It must have been a sore disappointment to the hard worker to see his structure suddenly swept to ruins. But he was a shrewd, common-sense, observing man, and, nothing daunted, he tried again. This time he determined to build one bold arch of 140 feet. The object was to obviate the necessity of raising piers for more arches, and so obstructing the water; these former piers having caused, or assisted in causing, the destruction of his first bridge.
But the second gave way from the proportionally heavy weights on the haunches, as Edwards, we imagine, told his friend, and once more he had to face ruins. Yet a third time he tried, and the third time he was successful. Generations have come and gone, the children who played about its abutments have grown grey and have passed away, but still the country mason’s bridge of 140 feet span stands its ground and serves the community.
He reduced the heavy weight on the sides by making openings in the spandrels—that is, the part above the curve of the arch; while, instead of filling up the interior space with rubble, he used charcoal. But the arch is very steep, and a chain and drag is kept to assist any horse when descending.
These bridges illustrate the principle of the arch. Passing by the fact that it is evidently safer to span a swelling river by a bridge of wide, rather than of several narrow arches, three powers or forces act on the row of stones or bricks forming the arch. There is first the force that would carry the stone downward—that is, the force of its own weight and of anything that might be placed upon it. But then there are stones or bricks pressing against it on either side, and in its turn it presses upon them. When, therefore, every part presses equally, one not heavier or weaker than the others, a support for all is gained by the contiguous pressure and by the balance of forces.
Long bridges were sometimes built in this way, and the longest in England in the Middle Ages was at Burton, over the Trent. It was 1545 feet long, and had 36 arches. It was not superseded till 1864, when a new bridge was built.
In an arched bridge, the higher it rises in proportion to the width of the arch, the easier is its construction, and the less is the stress upon its parts; moreover, any inaccuracy in design or in building is likely to be less harmful. We are not surprised, therefore, that Edwards, in his third attempt, decided upon that form.
One of the widest arches in the world is that of the famous Grosvenor Bridge at Chester. It has a span of 200 feet, with a rise of 42 feet. An arch, however, in the Washington Aqueduct extends to 220 feet span, while the central span in the Southwark Bridge, designed by Rennie, is 240 feet. This last, however, is of cast-iron.
The principle of the arch, however, does not appear first in the history of bridge building. Bridges are as old as mankind; that is, no one knows when first men began to cross streams and chasms by placing the trunk of a tree from one side to the other, and thus bridging the gulf.
Then, possibly, the next step was to build up a pile of stones in the centre of the stream—taking the stones there by coracle or canoe—and placing a tree trunk from the side to the central heap.
Yet another development would most likely be a simple cantilever bridge—though these early builders would not have known that Frenchified word. But they knew that after embedding a tree trunk firmly on each side of the bank so that a considerable portion should project over the stream, they could place a third log from one end to the other, and thus get a bridge much longer than when made of one tree trunk alone.