432 A Resultant System for the [Oct. 



chain, the lower link of which is attached near the centre to the longi- 

 tudinal beam at c 3 . In this position only can Mr. Dredge's theory of 

 a vanishing strain existing in the centre link (N, dotted line) be granted ; 

 but at the same time the roadway beam must be equal (nearly) to the 

 full section of iron in the upper link, as the result proved. The weights 

 Z and Y were alone necessary for this experiment, the weights a, d, d 1 , 

 d*, d 3 , being, as before, ^ cwt. each. 



The span of this half curve was only 40 feet, yet it required 1242 ibs. 

 at Y, and 1302 ibs. at Z, to produce equilibrium, being a greater weight 

 than in the former experiment, in consequence of greater tension being 

 called into action by the greater obliquity of the rods ; and a proof that 

 in Mr. Dredge's construction there is not iron enough in the centre of 

 the longitudinal beam to resist the tension existing there. This experi- 

 ment showed much more rigidity than the former one, being more 

 powerfully acted on ; but to have manufactured it sufficiently strong to 

 resist the tension, would have entailed a heavier outlay than the 

 former. 



There is no doubt but that this construction of making the longitu- 

 dinal beam act centrally as part of the chain would tend to stiffen the 

 structure, and might simplify the details in small spans ; but in large 

 spans, where the centre link is of great substance, and with a double 

 chain, practical difficulties occur which would render the centre link a 

 necessarily distinct feature, and prevent its absorption into the roadway 

 beam. 



The reason why the chains are drawn tangent to the railing is to 

 enable the railing to be placed centrally under the chains ; for if the 

 chains were tangent to the roadway, though there would be a decrease 

 in the height of the standards, there would be a loss of 2 feet in width 

 of platform ; for with a wide chain dipping below the railing, the stan- 

 chions supporting it must be placed 1 foot on each side, within the 

 central line of the chain, in order to avoid contact with it ; and an extra 

 2 feet of platform is more expensive in its consequences on the amount 

 of iron than an additional 4 feet of masonry on the standards. 



Experiment 3rd, of which Fig. 3 is illustrative, was a construction 

 on the resultant principle, similar to experiment 1, carried to a much 

 larger extent. The Fig. 3, shows only one half of it, as it was an entire 

 curve of 490 feet between the points of suspension, the lengths of the 



