G.—ENGINEERING 127 
If overloaded, long before rupture of the chains could occur, the excessive 
sagging or deformation of the platform would act as a warning. Where 
the ultimate strength depended mainly on the resistance to compression 
of the platform, as in Dredge’s bridge and others built about the 
same time, the failure by the buckling of the platform would be 
sudden and disastrous. The failure of Dredge’s bridges, in spite of 
the confidence with which they were recommended, created a strong 
prejudice against suspension-bridges of all kinds and retarded their 
development. 
One of the early suspension-bridges still in use, is that across the Thames 
at Marlow, built by W. Tierney Clark, F.R.S., in 1829. I examined 
and reported on this bridge some years ago and found it in a remarkably 
good state. In the development of the stability of suspension-bridges 
this one is of particular interest, for it was the first built with stiffening 
girders. The ends of the cross girders in this bridge are all stiffly 
connected by parapets made in the form of girders, and any cross girders 
on which a heavy load might rest cannot deflect the suspension chain, as 
it would do if the parapet girders were not there. 
In the modern suspension bridge the stiffening girder is as important 
a feature as the chain or cable, and its introduction has made it possible 
to construct the gigantic bridges in the United States. The interaction 
of the stiffness or flexibility of the girder with the curvature of the 
suspension cable, is the governing factor in the stability of the modern 
suspension bridge. It is a problem of considerable complexity and the 
calculations are laborious. 
It would be difficult to find more striking evidence of the advances 
made in the hundred years the British Association has been doing its 
work advancing science, than is provided by the comparison of great 
suspension bridges of to-day with the early ones I have referred to. The 
latest example with its span of 3,400 ft. and others of more than 1,500 ft. 
compare with Telford’s of 570 ft. and the others of 50 to 200 ft. Cables 
composed of thousands of steel wires, four times as strong as iron, laid 
side by side to form cables 3 ft. in diameter, take the place of the iron 
chains ; and the flexible timber platform, so easily deformed by moderate 
moving loads, is now replaced by deep steel stiffening girders with upper 
and lower decks, providing double tracks for both electric railways and 
street trams and road width for many cars. Instead of the stability of the 
structure being a matter of dispute, and of the extraordinary uncertainty 
I have tried to describe, the stability is now determined and gauged by 
calculation, some of which is very elaborate; but it is based on applied 
mechanics. 
The important aspect of stability which is governed by the character- 
istics of the materials—their resistance to compression and tension and 
to the repeated application of stress—on which economy of construction 
depends, I have not dealt with. That side of the problem has not ad- 
vanced as much as might have been expected. Engineers of a hundred 
years ago had much more confidence in their materials than we appear 
to have. In many of their structures, which are still standing and which 
