THE BRITANNIA TUBULAR BRIDGE.

“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.”

THE SUSPENSION BRIDGE.

In the same vicinity, and over the same strait, is the great suspension bridge, which, when it was finished in 1826, was deservedly esteemed one of the wonders of the world, and is still entitled to hold that rank. It is indeed a stupendous structure, of which the full details may be learned from the official reports; but the following are among the principal facts. It is one hundred feet above the water, so that the ships, even those of a large size, are not impeded, and can pass under it without lowering a sail or a spar. The bridge is built out upon arches from both sides of the river, to a certain distance, leaving the space between the points of suspension, five hundred and sixty feet. The platform is about thirty feet wide. The whole is suspended from four lines of strong iron cables, by perpendicular iron rods, five feet apart. The cables pass over rollers, on the tops of pillars, and are fixed to iron frames under ground, which are kept down by masonry. The weight of the whole bridge between the points of suspension is four hundred and eighty-nine tuns. The massy materials of which this bridge is composed, the admirable manner in which they are locked together, the great elevation at which it crosses this grand strait, its persistence without sign of failure during more than a quarter of a century, its importance as a connecting link between England and Ireland, and the result of this early effort to conquer formidable physical difficulties, fill the beholder with admiration and delight, and do lasting honor to Mr. Telford, the distinguished architect.

GREAT RAILWAY SUSPENSION BRIDGE AT NIAGARA FALLS.

We have before, on page 265, given some account of this vast structure as it was, when so far completed as to be used for ordinary passage. But we advert to it again here, both because it has since had added to it the superstructure for railway-trains, and also that we may bring in comparison, the first suspension bridge ever attempted, (an account of which has been given,) and one of the last and largest ever undertaken. The first train of cars passed over this bridge on the ninth of March, 1855, from the Canada to the American shore, the engine and tender being crowded with people, having the English and American colors flying, while bands of music were playing alternately the national airs of Great Britain and of the United States. The opening of this mighty and magnificent structure, well worthy of being classed with the world’s wonders, really forms an epoch in the history of the world. It unites with strong iron bands two countries, to the intelligence and enterprise of whose inhabitants the bridge owes its existence, and stands a fitting monument. Its strength can never be fully tested; the weight of a fully laden train being but a trifle in comparison to its capacity. A train of eight cars, filled with passengers, two baggage-cars, locomotive and tender, weigh but about one hundred and thirty tuns; this being only one-sixtieth of its immense capacity. The railway portion of the bridge is leased to and controlled by the Great Western railway company, and has laid upon it tracks of three different gauges, viz., the New York Central, four feet and eight and a half inches; the Elmira and Niagara Falls, six feet; and the Great Western, five feet and six inches, thus affording facilities for the transit of both passengers and freight, without change of cars. The following statistics will give some idea of this immense structure and its capacity.

Length of span from center to center of towers,822feet.
Hight of tower above rock on the American side,88feet.
Hight of tower above rock on the Canada side,78feet.
Hight of tower above rock on the floor of railway,60feet.
Number of wire cables,4
Diameter of each cable,10inch.
Number of No. 9 wires on each cable,3,659
Ultimate aggregate strength of cables,12,409tuns.
Weight of superstructure,750tuns.
Weight of superstructure and maximum loads,1,250tuns.
Maximum weight the cable and stays will support,7,200tuns.
Hight of track above water,234feet.

OTHER IMMENSE BRIDGES.

At Peru, in Illinois, is the great bridge of the Illinois Central railroad, which is thirty-five hundred feet, or nearly two-thirds of a mile long. This is perhaps the greatest work of the kind in all the western states. It reaches from bluff to bluff, is seventy-five feet in hight, and contains over one million feet of lumber, beside immense quantities of iron and stone. The top is covered with tin, and made water-tight; the trains of cars are to run on the top of all; and beneath them, and between the frames, pass the roads for wagons; while underneath all are the river and canal. An ornamental railing is placed on each side of the track.

Another large bridge, on the suspension principle, is that over the Mississippi, near St. Anthony and Minnesopolis, in Minnesota. The work consists of a wire suspension bridge, of one span of six hundred and thirty feet, having seventeen feet of roadway, connecting the western bank of the Mississippi river with Nicollet island, about one hundred yards above the first break of its waters into rapids above the falls.