416 A Resultant System for the [Oct. 



Taper Chain Bridge, that the result has been an encouragement to 

 combine the Taper Chain with the uniform system, possessing in con- 

 junction the advantages of each, with the positive defects of neither, 

 and which I will presently explain, after glancing at the evils which are 

 acknowledged to exist in both the above principles. 



9. The most important fact gleaned from the above experience and 

 research is one entirely overlooked by Mr. Dredge, viz. that where 

 strength or section of Iron is taken away from the chains, it should be 

 made good in the Longitudinal Beams to which they are connected. 

 Not that the precise quantity abstracted from the former should be 

 added to the latter, but that additional strength should be given to the 

 beams bearing a certain ratio to that taken from the chain. Mr. 

 Dredge, and the uniform chain system, afford instances of opposite 

 extreme cases. In the former, the section of the outer longitudinal 

 beams at the centre, where the chains are a minimum, should be nearly 

 equal to the entire section of the chains at the point of suspension, the 

 portion of beam in the centre of the bridge standing in place of the 

 chain theoretically, and almost so in practice ; in fact the longitudinal 

 beam is an indispensable item in the Dredgeian combination, whereas 

 in the uniform system the reverse is the case, for by the non-diminution 

 of the chain in the centre, there is no absolute necessity for the longi- 

 tudinal beam as a component portion of construction. 



10. The principal defects of Mr. Dredge's extreme Taper system are, 

 1st. The hazard of trusting a bridge, whatever the span may be, to 



the strength of one, or even two rods at the centre, for (admitting for 

 the sake of argument, that the section there may not be disproportion- 

 ed to the strain) yet the fracture of the link in the centre, (and being 

 so slender there is the greater probability of such an event there than 

 elsewhere) would be attended with very dangerous results ; the conclu- 

 sion therefore to be drawn from the admitted inexpediency of confiding 

 in the strength of so small a section of iron in the very centre of the 

 bridge is, that the chain should not diminish so rapidly as, in the 

 extreme Taper system, it does. 



1 1 . 2ndly. As noticed above, the section of iron in the longitudi- 

 nal beams is uniformly weak throughout with reference to the tension 

 at the centre, which, where the beam comes in place of the chain, is 

 infinitely great, as compared with that exerted near the standards. 



