of trees firmly secured at the ends lipon the banks, laid at thebreadth of the road apart, of length sufficient to cross each otherover the middle of the space, about 10 or 12 feet, and restingthere upon the cross-bearers D, D, which are kept in their placeby the pins F, F, &c. These bearers thus secured are then tobe covered with spars or planks F, which form the road, on whichalso a railing G may be fixed by way of parapet, and thus forma strong and useful bridge of the rudest materials, which may-be as durable as the materials, by giving proper attention thatthe ends of the beams C are well secured from shifting by theopposite banks, and that the cross bearers D are kept in theirplaces by the pins E, where they may also be fastened by a hoopof iron or rope.
Bridges of wood have been formed Over rivers of great widthfrom bank to bank, without any intermediate support. Thelargest of this kind was that over the Rhine at Schauffhausen,constructed by Ulric Grubenman in 1758, and destroyed by theFrench army in 1799. The extent from bank to bank was 36 Pfeet, the rise about 30 feet and 18 feet broad. It was formedupon the principles given by Palladio, in his designs for woodenbridges.
Ihe largest bridge of this kind in Scotland is that over theDon on the Banff road, seven miles from Aberdeen , construct-ed by James Burn of Haddington in i803. The span is 109feet 3 inches, with 13 feet 4 inches of rise, the width 18 feet.Mr Burn has also erected several othir similar bridges in thenorth of Scotland , all of which have great merit in the construc-tor. They consist of a series of frames in the form of an arch*laid horizontally across the soffit. The width of the bridgelias two rows of these frames, which form two ribs, upon whicha vertical framing is constructed to support the bearers, road,
* Such a construction might be useful in military services.