a Hanging Iron Bridge across the Menai Strait. i7 



continued at nearly the same angles as those of the bridge, to 

 the cast-iron frame, which embraces about 12,000 tons of ma- 

 sonry, and to which much more if necessary might be con- 

 nected. 



" With regard to any change bv contraction or expansion, it 

 is known from experiments, that with a difference of tempera- 

 ture of 90 degrees of Fahrenheit, the difference of length of iron 

 would only be tttI-ooT)'} ^^ about five inches upon 700 feet ; and 

 as the iron-work would most likely be put up at a mean tempe- 

 rature, the contraction would be two and a half inches, and the 

 expansion an equal (]uantity, which would not derange the bridge; 

 but if the main suspending cables were covered with some sub- 

 stance, which was an imperfect conductor of heat, and which is 

 intended, the above variation of 90 degrees of temperature could 

 not take place. 



" I have thus, for the satisfaction of the Committee, stated 

 the principal circumstances relative to this plan, and which have 

 induced me to recommend it. The numerous and tedious details 

 which are connected with such a work, I presume the Committee 

 do not expect me to go into here ; and as the having a suitable 

 and substantial bridge is the sole object in view, I shall most 

 thankfully receive any useful improvements that may be suggested 

 by others. (Signed) Thomas Telford. 



" Dated London, 23d April 1819." 



Mr. Rennie, on being asked if he had seen any instances of 

 bridges upon this construction of a large size ? answered, No, I 

 have not ; the only thing of that kind I have seen is a model that 

 was made by Captain Brown, who is an iron cable manufacturer 

 at Mill Wall, Isle of Dogs, of a bridge nearly of this construction, 

 of 120 feet span, and over which I was drawn in a carriage, and - 

 found myself perfectly safe and easy. 



Where was that bridge erected ? — At Captain Brown's manu- 

 factory, Mill Wall, Isle of Dogs, on the land, merely as an ex- 

 , periment ; and 1 suppose it is standing yet, where I believe it 

 may be seen at this day; I came out of the carriage, and made 

 the coachman drive several times over it, that I might see how it 

 acted. 



There was no vil)ration ? — Very little viliration. ,» 



Can you calculate the weight that would be distributed on such 

 a bridge as this of Mr. Telford's, supposing it was filled as full 

 as might be with a drove of oxen ? — I cannot answer that (jucs- 

 tion off hand. 



[lo Mr. Telford.] — Can you answer that (luestion? — The 

 weight, as far as I can calculate, of covering it with oxen will be 

 about three hundred tons, it dc|)ends on the weight of the cattle; 



Vol.54.No. 255. Jw/y 1819. B but 



