Now, here we are dealing with an altogether different principle. The arch is made up of a number of comparatively small pieces of material bound together by mortar, or cement, or even clamps, and by the power of gravitation. We doubt if that idea is realised by half the people using the multitudinous arches abounding to-day; yet it is true. Or to put it in another way, the various parts are arranged so that they keep up each other by pressure. If you take two cards, or bricks, or slabs of stone and lean them together at the top, while the other ends may be far apart, you will find they will bear a certain amount of weight. Here you have the principle of the arch in its simplest form; and it may be that out of that primitive performance the arch has grown. This kind of triangular arch is to be met with in ancient structures in Great Britain. The flanks or haunches of an arch are its sides, from the first stone to the keystone; and the crown is its highest part; while the central wedge-shaped piece of stone or brick is called the keystone. The stones or bricks are cemented together when being built over a framework of timber, called the centering, and when the keystone is placed and the arch is complete it ought to remain firm. 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. “What are the distances?”