THE BRITANNIA TUBULAR BRIDGE.

This wonder of modern engineering, a view of which is given in the cut, forms part of the railroad from Chester to Holyhead, and is thrown over the Menai strait which separates Caernarvon, in Wales, from the island of Anglesey. This strait appears to the eye as a beautiful river, half a mile wide, through which the tide, which here rises twenty and twenty-five feet, rushes with great rapidity and force. The tubular bridge over it is one hundred feet above high-water level, and formed of long, hollow, rectangular tubes, one for up, and the other for down trains, composed of wrought-iron boiler-plates riveted together, and resting on huge and massive towers of masonry. Of these tubes or galleries, eight in number, four for each line, the four shortest are each two hundred and thirty feet, and the four longest each four hundred and seventy-two feet in length. The middle and largest pier or tower, is sixty-two feet by fifty-two at the base, and rises majestically to a hight of two hundred and thirty feet. The workmen engaged upon this bridge, with their wives and families, were equal in number to the population of a moderately sized town, and had the usual provisions for large communities, of a clergyman, schoolmaster, surgeon, etc. The entire cost of the stupendous structure, was about three million, five hundred thousand dollars. The number of rivets used in fastening the tubes of this bridge was over two million; and the entire length of it is eighteen hundred and thirty-four feet. Silliman says of this immense and ingenious structure, that it “is wonderful. To construct,” he adds, “a vast tube of iron strong enough to admit of railroad trains passing safely through it; to build it in separate pieces down on the common level; to float them to the site, and there raise them to their elevation of one hundred feet, and place them on firm pillars of masonry as supports, and then to unite them into one continued tube, as part of the grand railroad connection between London and Holyhead and Ireland, is an achievement which must forever place the name of Robert Stevenson above all praise.” To show the immense strength of this bridge, he goes on to say, “An enormous weight of between three and four hundred thousand pounds, caused a depression of the level only three inches. The ordinary pressure of the railroad trains produces a depression of one-eighth of an inch, or even less, discernible only by instruments. A pressure of more than six hundred thousand pounds produced a deflection of less than an inch and a half. As works of art, this bridge, and the one next to be mentioned, are triumphs of mechanical skill and science, and they not only establish the connection which has been named between Wales, Anglesey and Ireland, but they afford the prospect of a still more important connection from Galway, in Ireland, to Nova Scotia, by steamers, thus bringing Europe and America within a week of each other. The most massy stone pier, the Britannia, was erected upon a firm rock which is in the middle of the river. The term tube, as here applied to the body of the bridge, may convey an erroneous idea; for instead of being round, it is square. It is an immense iron corridor, or parallelopiped, closed in, forming a horizontal iron gallery, or passage, in which the rails are laid. It is thirty feet high in the middle, and twenty-two feet toward the ends.

“This stupendous structure proves to be a very delicate thermometer. A little sunshine raises the center an inch, (as the expansion can not extend downward,) and produces a horizontal deflection or swelling of an inch and a half. For every fifteen degrees of Fahrenheit, it expands one ten-thousandth of its length, or half an inch. Alternate sunshine and showers of rain, cause the tubes to expand and contract. If one of the tubes was placed on end in St. Paul’s churchyard, London, it would rise one hundred and seven feet higher than the top of the cross. The rivets that unite the plates are an inch in diameter; they were put in red-hot, and beaten with heavy hammers, and in cooling, they contracted so strongly as to draw the plates together with a force requiring four to six tuns to make them slide on each other. The tubes were raised from their position afloat on the water, by means of a Brahmah hydraulic press, into which the water was injected by powerful steam-engines. The force exerted by this power would throw water nearly twenty thousand feet high; more than five times the hight of Snowdon, the highest mountain in Wales, and almost five thousand feet higher than the summit of Mont Blanc. The greatest number of men employed at any one time on this bridge, was two thousand, and the fatal casualties were seven. The second tube was floated to its place December fourth, 1849, and the opening of the bridge by the passage of cars took place March fifth, 1850. It may be deflected thirteen inches without injury, and would bear a weight of one thousand tuns.”