BRIDGE. 486 BRIDGE. way traffic, the engineers were obliged to devise some new form which should conform to the stipulated conditions. Mr. Stephenson, having decided on the tubular form, proceeded, in con- junction with Mr. Fairbairn. to make an elabo- rate series of experiments on tubes, to detennine the most suitable arrangement of the wrought iron of which they were composed. They found that a rectangular tube, of which the top and bottom were cellular, gave the greatest strength with the least material. The span of the Con- way tube was 400 feet, while the tubular part of "the Britannia Bridge consisted of two spans of 400 feet an0 bridges built of wrought iron began to be considered, and shortly afterwards the old cast and wrought iron combination structure had practically dis- appeared from use. To attempt to outline the development of the truss during all of the years in which it has been coming to its present state of perfection would involve the reader in techni- calities which hcbmg only in special treatises. It is also im|)ossible to record even the principal truss spans which have been built. In 18S9 Mr. Theodore Cooper estimated that the railways of the United States alone had over 29.000 iron bridge spans. A few of the most notable simple truss bridges of the world are the following: Cincinnati and Covington, double-track railway, over Ohio River, with one span 545 feet, two spans 490 feet: total length, over one mile; total steel contained. 10,180 tons. Henderson, Ky., over Ohio River; total length. 3088 feet; id spans: longest span. i>25 feet. Ohio Con- necting railway. Pittsburg, Pa.; longest span, 523 feet. Norfolk and Western Railroad, over Ohio River, at Credo, W. Va.; two spans 301 feet, two 304 feet, and one 521 feet. Cairo, 111., over Ohio River; two spans 518Vj feet, seven spans 400 feet, three spans of 249 feet; total length, with viaduct, 10,500 feet. Cin-
