electric railway outside of each truss, and a pedestrian-way outside of each wagon-way, the bridge crossing the river at right angles; while the present structure consists of the 520-foot draw-span, without the wings, and three single-track combination spans of 192 feet each, all the piers except the pivot-pier being built of piles and timber, and the centre line of structure making an angle of eleven degrees with the centre line of the final bridge. The deck carries a single railway track at the middle and an electric line by means of a third rail to one side. All four classes of travel use this deck. The only portion of the existing structure that is really finished is the pivot-pier, which consists of a double steel cylinder forty feet in diameter sunk by open dredging to bed-rock, which lies one hundred and twenty-two feet below extreme low water. The completion of the draw-span will be a very simple matter, consisting merely of adding the cantilever brackets with their stringers and flooring and laying the electric-railway rails thereon. The remaining piers for the final structure can all be put in, and the fixed span can then be placed on them without interrupting traffic, because of the deflection downstream of the present temporary structure. When the new bridge is completed, all that it will be necessary to do is to rotate the draw eleven degrees, so that the traffic may be transferred thereto. Afterwards the old pile piers and the combination spans can be removed at pleasure.

In designing combined bridges of all classes except No. 1, a considerable economy of metal may be effected legitimately by keeping the total live load as low as is proper with reference to the theory of probabilities. For instance, in Class No. 2 the live load for trusses can be determined by adding to the equivalent uniform live load, given in the diagram on Plate III or Plate IV, a much smaller highway floor load per lineal foot of span than that prescribed in the specifications for highway bridges, because when the greatest train load is on the bridge, the chances of having a heavy highway live load are very small. The longer the span the smaller may the live load per square foot of floor be taken when finding the total live load for the trusses.