I83S.] 



On the Construction of Suspe?ision Bridges. 



fastened to the wire ],,latform by diag-onal under ties which are strongly 

 bolted or nailed to the upper woodwork {PI. fig. 3). The cross beams 

 and side battens serve to support the posts, &c. of the railing", and the 

 centre cylinders being allowed to project beyond the road-way a few 

 inches on each side (if thought necessary) would yield additional 

 strength to the centre trussed rail by the application of an outer 

 buttress. (See Plate, fig. 4). 



From the amazing stiffness which w^ill arise by this mode of uniting 

 the chains and platform, judging from a very rough string model in my 

 possession, the necessity of a trussed railing with all its weight and 

 cost will I am certain be obviated, and any cheap light protection for 

 the sides, which can be conveniently had on the spot, may be substitut- 

 ed. I beg to observe that the wire platform is not to be much strain- 

 ed upwards, and should therefore be originally laid accordingly, and 

 as in all iron suspension bridges the sectional area of the material 

 used is calculated to bear three times the load that can be brought 

 upon them, so in the v/ire thus disposed and thus regulated, each wire 

 will really be almost as rigid as a bar of wood or iron. Could 

 a greater load than due to its " rigid pow'er," if I may be allows- 

 ed the term, be applied, then of course the wire by deflecting 

 would yield to the passing load, and the platform be springy 

 and unpleasant. Now, in gales of wind, suspension bridges of the pre- 

 sent best construction, having their platforms of disunited portions and 

 swinging below the chains, no under ties can entirely prevent the ruin- 

 ous undulations and jerking of the whole mass. With the wire plat- 

 form, however, as this cannot rise, and the chains cannot sink, one very 

 light but unyielding fabric is presented to the storm : and at Madras, 

 (should such a bridge or jetty be ever constructed) the planking might 

 (by a simple process), be rapidly removed before the monsoon, or when 

 threatened by a gale, and there would really not remain more than a 

 few inches of surface to be acted on by the wind. Again, when loaded 

 to the utmost, the platform will naturally sink a little, and thus those 

 wires be spared, while the weight will be properly borne by the sus- 

 penders and cables, which are designed accordingly, and from being 

 originally provided with a power three times greater than nny possible 

 load, cannot much alter their bearings. Deficient as we are in this 

 country of the opportunity and means of putting our notions to a fair 

 test, I cannot help fearing that some objections, not apparent to myself 

 or to those whom 1 have consulted, may exist to this simple and cheap 

 mode of constructing bridges, and I trust therefore that some of the 



