Bridges

Manner of laying a pontoon bridge acro a river. The bank on each side, where the ends of the bridge are to be, must be made solid and firm, by means of fascines, or otherwise. One end of the cable must be carried acro the river; and being fixed to a picket, or any thing firm, must be drawn tight by means of a capstan, acro where the heads of the boats are to be ranged. The boats are then launched, having on board each two men, and the nece ary ropes, . and are floated down the stream, under the cable, to which they are lashed endwise, by the rings and small ropes, at equal distances, and about their own breadth asunder; more or le , according to the strength required. If the river be very rapid, a second cable must be stretched acro it, parallel to the first, and at the distance of the length of the boats; and to which the other ends of the boats must be lashed. The spring lines are then lashed diagonally from one boat to the other, to brace them tight; and the anchors, if nece ary, carried out, up the stream, and fixed to the cable or sheer line acro the river. One of the che es is then laid on the edge of the bank, at each end of the bridge, bottom up; these serve to lay the ends of the baulks upon, and as a direction for placing them at the proper distances, to fit the che es that cover the bridge. The baulks should then be laid acro the boats, and keyed together: their numbers proportioned to the strength required in the bridge. If the gangboards are laid acro the heads and sterns of the boats from one side of the river to the other, they will give the men a footing for doing the rest of the work. Acro the baulks are laid the che es, one after another, the edges to meet; and the baulks running between the cro pieces on the under side of the che es. The gangboards are then laid acro the ends of the che es on each edge of the bridge. Precautions for pa ing a bridge of boats. Whatever size the bridge may be, infantry should never be allowed to pa at the same time with carriages or cavalry. The carriages should always move at a certain distance behind each other, that the bridge may not be shook, by being overloaded. The horses should not be allowed to trot over the bridge; and the cavalry should dismount and lead their horses over. Large flocks of cattle must not be allowed to cro at once. For the dimensions, weight, and equipage of a pontoon, see the word Pontoon . When bridges are made to facilitate the communication between different parts of the approaches at a siege, they should, if po ible, be placed above the town; or the besieged will take advantage of the current to float down large trees, or other bodies, in order to destroy the bridge. Two of such bridges should always be placed close to each other, in order to prevent the confusion of cro ing and recro ing on the same bridge; the one being intended to pa over one way, and the other to return. Pontoon bridges will generally not support a greater weight than 4 or 5,000 pounds. Pontoons, when united as a bridge, will no doubt bear more in proportion, than when acted upon separately: but the weight which a pontoon will bear may be easily ascertained, by loading it with water till it sinks to any required depth, and then by calculating the number of cubic feet of water it contains, ascertain the number of pounds required to sink it to that particular depth.