Miscellany: Suspension Bridges

The subject of Suspension Bridges 3 onc l of peculiar interest to the West. The time is i hastenirg on,vhen the Ohio must bc bridged ; over at different points, to meet the wants l of the population that is beginning to throng its banks. Before tliis time, had our sister ' Btate kept pnce wRh Ohio, a bridge wonld ' have spanned the stream between Cincinnati ' ond Covington. The amoürit of travel and ' trade would have demnndcd'it. ' lation on tlie other side is yet too sparse } ing to the m.ilign mfluence of slavery. ' Suspension bridges woiked tliernselves into ' favor very slowly. In 1800 an al tempt ivas 1 made on n moderate scale ii this country, but its success was uot such as to induce a speedy ' repetilion of it. It is ihougíit ho wever that Í it had something to do wilh the design of the ' groat bridge across the strait of Menai, be ( tween Wules and the Island of Anglesea. - l The widlli of the strait at tho point of erection ' was 1000 feet, and the deptli of water ut high ' tide, 48 feet. The d;stance between the ' points of support of the bridge, is 580 feet, ( and the flooring ia elevtted 128 feet above l low tide, so that sliips may pass under it in full ' sail. I In 1823, a series of cables, manufaclured of ' iron wire was substituted for the wrought iron ' chains, previously employee! for the support ) of the road-wny; and Tor Uiis [mpftTTèrhtnü v.-c j are indebted to Mr. Seguin, a practical f chanic of Lyons, who constructed a suspenBion bridge at his own cost, across the Rhone at Tournau. The experiment was repeated, ( and the improvement universally adopted, and ' there are now suspension bridges in every part ' of tlie continent of Europe. In 1838, the famous wooden bridge across the Schuylkill al Fairmount, which had cost ! $12C,000, was burnt down. The county of 1 Philadclphia purchased the title to the site, ' & C. Ellet,civil engineer.furnished the design ( of a suspensipn bridge. At first his projeef, on account of its apparent frailty, was ndiculed by some, distrusted by many; but at last, aftermuch inqniry, adopled. The work was finished, January lst, 1842, aftcr the lubor of ' four months. The lengih of the flooring is ' 345l feet; the diffèrènce between the points 5 of suspension, 357; its wiiUh between the parapete, 20; Ihe carringe way, 18 fcetwide, and cach of the foot-ways, rais-ed 8 inchos above the road-way, 4 feel wide. Four granite ; umns, 30 feet high, ' square at the base, ' sustain ihe bridge. There are 2816 strands of no. 10 wire in the ten cables, which uphold the flooring, and the average strength of the wire, is cqual to 51 tons per square inch of tsection; and the ultímale strength of the ten cables, 2100 tons. The weight of the entire woodwork is 115 tons; and the proper weight of the bridge, sustained b the cables, is 240 tons, a ninth part of the force recessary to break them. On the completion of the bridge, its strength was tested by placing 39 carts anc' horse, the former loaded each with two tons of stone-?, on the flooring, while some onc or two hundred people were on the footwaysjbul all this weight subjectcd the cables to butone fiflli of the force necessary to break them! - The whole cost of the bridge was $53,000. "The peculiar advantages of these structures," says Mr. Ellett, "is due to the power which they furnish of bridging rivers of great bread th, at moderate coat, and without the necestity of intermedíate supports. If the breadth of the stream is not over the third of n mile, and the object is suflïcicnt to justify the expense, a bridge may bo constructed without Ihe nced of a fier in the channel. - When it is expedient, thereforc, to avoid obetructing the waler-way, or impairing the navigalion, - where it is essential to elévate the flooring far above the surface of the river, or where the expense of permanent foundations for the oiers will be great, this system will be found to present remarkable claims.' Mr. Elletl shows conclusivcly, what every man rr.ust admit, that wooden bridges over the Miesissippi or Ohio, are out of the ques.tion. A suspension bridge over the Ohio, a Cincinnati, might be constructed, he believes with one pier, and at a cost of only $150,000 leaving the navigation completely unimpoded Mr. Ellett truly remarks that "the lighuicsbM- - - I ■ W ■ grace, and beauty of these structures, when lastefully designed and judiciously applied,can be only adequaiely appreciated when in place, and the edifice never parts with its beauty. - The forrn it assumes, when first thrown over the stream, is the result of natural laws,which are always in actio, and will preserve its