The general principle of the suspension bridge is exemplified in a chain hanging between two fixed points on the same level. If two chains were placed parallel to each other, a roadway for a bridge might be formed by laying planks across the chains, but there would necessarily be a steep descent to the centre and a steep ascent on the other side. And it would be quite impossible by any amount of force to stretch the chains into a straight line, for their weight would always produce a considerable deflection. Indeed, even a short piece of thin cord cannot be stretched horizontally into a perfectly straight line. It was, therefore, a happy thought which occurred to some one, to hang a roadway from the chains, so that it might be quite level, although they preserved the necessary curve. In designing such bridges, the engineer considers the platform or roadway as itself constituting part of the chain, and adjusts the loads in such a manner that the whole shall be in equilibrium, so that if the platform were cut into sections, the level of the road would not be impaired.
Public attention was first strongly drawn to suspension bridges by the engineer Telford, who, in 1818, undertook to throw such a bridge across the Menai Straits, and the work was actually commenced in the following year. The Menai Straits Suspension Bridge has been so often described, that it will be unnecessary to enter here into a lengthy account of it, especially as space must be reserved for some description of other bridges of greater spans. The total length of this bridge is 1,710 ft. The piers are built of grey Anglesea marble, and rise 153 ft. above the high-water line. The distance between their centres is 579 ft. 10½ in., and the centres of the main chains which depend from them are 43 ft. below the line joining the points of suspension. The roadway is 102 ft. above the high-water level, and it has a breadth of 28 ft., divided into two carriage-ways separated by a foot-track. The chains are formed of flat wrought iron bars, 9 ft. long, 3¼ in. broad, and 1 in. thick. In the main chains, of which there are sixteen, no fewer than eighty such bars are found at any point of the cross section, for each link is formed of five bars. These bars are joined by cross-bolts 3 in. in diameter. The main chains are connected by eight transverse stays formed of cast iron tubes, through which pass wrought iron bolts, and there are also diagonal ties joining the ends of the transverse stays. The time occupied in the construction was 6½ years, and the cost was £120,000. This bridge has always been regarded with interest for being the first example of a bridge on the suspension principle carried out on the large scale, and also for its great utility to the public, who, instead of the hazardous passage over an often stormy strait, have now the advantage of a safe and level roadway.