There are two tubes, to accommodate a double track (one would have done
in this country, but in England they do nothing by halves), and each is
1513 feet long. The total length of the bridge is 1841 feet. These tubes
are not round or oval, but nearly square at the termini; the bridge
being constructed on the principle of the arch. A section of one of the
tubes at the Britannia pier is in the form of a parallelogram, where it
is 30 feet high, gradually diminishing towards each end to 20 feet. The
tubes are riveted together into continuous hollow beams; they are of
the uniform width of 14 feet 8 inches throughout; they are constructed
entirely of iron, and weigh about 12,000 tons, each tube containing 5000
tons of wrought iron, and about 1000 tons of cast iron. The tubes were
constructed each in four sections; the sections extending from the
abutments to their corresponding piers, each 250 feet long, were built
_in situ_, on immense scaffolding, made of heavy timbers for the
purpose, even with the railway; but the middle sections, each 470 feet
long, were built on piers on the Carnarvonshire shore, then floated into
the stream, and elevated to their position; each of these sections
weighed 1800 tons.




CONSTRUCTION OF THE TUBES.


The sides, bottom, and top of these gigantic tubes are formed of oblong
wrought iron plates, varying in length, width, and thickness, according
to circumstances, but of amazing size and weight. They are so arranged
as to obtain the greatest possible strength, the whole being riveted