occupying the center of the bridge, and the electric railway lying close to one truss.

2. Structures having a single-track railway at the middle, a narrow foot- walk on each side thereof inside of the trusses, and cantilever brackets outside of the latter to carry roadways and electric lines. This arrangement may be varied by running the electric cars over the main railway track, thus leaving the wings free for vehicular traffic.

3. Structures having a double-track railway inside of the trusses, with long cantilever brackets outside carrying wagons and electric lines next to the trusses and pedestrians outside. This arrangement may be varied, as in Case 2, by carrying the electric trains on either one or both of the main railway tracks.

4. Structures having a double-track railway inside of the trusses, with short, cantilever brackets for wagon and electric-railway traffic outside, and either a single passageway overhead at the middle for pedestrians, or two passageways therefor on overhead brackets outside of the trusses. As before, this arrangement may be modified by running the electric trains over the main railway tracks.

5. Double-deck, single-track structures carrying a railway train on one deck and vehicles and pedestrians on the other. If electric cars also are carried, they should generally use the railway track on account of the narrowness of the bridge; but by putting the railway below and using cantilever brackets above, the electric cars may share the wagon-way and run over either one or two tracks. When the electric cars and the vehicles occupy jointly the upper deck, it is generally best to carry the pedestrians by cantilever brackets on the lower deck, as the structure might be too narrow to warrant caring for them above by footwalks outside of the joint wagon and electric car roadway and because permitting them to use the said joint roadway would be too hazardous.

6. Double-deck, double-track structures carrying railway trains on one deck and vehicles, electric trains, and pedestrians on the other, or with the electric trains using the steam railway tracks. The vehicles and electric trains may either occupy the same roadway, or the former may be carried on cantilever brackets, leaving the middle portion of the deck for the latter. In such a bridge the footwalks should be on cantilever brackets, either above or below, outside of the other roadways.

In double-deck structures where the steam railroad is below, it is necessary to use every precaution for keeping the locomotive fumes away from the upper deck, as smoke rising through the floor frightens horses even more than does the train itself. Moreover, smoke is exceedingly disagreeable to everybody passing over the structure. Again, the question of protecting the highway floor from being set on fire by sparks from locomotives must be satisfactorily solved in this combination.

Class No. 1 is the cheapest possible kind of combined bridge, and at the same time the most unsatisfactory, for when a railroad train is about