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