S.H.1825

DESCRIPTION

OF THE

IRON BRIDGES OF SUSPENSION

NOW ERECTING

OVER THE STRAIT OF MENAÏ, AT BANGOR,

AND

OVER THE RIVER CONWAY, IN NORTH WALES ;

WITH

TWO VIEWS:

ALSO SOME ACCOUNT OF THE

DIFFERENT BRIDGES OF SUSPENSION

IN ENGLAND AND SCOTLAND;

PARTICULARLY OF CAPTAIN S. BROWN'S IRON BAR BRIDGE OVER

THE RIVER TWEED.

WITH

Remarks on the proposed Suspension Bridge

OVER THE RIVER MERCY, AT RUNCORN GAP;

AND

SOME CALCULATIONS OF THE STRENGTH OF MALLEABLE IRON,
FOUNDED ON EXPERIMENTS.

S. GOSNELL, Printer, Little Queen Street, London.

A SIMPLE and economical principle of constructing bridges has lately been adopted, with success, in this country; which appears to be much superior to any other for wide spans, and peculiarly adapted for large navigable rivers. These advantages only require to be known, in order to be duly appreciated in a commercial nation, and, therefore, a considerable degree of interest has been manifested respecting the large works on this principle, which are now in progress; and also an account of the works already finished, is much desired. In this publication we propose to describe both the works in hand, and those which are completed; and also to notice the most extensive project of the kind that has yet been proposed. But as no popular illustration of the nature and advantages of the principle has yet appeared, perhaps a few re

marks on the subject will be useful, previous to entering on the description of the Bridges.

In the common principle of construction, an arch, with its convex side upwards, constitutes the support of the bridge. The weight of the arch itself, and of the load it supports, tend to compress it into a flatter curve.

In the newly-adopted principle a chain constitutes the support of the bridge; it forms a curve like the arch, but the convex side of the curve is, of course, downwards; and the weight tends to draw the chain apart. Any material of a durable kind, that is capable of resisting great tension, is applicable to construct bridges of this kind, and they are appropriately called, Suspension Bridges.

The mere difference of employing the tensile power of a material, instead of its resistance to compression, does not, however, present any advantage; in fact, a greater force, per square inch, is required to crush wrought iron, than to draw it asunder; therefore, the real advantage of employing the principle of suspension still remains to be explained.

In the common bridge, where the weight tends to compress the arch; if that arch were made only of a small depth, a slight force would derange it, and once deranged in a certain degree, it would fall. Hence in a bridge of this kind, the arch must necessarily have a considerable depth to render it stable, and sufficiently firm for resisting the stress of a loaded waggon; but this increase of the depth of the arch, to give stability, also increases the load to be supported, and, consequently, a much greater quantity of material must be employed than is necessary to resist the pressure in the direction of the arch.

In a suspension bridge it is different, for to whatever degree the equilibrium of the chain be disturbed, it resumes its natural state as soon as the disturbing force ceases to operate. That is, it is endowed with the power of resisting any change of form by its arrangement, and no excess of material is necessary to preserve that arrangement. This is the chief advantage of a suspension bridge, and it is one of immense