But should too great a weight fall on the crown the bridge will fall outwardly at the sides, and the crown will sink; while, curiously enough, if it be built with too little weight on the crown, it will be, as it were, forced upwards, and the sides will fall inwards, as in the case of the second of the famous Pontypridd bridges, which actually did this. The material in the middle of the arch was less in proportion than that over the sides or “haunches,” and these heavier weights on the sides caused the crown to be forced upwards.
Two causes combined to make changes in bridge building. These were the needs of railways and the introduction of iron as a building material. The first iron bridge was constructed over the Severn, near an appropriately named place, Ironbridge, in 1779. It had an arch of near upon a hundred feet span.
When, however, very wide span bridges were required, the question arose of the superiority of wrought-iron over cast-iron for such structures. The Menai Strait had to be crossed for the Chester and Holyhead Railway, and the greatest existing cast-iron span was Rennie’s Southwark Bridge, where 240 feet had been reached. But over the Conway and the Menai Strait, spans of 400 feet were involved. How were these yawning gulfs to be bridged?
CHAPTER II.
A NEW IDEA—THE BRITANNIA TUBULAR.
“We must cross the Strait at the Britannia Rock—that is settled.”
“And where is the Britannia Rock?”
“Nearly in mid-channel. It seems placed there for the purpose.”
And the great engineer smiled.