MENAI BRIDGES. The Menai Strait is the locality of two of the finest engineering works of our country; viz., the Suspension Bridge of Telford, and the Tubular Bridge of Stephenson.
This narrow channel, about 17 miles in length, which separates the island of Anglesey from the mainland of Wales, intervenes on the great mail-ronte from England, by way of Holyhead, to the eastern coast of Ireland. Telford's bridge was commenced Aug. 10, 1819, and the mail-coaches drove over it for the first time Jan. 30, 1826. The distance between the supporting pyramids or points of suspension is 560 feet, and the height of the carriage way above high water in the Strait is 100 feet. The roadway of the bridge is divided into two carriage ways, each 12 feet wide, with a footpath 4 feet wide between them. The main chains are 16 in number, with a deflec tion of 37 feet, their ends being secured in a mass of masonry built over stone arches between each of the supporting piers. The weight of the chains, plates, screw-pins, wedges, and transverse ties is 398 tons ; that of the suspending rods and platforms is 246 tons ; making a total suspended weight of 614 tons. It is estimated that the bridge will bear 732 tons more than its own weight.
Notwithstanding the excellence of this bridge, it was deemed inadequate to bear the trains of the Chester and Holyhead Railway ; and hence arose Mr. Stephenson's grand pro ject for the Tubular Bridge. This unequalled structure, called the Britannia Bridge, con. sists of two tubes of iron placed side by side, through which the up and down trains of the Chester and Holyhead Railway respectively pass. The entire length of each tube is 1516 feet, and the height above high water 102 feet. Each tube consists of four pieces, the ends of which rest upon and are joined together on a central pier built on the Britannia Rock in the middle of the Menai Strait, on two towers on the shores of Anglesey and Caernmonshire res pectively, and on two abutments farther inland on each coast. There are therefore eight tubes in all. The clear length of each tube reaching from the side-towers to the central tower is 460 feet, so that the entire length of tube across the Strait is 920 feet, leaving 506 feet for the entire length of the side tubes. The top of each tube forms a regular arch of slight curvature ; the bottom is quite straight and horizontal. The interior height is 26 feet
in the centre, and 18 feet 9 inches at the ends. The internal width is 14 feet from side to side. The tubes are formed of plates of wrought iron strongly riveted together. The weight of each tube is about 5000 tons, or 10,000 tons for the whole of the double tube of iron.
The lifting of these tubes was the grandest mechanical operation, perhaps, that the world ever saw. The hydraulic presses with which the lift was effected are displayed at the Great Exhibition ; they ought to be preserved, as a memento of this great engineering work. It may be interesting to add a record of the dates which marked the progress of the work. On June 30, 1845, the bill sanctioning the construc tion of the Britannia Bridge was passed by Parliament. In July the preliminary experi ments to determine the form of structure commenced ; Apri113, 1846, the first workmen were engaged on the bridge ; April 21, first stone of the Britannia tower was laid ; June 13, 1847, the first vessels arrived with iron at the Strait; August 10, the first rivet was inserted ; February 22, 181.9, the Caernarvon and Anglesey towers were completed; April, 1849, the pontoons were brought to bear; May 4, first tube was completed and platform cut away ; June 20, first tube floated; June 22, last stone in Britannia tower laid ; No vember 0, first tube deposited in permanent bed ; December 4, second tube floated; February 7, 1850, second tube deposited on permanent bed ; March 3, Caernarvon small tube lowered; March 5, first engine passed through tube, and last rivet inserted; March 18, single line of tube opened for public traffic ; June 10, third tube of second line floated ; July 11, third tube deposited ; July 25, last tube floated ; October 21, second lino of tube opened for public traffic.
Holyhead, at which the railway terminates, is a market-town with a good harbour, and is situated on the northern coast of Holyhead Island. Holyhead harbour is formed by a pier faced with hewn limestone 900 feet in length, at the head of which the water is 14 feet deep at low tides ; but the government and the company are about to incur a large expenditure in forming a magnificent harbour.
The direct distance across from Holyhead to Kingstown Harbour in the Bay of Dublin is 64 miles.