rom the great central tower; and,
again, the abutments stand at a distance from the side towers of 230
feet, giving the entire bridge a total length of 1849 feet,
corresponding with the date of the year of its construction. The side
or land towers are each 62 feet by 52 feet at the base, and 190 feet
high; they contain 210 tons of cast iron.

[Illustration: CONWAY CASTLE AND TUBULAR BRIDGE.]

The length of the great tube is exactly 470 feet, being 12 feet longer
than the clear space between the towers, and the greatest span ever yet
attempted. The greatest height of the tube is in the centre--30 feet,
and diminishing towards the end to 22 feet. Each tube consists of sides,
top and bottom, all formed of long, narrow wrought-iron plates, varying
in length from 12 feet downward. These plates are of the same
manufacture as those for making boilers, varying in thickness from
three-eighths to three-fourths of an inch. Some of them weigh nearly 7
cwt., and are amongst the largest it is possible to roll with any
existing machinery. The connexion between top, bottom, and sides is made
much more substantial by triangular pieces of thick plate, riveted in
across the corners, to enable the tube to resist the cross or twisting
strain to which it will be exposed from the heavy and long-continued
gales of wind that, sweeping up the Channel, will assail it in its lofty
and unprotected position. The rivets, of which there are 2,000,000--each
tube containing 327,000--are more than an inch in diameter. They are
placed in rows, and were put in the holes red hot, and beaten with heavy
hammers. In cooling, they contracted strongly, and drew the plates
together so powerfully that it required a force of from 1 to 6 tons to
each rivet, to cause the plates to slide over each other. The weight of
wrought iron in the great tube is 1600 tons.

Each of these vast bridge tubes was constructed on the shore, then
floated to the base of the piers, or bridge towers, and raised to its
proper elevation by hydraulic machinery, the largest in the world, and
the most powerful ever constructed. For the Britannia Bridge, this
consisted of two vast presses, one of which has power equal to that of
30,000 men, and it lifted the largest tube six feet in half an hour.

The Britannia tubes being in two lines, are passages for the up and down
trains across the Straits. Each of the tubes has been compared to the
Burlington Arcade, in Piccadilly; and the labour of pl