Had this stupendous fabric existed in ancient times, it would have been regarded as the first of the seven wonders of the world. Greater and more expensive structures have been raised, but none displaying more science, skill, and ingenuity, and none requiring such tremendous mechanical power to execute.

The Britannia Tubular Bridge was built to conduct the Chester and Holyhead Railway across the Menai Straits, to the island of Anglesea, in the Irish Sea.

The difficulties which the engineer had to overcome, were greatly augmented by the peculiar form and situation of the straits. Sir Francis Head says, "The point of the straits which it was desired to cross, although broader than that about a mile distant; preoccupied by Mr. Telford's suspension bridge—was of course one of the narrowest that could be selected, in consequence of which the ebbing and flowing torrent rushes through it with such violence, that, except where there is back water, it is often impossible for a small boat to pull against it; besides which, the gusts of wind which come over the tops, down the ravines, and round the sides of the neighboring mountains, are so sudden, and occasionally so violent, that it is as dangerous to sail as it is difficult to row; in short, the wind and the water, sometimes playfully and sometimes angrily, seem to vie with each other—like some of Shakspeare's fairies—in exhibiting before the stranger the utmost variety of fantastic changes which it is in the power of each to assume." The Menai Straits are about twelve miles long, through which, imprisoned between the precipitous shores, the waters of the Irish Sea and St. George's Channel are not only everlastingly vibrating, backwards and forwards, but at the same time and from the same causes, are progressively rising and falling 20 to 25 feet, with each successive tide, which, varying its period of high water, every day forms altogether an endless succession of aqueous changes.