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, arerequired 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.