=Frapping.=--The drawing together the several returns of a lashing so as to tauten it. The rope used for frapping is generally the running-end of the lashing itself; after having frapped, the end of the rope is made fast by two half-hitches, one around all the returns, the other on half only. BRIDGES. =Weight.=--Infantry in column of fours when crowded is allowed per lineal foot 560 pounds. Cavalry under same conditions is allowed 700 pounds. Field-artillery in column of sections weighs 400 pounds per lineal foot, so that a bridge built for infantry will carry it. In calculating the strength of parts of bridges the live load must be doubled to bring it to dead load if weight is to be applied suddenly. Formula for weight that may be borne safely by a beam supported at both ends and loaded in the centre: _W_ = _b__d²__S_/_l_, _W_ being the weight in pounds, _l_ the distance between the points of support, _d_ the depth, and _b_ the breadth, all expressed in inches. _S_ is a coefficient whose value for different kinds of wood is given in the following table: Ash 2000 Yellow Pine 1100 Beech 1700 Larch 1300 Birch 1900 Oak 1600 =Strength of Round Timber.=--The strength of a circular pole is six tenths that of a square beam whose side is equal to the mean diameter of the pole; so that the equation becomes _W_ = 6_d__S_/10_l_, in which _d_ is the mean diameter. The bridge must be capable of sustaining the weight when crowded; and while the load for infantry is usually distributed, the greatest strain is brought on the balks, or road-bearers, when the gun-wheels are at the centre of the bay; and a balk will only bear half the load concentrated at its centre that it will bear when distributed. =Roadway and Approaches.=-The roadway may be of planks 1½ to 2 inches for ordinary traffic, or of poles, fascines, hurdles, etc. Litter or earth should be scattered over it to deaden the rattle of planks, which is apt to frighten the horses, and preserve the roadway. If heavy loads are to be hauled, planks should be laid longitudinally to form wheel-tracks. Eight feet in the clear will suffice for width of roadway, but nine feet is to be preferred. A hand-rail should be provided, especially if horses are to pass, and ribbands on either side. At least five balks (road-bearers) should be used on a bridge nine feet wide. The bays generally run from ten to fifteen feet. A rise in the centre of the bridge, called the camber, is allowed for the subsequent settling. This allowance is usually one thirtieth of the bay. The approaches are very important; the exit must be fully as good as the entrance, to avoid crowding. If not, care must be taken not to admit troops at the entrance faster than they can pass off. Ramps at either end steeper than one on ten are inconvenient. TRESTLES, PIERS, ETC.