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Railroad bridges.

--The Romans built bridges which are now sound after twenty centuries. We presume few would wish to trust the ordinary class of railroad timber bridges for twenty years. The period is, indeed, about the limit assigned by Mr. Post, the former engineer of the Erie Railroad, for the duration of wooden bridges of the first class. A first class bridge, in Mr. Post's classification, is one the superstructure of which is composed of white pine, white oak, and iron; one of which the joints when fitted are inlaid with white lead and oil paint, and of which the exterior is painted as soon as the timber is perfectly seasoned; the seasoned checks cemented, and other exposed points protected, so as almost completely to secure the timber from the effects of the weather and preserve it from premature decay. Bridges built, as they often are, of inferior timber, with little or no mechanical skill and of light proportions, may eke out from eight to fifteen years, according to circumstances and the care bestowed in repairs. Trestle bridges, costing on an average, and aside from their foundation masonry, about eight dollars a lineal foot for single track, are found to hold out for about six years only. Stringer bridges --large timber girders supported at the ends by masonry and at intermediate points by braces below, resting upon the abutments — cost about five dollars a running foot, and appear to last about eight years. All these estimates are irrespective of the contingency of fire, to which all wooden structures are constantly exposed.

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