Where the depth of the stream is not great, the current slight, and the bottom smooth and hard, the passage may be effected by _fording_. If the bottom be of mud, or large stones, the passage will be difficult and dangerous, even where the depth and current are favorable. Under favorable circumstances infantry can ford a stream where the depth is not greater than four feet; cavalry to a depth of four or five feet; but artillery, and engineer trains, cannot go to a depth of more than two and a half feet, without greatly exposing their ammunition and military stores The fords should be accurately staked out before the passage is attempted, and ropes ought to be stretched across the stream, or cavalry and small boats stationed below, to prevent the loss of life. Ice may be crossed by infantry, in small detachments. Its strength may be increased by covering it with boards, or straw, so as to distribute the weight over a greater surface. By sprinkling water over the straw, and allowing it to freeze, the mass may be made still more compact. But large bodies of cavalry, and heavy artillery, cannot venture on the ice unless it be of great thickness and strength. An army can never trust, for any length of time, to either fords or ice; if it did a freshet or a thaw would place it in a most critical state. 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. A more substantial form may be given to the bridge by substituting for the trestles, piles, or the ordinary framed supports so much used in the newer parts of our country.