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 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