1848.] Construction of Iron Tension Bridges. 419 



18. 2ndly. The platform being wholly supported by the action of 

 gravity, the equilibrium of the system is disturbed by the most trivial 

 causes, the transit even of a single foot passenger over a bridge of 200 

 feet span produces a sensible vibration, whilst the motion of heavy 

 bodies is attended by effects actually injurious to the structure, and it 

 may therefore be readily conceded, that the effects of storms is very 

 much to be dreaded, of which the Menai, the Brighton Pier and Mon- 

 trose bridges are instances. 



19. Few, if any suspension bridges on the uniform system are 

 constructed on any very close calculations of the strength of the differ- 

 ent parts ; generally a very wide margin is allowed over and above the 

 power required by calculation ; thus the Menai bridge is equal to a 

 permanent load of nearly 400 tons above the weight of suspended 

 roadway, added to a full load of 75 ibs. per square foot ; and the bridge 

 at Montrose is equal to nearly 100 tons in excess of the entire load to 

 which it can be subjected, yet notwithstanding this excess of strength 

 in actual section of iron in the chains, these bridges have been in 

 imminent danger of total destruction when unloaded, from what may 

 safely be called the defects of construction ; surely nothing need be 

 added to show the inexpediency of providing a vast excess of strength 

 in any structure to meet a dead weight which it can never be subjected 

 to, and at the same time leave it unprotected to encounter the danger 

 of disruption to which at any hour it may be exposed from natural 

 causes ? 



The lately constructed bridge at Hungerford Market over the 

 Thames, 676" feet span, has a sectional area of 312 square inches, 

 and as the actual tension on the chains, even with the enormous as- 

 sumed weight of 170 ibs. per square foot of platform, could not exceed 

 1420 tons which @ 9 tons per square inch, requires 156 square inches, 

 there is exactly double the section or strength necessary for the struc- 

 ture. 



Resultant System. 



20. I will now proceed to explain a system which only proposes to 

 do what the formulae in Mr. Pratt's Memoir says may be done, which is 

 based on the experience and research I have above noticed, and which 

 proves what it engages to do, in a manner, I trust, unexceptionable. For, 



3 K 



