1848.] Construction of Iron Tension Bridges. 413 



own experience, and its demonstration is so clear, that I have been 

 induced from the wish to promote the advancement of such structures, 

 to place the following exposition of my system on record, feeling sure 

 that unbiassed minds will, on perusal, be divested of the timidity with 

 which the extreme, or Dredge's Taper Chain system has been received, 

 as its errors have been admitted and corrected ; whilst, if there be any 

 virtue in the present uniform chain system, the proposed " Resultant" 

 will be found to possess them in an eminent degree, and yet freed from 

 its acknowledged defects. 



The fact demonstrated in the above named fc Memoir" is simply this, 

 that in all Iron Suspension Bridges of equal span, and breadth of 

 platform, the quantity of Iron in the main parts must be the same, and 

 that quantity which " is necessary to enable each part to sustain the 

 greatest tension to which it may be subjected when the roadway is 

 loaded to the greatest extent, is altogether independent of the principle 

 of construction or form of the Bridge," provided of course that the 

 principle be sound. 



2. This is a very important conclusion, but whilst I freely admit 

 the soundness of the doctrine, I am not fully satisfied as to the correct- 

 ness of the writer's practical deductions therefrom, viz. that the 

 old system of suspension, consisting of a uniform chain and vertical 

 drop-bars, is the most proper for adoption under all circumstances. For 

 such an opinion the author of the above " Memoir" gives his reasons, 

 which, as might have been expected, are weighty enough, but " good 

 reasons must per force give way to better," and notwithstanding what 

 has been advanced above, I think the scale may yet be turned in favor 

 of the opposite opinion, viz. that the old, or uniform chain system is 

 by no means necessarily, and under all circumstances the most desire- 

 able for adoption. 



3. If the strength or stability of a structure to resist a constant 

 dead weight, were alone the points for consideration, the advantages 

 adduced in favor of the uniform chain system might be conclusive ; but 

 wherever failures of Suspension Bridges have occurred, they have in 

 almost every case been caused not by a steady, uniform dead strain, 

 exceeding the power of the materials to resist, but by the effect of a 

 much smaller load or weight in a state of motion. Not, for instance, 

 during a trial by means of a proof load uniformly distributed, but by 



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