This wonder of modern engineering, a view of which is given in the cut,
forms part of the railroad from Chester to Holyhead, and is thrown over
the Menai strait which separates Caernarvon, in Wales, from the island
of Anglesey. This strait appears to the eye as a beautiful river, half a
mile wide, through which the tide, which here rises twenty and
twenty-five feet, rushes with great rapidity and force. The tubular
bridge over it is one hundred feet above high-water level, and formed of
long, hollow, rectangular tubes, one for up, and the other for down
trains, composed of wrought-iron boiler-plates riveted together, and
resting on huge and massive towers of masonry. Of these tubes or
galleries, eight in number, four for each line, the four shortest are
each two hundred and thirty feet, and the four longest each four hundred
and seventy-two feet in length. The middle and largest pier or tower, is
sixty-two feet by fifty-two at the base, and rises majestically to a
hight of two hundred and thirty feet. The workmen engaged upon this
bridge, with their wives and families, were equal in number to the
population of a moderately sized town, and had the usual provisions for
large communities, of a clergyman, schoolmaster, surgeon, _etc._ The
entire cost of the stupendous structure, was about three million, five
hundred thousand dollars. The number of rivets used in fastening the
tubes of this bridge was over two million; and the entire length of it
is eighteen hundred and thirty-four feet. Silliman says of this immense
and ingenious structure, that it “is wonderful. To construct,” he adds,
“a vast tube of iron strong enough to admit of railroad trains passing
safely through it; to build it in separate pieces down on the common
level; to float them to the site, and there raise them to their
elevation of one hundred feet, and place them on firm pillars of masonry
as supports, and then to unite them into one continued tube, as part of
the grand railroad connection between London and Holyhead and Ireland,
is an achievement which must forever place the name of Robert Stevenson
above all praise.” To show the immense strength of this bridge, he goes
on to say, “An enormous weight of between three and four hundred
thousand pounds, caused a depression of the level only three inches. The
ordinary pressure of the railroad trains produces a depression of
one-eighth of an inch, or even less, discernible only by instruments. A
pressure of more than six hundred thousand pounds produced a deflection
of less than an inch and a half. As works of art, this bridge, and the
one next to be mentioned, are triumphs of mechanical skill and science,
and they not only establish the connection which has been named between
Wales, Anglesey and Ireland, but they afford the prospect of a still
more important connection from Galway, in Ireland, to Nova Scotia, by
steamers, thus bringing Europe and America within a week of each other.
The most massy stone pier, the Britannia, was erected upon a firm rock
which is in the middle of the river. The term tube, as here applied to
the body of the bridge, may convey an erroneous idea; for instead of
being round, it is square. It is an immense iron corridor, or
parallelopiped, closed in, forming a horizontal iron gallery, or
passage, in which the rails are laid. It is thirty feet high in the
middle, and twenty-two feet toward the ends.
[Illustration: THE BRITANNIA TUBULAR BRIDGE.]
“This stupendous structure proves to be a very delicate thermometer. A
little sunshine raises the center an inch, (as the expansion can not
extend downward,) and produces a horizontal deflection or swelling of an
inch and a half. For every fifteen degrees of Fahrenheit, it expands one
ten-thousandth of its length, or half an inch. Alternate sunshine and
showers of rain, cause the tubes to expand and contract. If one of the
tubes was placed on end in St. Paul’s churchyard, London, it would rise
one hundred and seven feet higher than the top of the cross. The rivets
that unite the plates are an inch in diameter; they were put in red-hot,
and beaten with heavy hammers, and in cooling, they contracted so
strongly as to draw the plates together with a force requiring four to
six tuns to make them slide on each other. The tubes were raised from
their position afloat on the water, by means of a Brahmah hydraulic
press, into which the water was injected by powerful steam-engines. The
force exerted by this power would throw water nearly twenty thousand
feet high; more than five times the hight of Snowdon, the highest
mountain in Wales, and almost five thousand feet higher than the summit
of Mont Blanc. The greatest number of men employed at any one time on
this bridge, was two thousand, and the fatal casualties were seven. The
second tube was floated to its place December fourth, 1849, and the
opening of the bridge by the passage of cars took place March fifth,
1850. It may be deflected thirteen inches without injury, and would bear
a weight of one thousand tuns.”