.

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.

THE TUBES.

The tubes forming the viaducts, rest upon two abutments and three piers,
called respectively the Anglesea abutment and pier, the Carnarvon
abutment and pier, and the Britannia or central pier, built upon the
Britannia rock in the middle of the straits, which gives name to the
bridge. The Anglesea abutment is 143 feet 6 inches high, 55 feet wide,
and 175 feet long to the end of the wings, which terminate in pedestals,
supporting colossal lions on either side, 25 feet 6 inches in length, 12
feet 6 inches high, and 8 feet broad, carved out of a single block of
Anglesea marble. The space between the Anglesea abutment and pier is 230
feet. This pier is 196 feet high, 55 feet wide, and 32 feet long. The
Carnarvon abutment and pier are of the same dimensions as those above