4 GIRDERS AND SUSPENSION CHAINS COMBINED. 245 
to the Londonderry Bridge, will not exceed ;/;th of the deflection of the same 
girder not attached to the chain. 
3rd. That theoretically the saving of metal to give equal strength in a 
suspension bridge is only one-half of that of a girder ; but as it can be made 
of great depth without practical difficulty, and as the deflection varies as the 
cube of the depth, a bridge on this principle of such span as the Londonderry 
Bridge may be made under average circumstances with at least one-fourth 
of the metal of an ordinary girder bridge having equal rigidity. 
- The results Nos. 1 and 2, although at variance with the general practice of 
engineers, are still in accordance with such experience as we possess. 
Suspension bridges, with a few exceptions, have been not only built of 
small depth without stiffening girders, either vertically or horizontally, but 
the points of suspension have not been fixed, but simply resting cn rollers, 
so as to give every facility for movement ; and thus arises the motion generally 
‘complained of in suspension bridges. 
_ Moreover, suspension bridges have been built without any rule or super- 
vision, and as they will bear their own weight, however lightly constructed, 
they have been in most cases of insufficient strength, many now existing 
‘not having 4th or 3th the strength given in the Derry Bridge. 
In a few cases where a girder has been used, the results accord with my 
experiments. The Niagara Bridge of 820 feet span has a girdey very little 
deeper than the Derry Bridge, and is built of timber only ; yet the deflection 
from a train is not more than 5 inches, as appears from the Report of Mr. 
Roebling ; an amount much less than my experiments would indicate, whea 
it is considered that the girder is of timber only. 
Another case is that of the Inverness Bridge, which has a wrought-iron 
parapet 3 feet 6 inches deep, and is nearly represented by the smal! wrought- 
iron model. 
_ This bridge has been subjected to the test of a locomotive passing over it 
_ ona truck drawn by fourteen horses, which produced so little deflection, as 
_ appears from the Report of Mr. Rendel, that a member of the Institution of 
Civil Engineers, when the subject was mentioned -at the recent discussion, 
pressed his doubt of the fact. 
_ It is however satisfactorily explained by the preceding experiments, which 
_ prove that such a parapet is sufficient to render a suspension bridge so nearly 
rigid that no deflection would be observable without measurement. 
__ There are other cases of suspension girder bridges, viz. the Montrose 
Bridge in Scotland, the Kief Bridge in Russia, and more recently the Chelsea 
_ Bridge over the ‘Thames at London, in all of which it is reported that objec- 
tionable movement is cured; and I am informed by Mr. Vignoles, the engi- 
r, that the Kief Bridge has been passed over by Russian artillery at a 
lop without any objectionable oscillation or deflection. In America sus- 
ion bridges have been used for aqueducts, the trough acting asa girder, 
the success of which proves that all vertical and horizontal oscillation has 
been cured. 
_ I will conclude my paper by remarking, that it has been necessary in the 
preceding investigation to make reference to the existing works of eminent 
engineers. I am desirous to observe that such comparisons have been 
essential to the elucidation of the question, and that I have no intention for 
one moment to detract from the engineering merit of these great works. The 
‘genius exhibited in overcoming the various difficulties which presented them- 
selves during their execution must be evident to all, but especially to those 
whose profession renders them acquainted with what had to be contended 
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