and this is usually done by means of portable
bridges. These may be built of light trestles with adjustable legs to
suit the different depths, or of wooden or canvas boats supporting a
light roadway wide enough for a single line of ordinary wagons or
artillery carriages. The materials for these bridges, which are known
as Ponton Bridges, are loaded upon wagons and accompany the army on
its marches, and when required for use the bridge is rapidly put
together, piece by piece, in accordance with fixed rules, which
constitute, in fact, a regular drill. The wooden boats are quite heavy
and are used for heavy traffic, but for light work, as, for example,
to accompany the rapid movements of the cavalry, boats made of heavy
canvas, stretched upon light wooden frames, that are put together on
the spot, are used.
During Gen. Sherman's memorable Georgia campaign and march to the sea,
over three miles of Ponton bridges were built in crossing the numerous
streams met with, and nearly two miles of trestle bridges. In Gen.
Grant's Wilderness campaign the engineers built not less than
thirty-eight bridges between the Rappahannock and the James Rivers,
these bridges aggregating over 6,600 feet in length. Under favorable
circumstances such bridges can be built at the rate of 200 to 300 feet
per hour, and they can be taken up at a still more rapid rate. When
there is no bridge train at hand the engineer is obliged to use such
improvised materials as he can get; buildings are torn down to get
plank and trees are cut to make the frame. Sometimes single stringers
will answer, but if a greater length of bridge is required it may be
supported on piles or trestles, or in deep water on rafts of logs or
casks. But the heavy traffic of armies, operating at some distance
from their bases, must be transported by rail, and the building of
railway bridges or rebuilding those destroyed by the enemy is an
important duty of the engineer. On the Potomac Creek, in Virginia, a
trestle bridge 80 feet high and 400 feet long was built in nine
working days, from timber out of the neighborhood. Another bridge
across the Etowah River, in Georgia, was built in Gen. Sherman's
campaign, and a similar bridge was also built over the Chattahoochee.