1 8 Plan for building an Iron Bridge across the Menai Strait. 



but in the usual waj- of driving cattle, there is never that quan- 

 tity together, that is, supposing the whole bridge covered from 

 end to end, and no void space left, which is a very unusual thing. 

 In driving cattle, nobody ever thinks of driving 200 head all in 

 a heap. 



[To Mr. Rennie.'] — What power of resistance beyond the pro- 

 bable weight that could be put upon such a bridge, by cattle or 

 otherwise, should you conceive to be necessary to bear up a bridge 

 of this description ? — I should not think it secure unless it was 

 capable of carrving at least four times its own weight ; for in- 

 stance, suppose the carrying itself was 300 tons, I should make 

 it capable of carrying four times that weight at least, in addition 

 to its own, that is, 1500 tons; but this proportion would not hold 

 if it was a light bridge. If you vvill allow me to state with re- 

 spect to lateral pressure, or rather lateral tension, I should think 

 there is no difficulty in that respect ; for in the bridge I have 

 constructed over the Thames, at Queen-street, the lateral pres- 

 sure of the side arches is about 3700 tons, which is infinitely 

 greater than any thing that would ever be wanted here. 



That being an arched bridge, how does the comparison hold? 

 — The pressure of an arched bridge is outward, the draft of the 

 other is inward, and I conceive it equally practicable to make a 

 pier to sustain the weight inward as outward ; and in the expe- 

 riments I have made, I have found that the force necessary to 

 crush the materials against which the iron acts, is between twenty 

 and thirty times the lateral pressure. I made a set of jexperi- 

 ments to ascertain the fact as to the particular kind of stone used 

 in the abutments and piers. 



Did you ever see the chain bridge over the Tees ?^ — I never 

 saw it. When the Bell Rock light-house was building, which is 

 about ten miles out at sea, a smith's forge and other matters were 

 erected on a different part of the rock to which the light-house 

 was ; and, in order to have a communication for the heavy ma- 

 terials from the forge to the light- house, we had one of those 

 bridges, which answered very well. 



What length was it? — Fifty or sixtv feet; I do not know ex- 

 actly, but it was full that, I know. I would observe, that this 

 bridge being to be covered with either planks or wood, it makes 

 a single plank 522 feet long and 30 feet wide, from which I con- 

 ceive that the shake can scarcely be anything sideways. 



Do you entertain any doubts with respect to the practicability 

 of constructing such abridge as that which is proposed by Mr. 

 Telford, according to the plan which he has just stated to the 

 Committee ? — I have no doubt of the practicability of construct- 

 ing such a bridge 3 and I presume that Mr. Telford has taken' 



care 



