The tubes of the Britannia bridge were raised by means of three
hydraulic presses of the most prodigious size, strength, weight, and
power; two of which were placed in the Britannia pier, above the points
where the tubes rest, and the other alternately on the Anglesea and
Carnarvon piers.
In order that all who read these pages may understand this curious
operation, it is necessary to describe the principle of the hydraulic
press. If a tube be screwed into a cask or vessel filled with water, and
then water poured into the tube, the pressure on the bottom and sides of
the vessel will not be the contents of the vessel and tube, but that of
a column of water equal to the length of the tube and the depth of the
vessel. This law of pressure in fluids is rendered very striking in the
experiment of bursting a strong cask by the action of a few ounces of
water. This law, so extraordinary and startling of belief to those who
do not understand the reasoning upon which it is founded, has been
called the _Hydrostatic paradox_, though there is nothing in reality
more paradoxical in it, than that one pound at the long end of a lever,
should balance ten pounds at the short end. This principle has been
applied to the construction of the Hydrostatic or Hydraulic press,
whose power is only limited by the strength of the materials of which it
is made. Thus, with a hydraulic press no larger than a common tea-pot, a
bar of iron may be cut as easily as a slip of pasteboard. The exertion
of a single man, with a short lever, will produce a pressure of 1500
atmospheres, or 22,500 pounds on every square inch of surface inside the
cylinder. By means of hydraulic presses, ships of a thousand tons
burthen, with cargo on board, are lifted out of the water for repairs,
and the heaviest bodies raised and moved, without any other expense of
human labor beyond the management of the engine.
The tubes on the Anglesea side were raised first. The presses in the
Britannia tower were each capable of raising a weight of 1250 tons; that
in the Anglesea tower, larger than the others, 1800 tons, or the whole
weight of the tube. These presses were worked by two steam engines of 40
horse power each, which forced the water into the cylinders, through a
tube half an inch in diameter. These steam engines were placed in the
Britannia and Anglesea piers. The press in the Anglesea pier is thus
described, the others being constructed in the same manner. The
hydraulic press stands on massive beams of wrought iron plates
constructed on the principle of the arch, placed in the tower above the
points where the tubes rest. The press consists of a huge cylinder, 9
feet 2 inches in length, 3 feet 6 inches outside diameter, and the ram 1
foot 8 inches in diameter, making the sides and bottom of the cylinder
11 inches thick; it was calculated that it would resist a pressure of
8000 or 9000 pounds to the square inch. The ram or piston was attached
to an exceedingly thick and heavy beam of cast iron, called the
cross-head, strengthened with bars of wrought iron. To the cross-head
were attached the huge chains that descended to the tubes far below, to
which they were secured, so that, as the ram was forced up 6 feet at
each stroke, the tube was raised the same distance. "The power of the
press is exerted on the tube by aid of chains, the links of which are 6
feet in length, bolted together in sets of eight or nine links
alternately.--The ram raises the cross-head 6 feet at each stroke, and
with it the tube, when that height is attained, a lower set of chains on
the beams grip the next set of links, and thus prevent them from
slipping down, whilst the clamps on the cross-heads are unscrewed, the
upper links taken off, and the ram and cross-head lowered to take
another stroke." To guard against all chances of injury to the tubes in
case of accident to the machinery, a contrivance was adopted by which
the tubes were followed up with wedges. The importance of this
precaution was fully proved on the very first attempt to raise the tube
on the Anglesea side, when the huge cylinder broke, almost at the
commencement of the operations. The following is the engineer's
interesting report of the accident:
"On Friday last (August 17, 1849), at a quarter to twelve o'clock, we
commenced lifting the tube at the Anglesea end, intending to raise it
six feet, and afterwards to have raised the opposite end the same
height.
"The tube rose steadily to the height of two feet six inches, being
closely followed up by inch wooden boards packed beneath it, when
suddenly, and without any warning, the bottom of the hydraulic press
gave way, separating completely from the body of the press.
"The ram, cross-head, and chains descended violently on the press, with
a tremendous noise, the tube sinking down upon the wooden packing
beneath it. The bottom of the press, weighing nearly two tons and a
half, fell on the top of the tube, a depth of eighty feet.
"A sailor, named Owen Parry, was ascending a rope ladder at the time,
from the top of the tube into the tower; the broken piece of press in
its descent struck the ladder and shook him off; he fell on to the tube,
a height of fifty feet, receiving a contusion of the skull, and other
injuries, of so serious a nature that he died the same evening. He was
not engaged in the raising, and had only chosen to cross the tube, as
being the nearest road from one tower to the other. An inquest was held
on the following day, and a verdict of accidental death returned. No one
actually engaged in the operation was injured, although Mr. Edwin
Clark, who was superintending the operation, on the top of the
cross-head, and his brother, Mr. L. Clark, who was standing beneath it,
had both a very narrow escape.
"The tube is not at all injured, but some portions of the cast iron
lifting frames are broken, and require repairing; some weeks must elapse
before a new cylinder is made, and the operation continued."
Sir Francis Head, when he saw one of the tubes raised, and in its place,
observed, "It seemed surprising to us that by any arrangement of
materials, it could possibly be made strong enough to support even
itself,--much less heavily laden trains of passengers and goods, flying
through it, and actually passing each other in the air at railway speed.
And the more we called reason and reflection to our assistance, the more
incomprehensible did the mystery practically appear; for the plate iron
of which the aërial gallery is composed is literally _not so thick_ as
the lid, sides, and bottom which, by heartless contract, are _required_
for an elm coffin 6½ feet long, 2¼ wide, and 2 deep, of strength
merely sufficient to carry the corpse of an emaciated pauper from the
workhouse to his grave! The covering of this iron passage, 1841 feet in
length, is literally not thicker than the hide of an elephant; lastly,
it is scarcely thicker than the bark of the good old English oak,--and
if this noble sovereign, notwithstanding 'the heart' and interior
substance of which it boasts, is, even in the well-protected park in
which it has been born and bred, often prostrated by the storm, how
difficult is it to conceive that an attenuated aërial hollow beam, no
thicker than its mere rind, should, by human science, be constructed
strong enough to withstand, besides the weights rushing through it, the
natural gales and artificial squalls of wind to which, throughout its
entire length, and at its fearful height, it is permanently to be
exposed."
Notwithstanding these "incomprehensible" speculations, the tubes are
abundantly strong to sustain the pressure of the heaviest trains, even
were they to stand still in the middle of the bridge. It is calculated
that each tube, in its weakest part, would sustain a pressure of four or
five thousand tons, "support a line of battle ship, with all her
munitions and stores on board," and "bear a line of locomotives covering
the entire bridge." The bridge was completed, and the first train passed
through it March 5th, 1850. The total cost of this gigantic structure
was only £601,865.