“Well, I wish you good luck, friend Edwards, for we need a bridge sorely over the brawling Taff.” “You shall have it, neighbour. I shall succeed this time. I have gripped the right principle at last.” He had indeed, for the bridge he then built lasts to this day. It was the famous Pontypridd bridge over the Taff on the Llantrissant and Merthyr road, and was called the Pont y du Prydd, or the bridge by the earthen house, for a mud hut stood near. [Illustration: PONTYPRIDD BRIDGE. _From Encyclopædia Britannica._ ] About the year 1745 it was determined to build a bridge over the rushing Taff, and William Edwards, a self-taught mason of the country, undertook the task. The first bridge he built was of three arches, which, in less than three years, was dashed away by a great flood. The water rose so high as to surge over the parapet. 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.