table is
built, lashed, and anchored. This bridge has remarkable portability, but
it has also serious defects. The oxidation of the sulphur in vulcanized
rubber produces sulphuric acid in sufficient amount to impair the
strength of the canvas-fibres, thus causing eventual decay, rendering it
prudent to renew the pontoons after a year's campaigning. The pontoons
are required to be air-tight, and are temporarily made partially useless
by punctures, bullet-holes, rents and chafings, although they are easily
repaired. Hence this bridge, despite its portability, is hardly equal to
all the requirements of service, though it was the main dependence in
Banks's operations in Louisiana, and was successfully used in Grant's
Mississippi campaign.

General Cullum briefly describes the various bridge-systems employed in
the different services of the world, including the galvanized iron boat
system, the Blanchard metal cylinder system, the Russian and Fowke's
systems of canvas stretched over frames, the Birigo system, the French
_bateau_ system, the various trestle systems, and many others. The
French wooden _bateau_ is the pontoon chiefly used in our service, and
it is specially commended by its thoroughly proved efficiency, and by
its utility as an independent boat. Its great weight and the consequent
difficulty of its transportation are the great drawbacks, and to this
cause may well be ascribed much of the fatal delay before the
Fredericksburg crossing.

It is a hopeless problem to devise any bridge-equipage which shall
overcome all serious objections. All that should be expected is to
reduce the faults to a practical minimum, while meeting the general
wants of the service in a satisfactory manner. The lack of mobility in
any bridge-train which can be pronounced always trustworthy may,
perhaps, compel the adoption, in addition to the _bateau_-train, of a
light equipage for use in quick movements. This will, however, create
complication, which is nearly as objectionable here as in the calibre of
guns. Thus it is that any solution may prove not exactly the best one
for the particular cases which may arise under it. All that should be
demanded is, that, by the application of sound judgment to the data
which experience and invention afford, our probable wants may be as well
met as practicable. Some system we must have; and, on the one hand, zeal
for mobility, commendable as it is, must not be permitted to invite
grand disasters thro