When reconnoitring a river with a view of effecting a passage, observe: the nature of the banks, the nature of the bed, position and depth of fords, strength of current, whether tidal or otherwise, probability and extent of floods. =Fords.=--The following depths are fordable: For infantry, 3 ft.; for cavalry, 4 ft.; wagons containing ammunition, 2⅓ ft. Gravelly bottoms are best; sandy bottoms are bad, as the sand stirs up and increases depth of water. Fords should be clearly marked by long pickets driven into the bed of the river above and below the ford, their heads being connected by a strong rope. It is well to mark the pickets, in order that any rise of the water may be at once evident. The depth of a river is generally most uniform in straight parts; at bends the depth will generally be greater at the concave bank, and less at the convex. For this reason a river which is not anywhere fordable straight across may be found passable in a slanting direction between two bends. To measure the velocity of a stream, use a light rod weighted at the end so as to stand vertically in the water; note the distance it floats in a given number of seconds; then seven tenths the mean number of feet a second gives the number of miles an hour. =Ferries and Flying Bridges.=--The simplest form of permanent ferry consists of ropes stretched across the river by means of which rafts can be hauled from bank to bank. The flying bridge can be used if the velocity of the current is two miles an hour or more. The current moves the boat or raft across the stream by acting obliquely against its side, which should be kept at an angle of about 55 degrees with the current. The cable, whose length should be 1½ to 2 times the width of the river, and float if possible, can either be anchored in midstream (in which case the boat can swing between two landing-places), or two cables may be used, one anchored on either bank. Or a cable may be stretched from bank to bank as taut as possible and six feet above the water at the lowest point. The boat (a double-ender) is attached to travellers, which are small wheels grooved on the circumference to fit the cable on which they ride, maintained in their position by a counterpoise below, to which the stem-and stern-lines of the boat are attached. Long, narrow, deep boats with vertical sides, to which leeboards can be attached, are the best for the purpose, and straight reaches of a river the most suitable places for flying bridges. =Floating Bridges.=--These can be made of boats, barrels, timber, etc. Each pier must have enough available buoyancy to support the heaviest load that can be brought on one bay of the bridge. The length of the piers should be at least twice the breadth of the roadway, for steadiness, and they should be connected together at their extremities by tie-balks or lashings. The waterway between the piers should never be less, and should if possible be more, than the width of those piers.