=Transoms.=--They must be sufficiently strong to bear the weight that can be concentrated on one bay, reckoned as distributed load on the transom. On a 9-foot roadway for light artillery, with bays of 15 feet, the transoms should not be less than 9 inches in diameter. Trestles are used for bridges across shallow rivers having sound hard beds and not liable to sudden floods; and for gaps, etc. They are not suitable for deep muddy rivers. They may be used in streams 9 feet deep with velocity of 4 miles an hour; or in deeper streams if the current be less. =Two-legged Trestle= (Fig. 130).--The standards, transom, and ledger are marked at the proper intervals, the heights of the first having been obtained by a careful inspection, and the lengths of the two latter depending on the width of the proposed roadway. The transoms should be about 3 feet longer than the width of the roadway in the clear, and the standards of the trestle should have a splay outward of six on one. Square lashings must be used. The braces are put on the frame with both butts and one tip on the same side, the second tip on the reverse side; the butts can be lashed simultaneously with the ledger and transom. The frame must then be squared by testing the diagonals, measuring from the butt of each standard, and the frame must be adjusted until these measurements are equal. The braces can then be lashed at the tips and crossing-point. If the timber is weak, both legs and transom can be doubled. Ledgers and diagonal braces can be of light material, as little strain is brought upon them; but they should be well lashed. [Illustration: FIG. 130.] =Placing Trestles.=--They are most easily placed by hand; but with those with two legs only care must be taken that they do not fall over. Each frame should be fastened to either bank; cables or struts may be used. =Three-legged Trestles= (Fig. 131), two of which are required for a single transom, have the advantage of standing without bracing, admit of ready adjustment, and utilize light material. They are, however, unsuitable for an uneven bottom, are extremely difficult to place, excepting by hand, and the legs require weighting in water. To make them, lash two legs together by a sheer lashing, open them out and then add the third leg; the trestle must then be up-ended, the feet placed on the angles of an equilateral triangle with sides about half the height, and three light ledgers attached. [Illustration: FIG. 131.] =Cribs=, =Gabions=, and =Fascines=.--These are also used for supporting roadways, the two latter are particularly useful for short gaps. =Limbers.=--Bridges supported on transoms lashed across the tops of limber-wheels may be used. FRAME BRIDGES. If a trestle bridge is impracticable, owing to depth of gap or stream, unevenness of bottom, or swiftness of current, a frame bridge may be used. [Illustration: FIG. 132.]