1364
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Fig. 2210.
Fig. 2209.
Such a bridge as the last may be made of very considerable strength by introducing underthe longitudinal timbers an additional thickness, against which the two struts abut; and
Fig. 2211. BRIDGE OVER THE DANUBB, NBAS ULM.
taking the precaution of having a hard material between the ends of the timbers, where theirfibres are likely to penetrate, and iron bolts to secure the whole together: abutments of
brick or stone are always preferable when they can be constructed.
Where the bearing is upwards of 40 feet, additional strength is obtained by the intro-duction of stout pieces of timber under each beam; these may be secured by iron pins; sostrutted, they are capable of sustaining considerable weight.
Fig. 2212.
In many instances it is found that the construction of a timber bridge is rendered lessdifficult when the piles that support the platform are in two lengths, or that the first aredriven into the bed of the stream, and cut off a little below the level of the water : uponthe heads of these lower piles rests a plate, laid in the direction of the current, and on•which the piles that form the piers are secured: should these require any reparation, it canbe readily effected without the aid of the pile-driving machine.