432 Mr. Seaward on Suspension Chain Bridges. 



hinder wheels of a waggon are generally 7 feet apart from 

 the otliers ; and therefore when the fore wheels of the two wag- 

 gons come in a line with a set of rods, it follows that the whole 

 weight will 7wt press upon that set of rods only, but a con- 

 siderable part will be thi'own on the contiguous rods : and 

 place the waggon in any other relative position, the effect pro- 

 duced will be nearly the same. 



In the next place, it is assumed that the platform of the 

 roadway will be made of such flexible matei'ials, that a weight 

 coming upon any particular point, that point must necessarily 

 be pressed down, without communicating any motion to the 

 adjacent parts, and thus all the weight would be thrown on 

 one set of rods only. But this is not true of the timber plat- 

 forms of suspension bridges ; for it is usual to make them so 

 very firm and stiff, that no point can be borne down without 

 carrying a considei-able length each way of the platform 

 with it. 



The platform of the proposed bridge would be constructed 

 of longitudinal planking, three and four inches thick, spiked 

 down to transvei'se joists 10 inches deep and 6 inches thick: 

 the latter being supported by four longitudinal beams, 17 

 inches deep and 85 inches thick each, reaching from one end 

 of the bridge to the other ; — the whole being firmly bolted 

 and secured together, and forming a strong compages of tim- 

 ber work 27 inches deep and 22 feet wide. If a portion of 

 such a platform 37 feet long were to rest upon its two ends, 

 and without being supported in any other part, it would ad- 

 mit a load of 1 5 tons to pass over it and not sink in the mid- 

 dle in any sensible degree. 



It is therefoi'e quite evident, that the whole weight of the 

 two waggons could not bear upon one set only of the diagonal 

 rods ; on the contrary, it could be satisfactorily proved that 

 not more than half the load could be thrown, under any cir- 

 cumstances, upon a single set of rods. Now half the weight 

 of the waggons would be 1\ tons, which added to 7^ tons, 

 the weight of a portion of the platform, would make 15 tons 

 pressing perpendicularly ; and this, as has been already shown, 

 would produce a strain of 7 tons per square inch of sectional 

 ai'ea in the diagonal rods. 



It could be very easily demonstrated, that the objection, if 

 it have any value at all, would apply with equal force against 

 a bridge built on the principle of the catenary curve, in which 

 it is known that all partial sinking of the road, in consequence 

 of accumulated weight, is prevented by the stiffness of the 

 platform, which causes the weight to be distributed over a 

 considerable extent of the chain and vertical rods. 



It 



