150 Mr Buchanan's Report on the Bridge of Suspension 
thick or more. The weight of the iron-work in the plates, 
frames, side-rails, suspending amd cross rods, and, in short, all 
the iron-work supported by the chains together, I estimate at 
180 tons, and the weight of four inches thickness of gravel at 
220 tons. 
The greatest load that can ever be laid on the bridge is when 
it is crowded with people, and I estimate the utmost number that 
it will hold at 7000, which is more than half the whole inhabi- 
tants of the town. Suppose each person 150 lb., or 12 stones 
and upwards, the total will be nearly 4?0 tons weight. The 
weight of the chains themselves will be 100 tons, so that, on the 
whole, in the most extreme case that can occur, we shall have 
loading and Straining every inch of the chains,— -viz. 
Iron in roadway and in suspending rods, 180 tons. 
Gravel, - 220 
People, - » - - 470 
Chains, - - - - 100 
Total, 970 tons. 
We shall be enabled to lay this great load upon the chains 
with perfect safety, from the circumstance that, owing to the 
deepness of the curve, this load will produce no undue strain 
upon the arch ; it will just stretch the chains as if they had been 
hung perpendicularly, and the weight suspended by their extre- 
mities. In the middle of the arch, indeed, the strain on the 
chains will scarcely be so much as this. This advantage is ow- 
ing entirely to the height of the towers ; for had these been so 
low as those of the Tweed Bridge, compared with its span, the 
weight of the roadway would have produced on the chains, ow- 
ing to the hatness of the arch, a strain almost double that of 
their natural weight, and the carriages and people would all of 
them have produced an augmented strain in proportion ; so that, 
instead of 1000 tons, w r e should have had a strain of nearly 2000 
tons on every inch in the length of the chains from pillar to pil- 
lar, and from thence to the ground *. 
® This circumstance of the strain on the chains varying with the depth or 
flatness of the arch, even though the natural weight hanging upon them be in 
either case the same, arises, it is well known, from the principle of oblique 
action. 
When 
