The truss is a distinct evolution from those old timber bridges of which we have already spoken. Burr and Latrobe and Bollman and Howe and Squire Whipple--those distinguished engineers of other days--have evolved it, step by step. It is, in one sense, no more than an enlarged form of lattice girder, the work of the different designers having been to accomplish at all times, a maximum of strength with a minimum of weight. It is built of members that stand pulling-strain, and those that stand pressure-strain; and these are respectively known as tension and as compression members. In them rests the real strength of the truss. But in addition to the structure are the bracing-rods, generally placed as diagonals and built to sustain the structure against both lateral and wind-strains. The members that form the trusses are stoutly riveted together; the rapid rat-a-tap-tap of the riveter is no longer a novelty in any corner of the land. Sometimes certain of the important bearing-points are connected by steel pins instead of rivets--another survival of the old days of the timber bridge. As a rule, the railroad is carried through the truss--and this is known as the through span. Sometimes it is carried upon the top of the structure, and then the truss becomes known as a deck span. A long bridge may effectively combine both of these types of span. The splendid new double-track truss bridge recently built by the Baltimore & Ohio Railroad over the Susquehanna River between Havre-de-Grace and Aiken, Md., to replace a single-track bridge in the same location, is a splendid example of the best type of such structures. At the point of crossing, the river is divided into channels by Watson Island; the width of the west channel being approximately 2,600 feet and that of the east channel being approximately 1,400 feet. The distance across the low-lying island is 2,000 feet--making the length of the entire bridge about 6,000 feet. The bridge, as originally constructed when the line from Baltimore to Philadelphia was built, in 1886, had a steel trestle over Watson Island. In building the new structure, this viaduct was eliminated in favor of a bridge structure of 90-foot girder spans, placed upon concrete piers. Additional piers were placed in the west channel, shortening the deck spans from 480 to 240 feet; the through span over the main channel was kept at the original length--520 feet. In the east channel, the span lengths remained unchanged, with a single slight exception. The changes in the span lengths involved new masonry, and all piers were sunk to solid rock, those in the west channel being carried by caissons to a depth of more than seventy feet beneath low-water. The total amount of new masonry and concrete approximated 62,000 cubic yards. The long span-lengths of the deck span over the east channel and the through span over the navigable portion of the west channel--each 520 feet in length--occasioned heavy construction.