Many persons suppose, and maintain, that the grandeur of the monuments of the ancients, and the great size of the stones they employed for building purposes, prove that they understood mechanics better than the moderns. The least knowledge in mechanics, however, will show this opinion to be erroneous. The moderns possess powers which were unknown to the ancients, as the screw, and the hydraulic press, the power of which last is only limited by the strength of the machinery. The works of the ancients show that they expended a vast deal of power and labor to gratify the pride and ambition of kings; but the moderns can do all these things much easier, and in far less time, whenever they deem it proper. There was nothing in ancient times to be compared with that daring, ingenious, and stupendous monument of engineering skill—the Britannia Tubular Bridge, across the Menai straits—projected, designed, and built by Robert Stephenson, the famous English engineer. He had previously built a similar but smaller structure—the Conway Tubular Bridge.
THE BRITANNIA TUBULAR RAILWAY BRIDGE.
Had this stupendous fabric existed in ancient times, it would have been regarded as the first of the seven wonders of the world. Greater and more expensive structures have been raised, but none displaying more science, skill, and ingenuity, and none requiring such tremendous mechanical power to execute.
The Britannia Tubular Bridge was built to conduct the Chester and Holyhead Railway across the Menai Straits, to the island of Anglesea, in the Irish Sea.
The difficulties which the engineer had to overcome, were greatly augmented by the peculiar form and situation of the straits. Sir Francis Head says, "The point of the straits which it was desired to cross, although broader than that about a mile distant; preoccupied by Mr. Telford's suspension bridge—was of course one of the narrowest that could be selected, in consequence of which the ebbing and flowing torrent rushes through it with such violence, that, except where there is back water, it is often impossible for a small boat to pull against it; besides which, the gusts of wind which come over the tops, down the ravines, and round the sides of the neighboring mountains, are so sudden, and occasionally so violent, that it is as dangerous to sail as it is difficult to row; in short, the wind and the water, sometimes playfully and sometimes angrily, seem to vie with each other—like some of Shakspeare's fairies—in exhibiting before the stranger the utmost variety of fantastic changes which it is in the power of each to assume." The Menai Straits are about twelve miles long, through which, imprisoned between the precipitous shores, the waters of the Irish Sea and St. George's Channel are not only everlastingly vibrating, backwards and forwards, but at the same time and from the same causes, are progressively rising and falling 20 to 25 feet, with each successive tide, which, varying its period of high water, every day forms altogether an endless succession of aqueous changes.
THE TUBES.
The tubes forming the viaducts, rest upon two abutments and three piers, called respectively the Anglesea abutment and pier, the Carnarvon abutment and pier, and the Britannia or central pier, built upon the Britannia rock in the middle of the straits, which gives name to the bridge. The Anglesea abutment is 143 feet 6 inches high, 55 feet wide, and 175 feet long to the end of the wings, which terminate in pedestals, supporting colossal lions on either side, 25 feet 6 inches in length, 12 feet 6 inches high, and 8 feet broad, carved out of a single block of Anglesea marble. The space between the Anglesea abutment and pier is 230 feet. This pier is 196 feet high, 55 feet wide, and 32 feet long. The Carnarvon abutment and pier are of the same dimensions as those above described, on the opposite shore. The Britannia pier is 240 feet high, 55 feet wide, and 45 feet long. This pier is 460 feet clear of each of the two side piers. The bottom of the tubes are 124 feet above low water mark, so that large ships can pass under them, under full sail.
There are two tubes, to accommodate a double track (one would have done in this country, but in England they do nothing by halves), and each is 1513 feet long. The total length of the bridge is 1841 feet. These tubes are not round or oval, but nearly square at the termini; the bridge being constructed on the principle of the arch. A section of one of the tubes at the Britannia pier is in the form of a parallelogram, where it is 30 feet high, gradually diminishing towards each end to 20 feet. The tubes are riveted together into continuous hollow beams; they are of the uniform width of 14 feet 8 inches throughout; they are constructed entirely of iron, and weigh about 12,000 tons, each tube containing 5000 tons of wrought iron, and about 1000 tons of cast iron. The tubes were constructed each in four sections; the sections extending from the abutments to their corresponding piers, each 250 feet long, were built in situ, on immense scaffolding, made of heavy timbers for the purpose, even with the railway; but the middle sections, each 470 feet long, were built on piers on the Carnarvonshire shore, then floated into the stream, and elevated to their position; each of these sections weighed 1800 tons.