In England, the progress of bridge ar chitecture has kept pace with that of the Continent ; and if her bridges cannot boast the elegance in design of her lively neighbors, they are fully equal to them in boldness of conception and exe cution of the work. Indeed, if the de signs of the late Messrs. Telford and Rennie had been equal to their engin eering skill, no country in the world would have been able to compete with what she would have been able to exhibit. And here must not be forgotten the bridge over the river Taaf, near Llan trissart, in Glanaorganshire, celebrated for its great span ; the work of William I Edwards, a country mason, in 1755. The chord line is 140 feet, and the versed sine 35 feet.
Of timber bridges the boldest and most ingeniously constructed was that at Schaffhausen, in Switzerland, destroyed by the French in 1799. It was designed and executed by Grubenman, a common carpenter, in 1758. The total length of the bridge was 364 feet, but it was re lieved by a pier in the middle of the river.
This country has some of the grandest specimens of timber bridges in the world. The Colossal bridge over the Schuylkill, at Philiiiielphia (since burnt) was 340 feet span, with a rise of only 20 feet, and the thickness of the timber at the vertex only 7 feet. That at Piseataqua, over the river of same name, has a span of 250 feet, and a rise of 27, built in 1794 by Palmer.
The bridge of the Kennebec and Port land Railroad, over the Androscoggin River at Topsham, is one of the largest and most substantial structures of the kind in the United States. It is a deck bridge, the upright posts and rods being about 18 feet from the lower to the upper deck. One of them reaching from centre to cen tre of the piers, is one hundred and eighty feet. The piers are of granite laid in the most durable manner. The whole length of the bridge is over seven hundred feet. The track of the road along the upper deck will be about fifty feet above tide water. The large lower and upright timbers, and the iron work, together with the X work between decks, give the bridge an appearance of strength and solidity sufficient for any weight. Suspension bridges derive their chief value from the fact of their independence of the bed of the river. Hence they may be used where it is impossible from the force of the current or the altitude of the banks to erect centreing for a stone bridge. They are built with ease and expedition, and are economical. The celebrated suspension bridge over the Menai Strait, by the late Mr. Telford,
is almost the giant of its species, and renders unnecessary any enumeration of others. It consists of one opening of 560 feet between the points of suspen pion, the height between high water and the under side of the roadway being 100 feet. The platform is 30 feet in breadth. The whole is suspended from four lines of strong iron cables by perpendicular rods 5 feet apart. On the tops of the pil lars the cables pass over iron rollers, and are fixed under ground to iron frames, which are retained in their places by ma sonry. The weight of the whole bridge between the points of suspension is 489 tons. In suspension bridges it has been found that the most trying circumstances under which they can be placed, as af fecting the stability of their equilibrium, is the heavy and measured tread of a long line of infantry, whose feet drop at the same instant of time.
The wire suspension bridge at Wheel ing, Va., over the Ohio, has been com pleted by Charles Ellett, Jr., Architect. The flooring is supported by 12 cables of iron, each cable 4 inches in diameter, composed of 550 strands of No. 10 wire, and is 1,380 feet long ; and from centre to centre of the abutments, the flooring is 1,010 feet long, 24 feet wide, with two foot-ways, each 31 feet, and an interme diate carriage-way 17 feet wide. The cables rest on iron rollers, placed on the summits of the towers, the movements of which will relieve the towers of the strain, and are anchored into the heavy masonry of the wing walls at each end of the bridge. The length of the wood work which rests on the cables is 960 feet ; its weight 546 lbs. per lineal foot, or 524,160 pounds of 262 tons in the whole. The weight of each lineal foot of the 12 cables, composed of 6,600 strands, is 330 pounds, making, with the weight of timber, bolts, castings, sus penders, &c., 920 lbs. per lineal foot, or 441 tons as the permanent weight of the bridge itself. Above its own weight the bridge is constructed to support the greatest transitory weight that is ever likely to be, or we may say, can possibly be brought upon it, such as two columns of teams, and the sides loaded with men, so as to weigh, ;jointly, 297 tons, or the average weight of 4,000 men, and the strength of the bridge is calculated to support three times tie amount of ten sion that ever can be brought to bear upon it. This bridge will no doubt last long as a monument of American skill and enterprise.