est upon the pontoons, being
taken through 'cable-stoppers' which are contrivances for embracing and
gripping the hawser extended across the stream, and thereby retarding,
or if necessary entirely destroying, the speed induced by the current."

RAISING THE TUBES

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