the timber to the metal-work through each alternate tie, the other ties being bolted to the inner guard-rails.

The ties should be spaced with openings not greater than six inches, their section for a five-foot stringer being 6" X 8" laid on flat; but where cross-overs are employed, the depth should be properly increased to withstand the bending moment due to the greatest load from the wheels.

The least allowable overhead clearance for most cities is fourteen feet; but there are sometimes special crossings requiring a greater height. The width of right of way beyond the centre line of the outer track should not be less than seven feet. The proper depths of longitudinal girders are to be determined very carefully. For the sake of appearance it is generally not well to use more than one depth, but such an arrangement cannot always obtain. The general depth should, if possible, be the economic one for the average span length. For plate-girder spans it is about one twelfth of the length, while for open-webbed, riveted spans it is much greater—so much greater, in fact, that for deck-spans the economic depth cannot be adopted, because of the raising of the grade which would be caused thereby.

Before the designing of the metal-work for an elevated railroad is started there are certain important matters which should be fully determined, viz., the dimensions and weights of rolling stock, sizes and number of trains, method of traction and the proper track to suit same, the locations of all stations and their leading dimensions, the storage capacity for the terminals, the capacity of the repair-shops, and the method of operating the road. Unless all these questions be settled conclusively at the outset and before the designing is begun, trouble is sure to ensue because of changes that will have to be made from time to time during the course of construction.

In designing elevated railroads according to the specifications given in this treatise, it must not be forgotten that the entire line is to be proportioned by the specifications for railroad bridges (Chapter XIV), while the stations are to be proportioned by the specifications for highway bridges (Chapter XVI).