Six stringers and fifty flooring-poles, 6 inches in diameter and 12 feet long, will be required. Two axemen to each stringer will fell and prepare them in a few minutes, and while they are being carried and placed in position the axemen can prepare the poles. Stringers should have 4 feet extra length. Their diameter will depend on the weight to be borne, how distributed, and kind of wood. It is determined by the formulas already given. A good rough rule for calculating the live load which can be borne by rectangular timber of given length and scantling is: For larch safe distributed live load in cwts. (112 lbs.) = _b__d²_/_L_, in which _b_ = breadth in inches, _d_ = depth in inches, _L_ = length of span in feet. For fir the load may be 4/3, for cedar 5/3, for beech, oak, and pitch-pine 2, and for teak 7/3 of load of larch. This formula gives a theoretical factor of safety of about 3. The stringers are placed in position by means of jumping-poles, each of which should be strong enough to bear the weight of the stringer and a little longer than the hypothenuse of the right-angled triangle, the base and perpendicular of which are respectively three fourths the width and depth of the chasm. Place first stringer on abutment as a temporary wall-plate and chock it; slide the second and third over the chasm a little more than one fourth their length for a footway; push out one jumping-pole, butt first, until nearly balanced on wall-plate stringer, and pass the bight of a rope around it a little in advance of its centre of gravity, by means of which a man on the footway supports the butt while the pole is being slipped forward until it reaches its proper resting-place (while this is being done the ends of the stringers forming the footway are held down); then place second jumping-pole in position; draw back the two stringers forming footway; cross jumping-poles, lashing them about 2 feet above the level of the abutment, and attach guy-rope long enough to reach across the stream; place stringer in crutch and push forward until nearly balanced; then raise end and push so as to throw it forward and cause the other end to rest on the opposite abutment. Four men now cross over on the stringer, steadying themselves by means of the guy-rope held taut for the purpose, lift the stringer off the jumping-poles, and, with assistance from opposite side, roll the stringer in position. Pull back jumping-poles and place the other stringers. The flooring is laid as for corduroy roads, and the bridge is finished by pinning on a ribband of poles to hold down the ends of the flooring and erecting hand-rails if required. It is well to cover the roadway with straw to lessen the jolting of carriages. FORDS, FLYING AND FLOATING BRIDGES.