Chap. XX111.
379
suspended to the timber arch : this kind of hang werck , as it is termed in Germany , is thesame as that employed at Feldkirch and Mellingen .
WCRTEMBERG BRIDGE, ON THE NECKER.
Before the principle was adopted of jagging or notching thethree timbers which form the ribs, to enable them more firmly tolock into and abut against each other, it was the custom to buildup the arches with courses of solid logs of oak, in length from 12to 15 feet, and about 16 inches square ; those were selected fromthe forest which had a curve most suitable to the arch to whichthey were to be applied; and in producing the requisite formthe timbers were never cut across the natural grain ; they wereso laid that their abutting joints did not come over each other,and were afterwards secured together by straps or hoops, whichsurrounded them at every 5 feet distance. Much inconvenienceresulted from this method, and an arch constructed with threebent timbers notched into each other is a rash improvement uponit: in one instance the platform of the bridge was supported bythe sides of the polygon, and in the other by the arc of acircle ; both required that the abutments should not yield, butthe strength of an arch formed with bent timbers is far less likely to be interfered with thanthat consisting of a number of timbers arranged along the portion of a polygon ; for someof the sides lying horizontally, and others perpendicularly, it is not possible to make themall bear equally, and resist the effort occasioned by a load placed on the centre of thebridge.
The Bridge of Feldkirch, on the Rhine , is perhaps one of the earliest constructed upon theprinciple of suspending the roadway to one or more timber centres or arches; the span isabout 65 feet 6 inches : each centre is formed of two timbers, cut and indented so as
Fig. 2273.
Fig. 2274.
A t 2