1848.] Construction of Iron Tension Bridges. 421 



ed with a power of (the weight % by cosecant of the angle b)=16 

 tons, whilst the horizontal force or (weight X cotangent of the angle 

 b)=14 tons. 



Now although in the first instance the actual tension on the rod B is 

 only 8 tons, and by that the weight is upheld, whilst in the second the 

 total amount of sustaining power is 16-{- 14=30 tons, yet mark the 

 difference of effect on the chains from which such rods are suspended. 

 In a bridge of 1 60 feet span and 20 feet width of platform (for exam- 

 ple) the area to be supported will be 3200 square feet, which, at 1201bs. 

 per square foot will be 172 tons. With an angle of suspension of 15° 

 the tension on the chain in the uniform system will be J weight X by 

 cosecant of the angle of suspension, or 1 ^ 2 X 3.86=332 tons. 



In the "Resultant" system (vide Fig. 17, in which the entire series 

 of strains have been worked out as shown in the table) the extreme 

 tension on the chain, or that due to the upper link, is 192.82 tons, the 

 difference being made up in the tension on the horizontal beam, for 

 which a proportionate section of iron is allowed, and this horizontal 

 beam is not an extra item introduced merely to meet the strain, but is 

 a component part of the system of framing of the platform, and as 

 necessary to the whole as the platform of any ordinary suspension 

 bridge. 



Here then it is apparent that, in Fig. 5, the weight supported verti- 

 cally causes a tension of 332 tons on the upper link of the example 

 above mentioned, and that a proportional section of iron must be given 

 to meet that strain, and not only that, but the same section must be 

 continued throughout the whole series of links ; whereas, as in Fig. 6, 

 the extreme tension on the chain, with an equal load, is only 192.82 

 tons, so that its section can be reduced in the proportion of 1 to 1.72 

 in the upper link, each link in the descending curve becoming lighter 

 in proportion to the extent of diminution allowed ; in addition to which 

 advantages the chain links, by the oblique position given to the suspend- 

 ing rods, are strained in the direction of their length, the most favora- 

 ble to which they can be exposed. Finally if the weight of the 

 whole series of chains, links, and vertical rods in the old system, be 

 compared with the chains, oblique rods, and longitudinal beams of the 

 " Resultant" system, for any given bridge, it would be seen that the 

 two correspond as nearly as can be obtained in practice. This I have 



3 k 2 



