il8 Oil the Con^.trnclion of Suspension Bridges. [Jai^ 



" For a bridge may be, froin its size, just so heavy, that by being put 

 " in motion iL will acquire great momenium, and just so light, and slight^ 

 " that it will be unable to resist the effects of its own vibration. 

 *' Therefore, when it becomes necessary to make the chains of a bridge 

 so heavy that vibration would be dangerous, it is advisable boldly to 

 *' increase their weight, rather than attempt to diminish it, and to 

 *' bind and connect the several chains and the road-way firmly togethevr 

 " in order that there may be sufficient mass and stiffness in the bridge 

 *' to resist motion, rather than yield to it readily." The Dryburgh 

 bridge, across the Tweed, was entirely ruined by violent vibration 

 during a storm, but it was of a very faulty construction, and by no 

 means so well adapted to resist motion as those now in use. It is 

 needless to observe, that the greater the weight suspended be- 

 tv.cen the piers, the greater the cost, and (vibration apart), the 

 greater the risk. Under these circumstances therefore, I venture,- 

 with diffidence, to suggest a iiiode of forming the platforms of suspen" 

 gion bridges, v.diich shall not only prevent all motion in the chains, but 

 reduce the cost, while, at the same time heavy wooden joists are avoid- 

 ed ; and, of perishable material, a little planking only made use of. My 

 plan is this {PL fig' 1) : — Having fixed strong iron beams horizontally 

 to each basement of the suspending piers, transversely at the level of the 

 roadway, iron wires are to be slightly stretched from the one to the other 

 of these straining beams, in parallel lines. The number of these bands, 

 and whether composed of one or more W'ires each, must of course 

 depend on the load of planking and other weight, to be allowed on the 

 bridge. A temporary beam, placed under the middle of this wire plat- 

 form, will now be drawn up by tackle fastened to a frame resting on the 

 suspending chains, to a height sufficient to. admit of the insertion tra?is- 

 versely of a strong hollow cylinder of iron, beloiv the wires,and upon the 

 chains. Thus the wire platform will be strained (but not too strongly) 

 into a curve upwards {PL fig 2), forming any desired slope for the road- 

 way, from the piers to tlie centre. Other hollow cylinders must be 

 inseried in the spaces on either side of the centre transom, and at every 

 10 feet distance, additiouril cylinders, or beams, supporting the wire 

 platform, may be suspended to tlie main chains, and these can be readi- 

 ly and easily drawn up as tighlly as required by the common swivel 

 fccrews, &c. Three inch planks, six inches wide, to prevent warp - 

 ing, are now placed transversely along the bridge, firmly connected 

 together by upper battens on each side of the road-way and on each 

 side of a central space of seven feet fjr carriages, and ultimately 



