We learn by the Troy, (N. Y.) papers, that a bridge has been erected over the creek in Second street, that city, by the inventor, Dudley Blanchard, in company with Louis Felloes, of that city. It is an iron trass bridge of 73 feet span, composed of 24 separate castings, after six different patterns—four to each. It weighs about 5 tons, of cast-iron, and has about 2 tons of bolting. It has been tested with 40 tons on it, and no sign of deflection exhibited. The usual plan of making truss frames, is to have all the braces equal with a atop and bottom cord of uniform size throughout the whole length. This bridge is constructed with braces and chords of various proportions—each part of tne truss frame being made and proportioned to the strain which it has to sustain. He employs less , material in making a bridge of equal strength to that of the uniformed trussed bridges.— Messrs. Blanchard & Fellows are good practical mechanics, and are now engaged roofing the extensive rolling mill of the Albany Iron Works, a building 336 feet long by 135 feet wide, with an iron roof, supported on theisms principle