244 REPORT—1857. 
will show a comparatively better result than with the entire bridge loaded, 
but not to any sensible amount, as the same property which renders the 
suspended girder rigid will prevent the movement of the point of suspension. 
The weight on one opening will create a disposition to straighten the chain 
in the adjoining opening, which will be resisted by the girder so effectually 
from being united with it, that little motion of the point of suspension will 
occur, even if no assistance were given by the tower. 
We may make a similar comparison deduced from other large girders, of 
which the next largest actually erected is the Boyne Viaduct: here the 
Spin is) ) yee) tay OL. SL BGA feet. 
Weight of effective metal 300 tons. 
To find a girder of equal depth and rigidity cf 440 feet span, we have 
as— 
264° : 440% :: 300 : 1388 tons; the weight of a girder being continuous 
that would deflect 1:9 inch with 540 tons all over, or about two-thirds of the 
rigidity of the Londonderry Bridge. 
The Boyne Viaduct thus indicates a much more favourable result than 
the tube; and, as the system would admit of greater depth, much less metal 
would suffice for this span. 
A similar deduction may be made from the Newark Dyke Bridge, which 
has— 
a 
Span of opening. . . . 240 feet. 
Weight of metal. . . . 244} tons. 
Here we have as— 
240° : 440% :: 2444 : 1506 tons, the weight required to construct a girder 
that will deflect ox inches with 240 tons, and indicates also a more favourable 
result than the solid-sided girder, but not equal to the Boyne Viaduct. 
I must not conclude these comparisons without referring to Mr. Brunel's 
new system of combining an arch and a suspension chain, giving each half 
the duty. 
There is no doubt, in the case of the proposed Londonderry Bridge, if the 
chain was reduced to half the section, and an arch of the depth of the chain 
was substituted. and the suspension rods extended to the arch, that theoretically 
with the same metal there would be equal strength and rigidity ; but the real 
difficulty is the impracticability of such a construction: the metal in an arch 
of 451 feet span and 80 feet rise cannot be measured by the section as ina © 
chain, from the tendency to buckle, and from having to contend with its 
own weight. 
Thus in the Saltash Bridge, which is now in course of construction on this 
principle of 451 feet span, the depth is only 56 feet, or little more than 3rd 
of the Londonderry Bridge if of that construction, and thus nearly three 
times the metal is required to give equal strength, and nearly nine times to 
give equal rigidity, from the deflection varying as the cube of the depth. 
It will be observed that there will be no difficulty in giving even a greater 
depth to a suspension bridge ; the vertical pressure or weight of the bridge is 
small compared with the pressure on the arch of Mr. Brunel’s girder, and as 
the height is only 88 feet no practical difficulty arises. 
Concluding Observations. 
The important practical results of the preceding experiments are:— 
lst. That in suspension bridges it is essential that the platform should be 
stiffened with a girder to prevent vertical undulation. 
2nd. That the deflection of the wave of a girder attached to a chain simile 
