1392
THEORY AND PRACTICE OF ENGINEERING.
Book II.
above is another, which lifts up the platform from j y -
the recess into which it is laid when the bridge is level; I / I
when so lifted it is drawn back, as shown by the dotted || j ' yf
lines, and turning on the upper pivot assumes an upright ^ j
position, dropping into the ditch, where it is lodged in ^-*
a recess of the masonry prepared to receive it. ^
Spherical Rollers for bridges of this description unitemany advantages over the pivots, as they produce less friction and can be more easilyturned.
Fig. 2311.
Fig. 2310.
Rope Bridges are of very simple construction : upon a number of cords stretched across aravine or river may be placed a timber tripod framed together in a triangular shape, uponwhich timber bearers may be laid, to sustain the planks that are to constitute the road-way :the weight to be transported must guide the engineer in the number and strength of theropes employed, which are to be secured by piles driven firmly on each bank.
Fig. 2312.
ROPE B1UDGES.
Or a Must may be used to sustain the ropes, when the breadth of the river is too great toadmit of the ropes being carried over in one length ; the height must be governed by thewidth of the stream. After ropes are secured to the shores and mast, others are suspended
ROPE BRIDGES.