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 as light as possible, as it was known that the strength of the bridge depended on its lightness. This fact sounds rather paradoxical; but if the reader will reflect a moment he will find that a bridge has to support itself, as well as the things passing over it. A beam of solid iron, of the dimensions of the Britannia Bridge, would be useless if placed across the Straits, as it would infallibly break down under the enormous pressure of its own weight. Stephenson's beam, as we have already intimated, has all the elements of strength, but none of the elements of weakness of a common beam.

While the monster tubes were being constructed, the masons were heaping up sandstone and marble into the huge piers upon which they were to rest. The central pier or tower was built upon a little rock in the middle of the stream. This rock, which was only exposed at low-water, had long been a trouble to sailors and nothing else, but it is now world-famous as the Britannia Rock, the chief support of Stephenson's magic aërial galleries. Two other piers were constructed, one on the Anglesey, and the other on the Carnarvon shore, each at a distance of 472 feet from the Britannia tower.

The bridge was to consist of two tubes, placed side by side, one for the down and the other for the up trains. Each tube was formed in four lengths, and when completed these lengths had to be joined together, like the pieces of a huge dissected puzzle. A huge puzzle, indeed! When these immense tubes