ought-iron plates, varying in
thickness from a quarter of an inch to one inch, riveted together, and
strengthened by irons in the form of the letter T; and, to give
additional strength to the whole, a series of cells is formed at the
bottom and top of the tube, between an inner ceiling and floor and the
exterior plates; the iron plates which form the cells being riveted and
held in their places by angle irons. The space between the sides of the
tube is 14 feet; and the height of the whole, inclusive of the cells, is
22 feet 3-1/2 inches at the ends, and 25 feet 6 inches at the centre.
The total length of the tube is 412 feet. One end of the tube is fixed
to the masonry of the pier; but the other is so arranged as to allow for
the expansion of the metal by changes of the temperature of the
atmosphere, and it therefore, rests upon eleven rollers of iron, running
upon a bed-plate; and, that the whole weight of the tube may not be
carried by these rollers, six girders are carried over the tube, and
riveted to the upper parts of its sides, which rest upon twelve balls of
gun-metal running in grooves, which are fixed to iron beams let into the
masonry.

The second of these vast railway bridges crosses the Menai Straits,
which separate Caernarvon from the island of Anglesey. It is constructed
a good hundred feet above high-water level, to enable large vessels to
sail beneath it; and in building it, neither scaffolding nor centering
was used.

The abutments on either side of the Straits are huge piles of masonry.
That on the Anglesey side is 143 feet high, and 173 feet long. The wing
walls of both terminate in splendid pedestals, and on each are two
colossal lions, of Egyptian design; each being 25 feet long, 12 feet
high though crouched, 9 feet abaft the body, and each paw 2 feet 1
inches. Each weighs 30 tons. The towers for supporting the tube are of a
like magnitude with the entire work. The great Britannia Tower, in the
centre of the Straits, is 62 feet by 52 feet at its base; its total
height from the bottom, 230 feet; it contains 148,625 cubic feet of
limestone, and 144,625 of sandstone; it weighs 20,000 tons; and there
are 387 tons of cast iron built into it in the shape of beams and
girders. It sustains the four ends of the four long iron tubes which
span the Straits from shore to shore. The total quantity of stone
contained in the bridge is 1,500,000 cubic feet. The side towers stand
at a clear distance of 460 feet f