Military bridges will, therefore, become its only safe reliance for keeping open its communications.
Military bridges are made with trestles, rafts, boats, and other floating bodies.

Rope bridges are also sometimes resorted to by troops for passing rivers.
_Trestle bridges_ are principally used for crossing small streams not more than seven or eight feet in depth: they also serve to connect floating bridges with the shore, in shallow water.

The form of the trestle is much the same as that of an ordinary _carpenter's horse,_ i.e., a horizontal beam supported by four inclined legs.

These trestles are placed in the stream, from twelve to twenty feet apart, and connected by string-pieces, (or _balks_ as they are termed in technical language,) which are covered over with plank.

The action of the current against the bridge may be counteracted by anchors and cables, or by means of boxes or baskets attached to the legs of the trestles, and filled with stones.