238 REPORT—1857. 
On the Mechanical Effect of combining Girders and Suspension Chains, 
and a comparison of the weight of Metal in Ordinary and Suspension 
Girders, to produce equal deflections with a gwen load. By PETER 
W. Bartow, F.R.S. 
My attention has been recently directed to this subject from having been 
required to investigate, as engineer of the Londonderry and Enniskillen, and 
Londonderry and Coleraine Railways, the best mode of effecting a junction 
between the lines at Londonderry, to be combined with an improved road 
communication, for which an Act has been obtained by the Corporation of 
the city ; and the Commissioners having determined to advertise for plans, 
leaving the decision to Sir William Cubitt, an engineer justly occupying a 
position so eminent, and in whose judgment I had the greatest confidence, I 
determined to submit the result of my investigation to him, although the 
principle which I concluded would best meet all the circumstances of the 
case, viz. the suspension girder, was one with reference to which consi- 
derable prejudice had existed. 
Sir William Cubitt, after devoting much attention to the subject, has fully 
sanctioned the principle, and recommended the Bridge Commissioners to 
carry out ny design, with some modifications suggested by him. 
In order to verify my calculations, I have caused a series of experiments 
to be made, the results of which are of so much practical importance, and so 
fully confirm my investigations, that I determined to lay them before the 
British Association, in order that the simple question of the mechanical effects 
of combining a girder with a suspension chain, on which no difference of 
opinion ought to exist, should be fully decided ; but before describing these 
experiments I will make a few general remarks upon the systems which have 
been adopted in bridge constructions. 
General remarks upon the construction of Bridges of large span—Bridges 
may be divided into three classes :-— 
Ist. The Arch, astructure in which the supporting material is subjected to 
compression alone, but which contains no rigidity in itself. 
2nd. The Suspension Bridge, in which the supporting material is sub- 
jected to extension alone, which also contains no rigidity in itself; and 
3rd. The Girder, in which the material is subjected to both extension and 
compression, of which there are two varieties; one, which is subjected to 
diagonal strains, as the lattice, Warren, and tubular girders; and a second, 
in which all the strains are confined to the upper and lower webs, as in the 
bow and string; and Mr. Brunel’s new girder, which is a combination of 
an arch and a suspension chain, each doing half the supporting duty. 
This second variety is the most simple form, but has no more rigidity in 
itself than an ordinary arch or suspension bridge. 
Of these three systems, the girder necessarily requires, from combining 
compressive and extensive resistances, a much larger amount of metal than 
either of the other systems, which will be rendered evident by a simple inves- 
tigation, and by reference to existing structures. 
In an ordinary arch the compressive force is resisted by the abutments, 
which in no way add weight or strain to the metal ; but if the arch is converted 
into a girder, it can only be done by adding a tie-bar, the arch having then 
to support its own tie or substitute for an abutment, in addition to its own 
weight. 
In a suspension bridge the tensile force is resisted by back chains, and 
if these are taken away to make it a girder, a compression-tube or bar has to 
be used as a substitute for them (as in the Chepstow bridge), which tube 
