have been duly considered in making the economic studies for the preparation of this memoir.

When the layout of the investigation for the solution of the economic problem herein discussed was first considered, it was intended to make the said investigation comparatively short by confining it to spans of only one length and having but one live load. That span-length was 1,750 feet,-selected because it was used by the author in his preliminary study of the proposed crossing of the Delaware River between Philadelphia, Pa., and Camden, N. J., made for the Camden Bridge Commissioners.

The proposed structure consisted of a single suspension span with backstays, the approaches to it being either steel or reinforced-concrete trestlework, entirely independent of the main span. The deck, in a later modification, was laid out for a double-track electric railway at the middle, a paved roadway supported by a reinforced-concrete base twenty-two feet wide in the clear on each side thereof, and two sidewalks, each ten feet wide, cantilevered beyond the trusses, of which there were four lines. In the first design each of these trusses consisted of two eye-bar cables forming two crescents, each with a web system between composed of vertical posts and adjustable tension diagonals, but later there was also figured a wire-cable structure with stiffening trusses. In each case the towers consisted of braced steel columns with segmental-roller pedestals resting on concrete shafts supported by pneumatic caissons sunk to bed rock at a depth of about one hundred feet below low water. The anchorages were to be of plain concrete either supported by piles driven to bed rock or else resting directly on a foundation of satisfactorily-hard material, should such be encountered when making the borings. Fig. 29a gives the layout for the wire-cable type, and Fig. 29b that for the eye-bar-cable structure.

The first set of calculations prepared for this paper was based on the preceding data and consisted of five designs and estimates of cost of finished bridge upon the following lines:

First. Wire cables of very high elastic limit and ultimate strength.

Second. Mayarí-steel eye-bar-cables having an elastic limit of 50,000 lbs. per square inch and an ultimate strength of 85,000 lbs. per square inch.

Third. High-grade, nickel-steel eye-bar-cables having an elastic limit of 60,000 lbs. per square inch and an ultimate strength of 100,000 lbs. per square inch.

Fourth. Heat-treated, alloy-steel eye-bar-cables having an elastic limit of at least 75,000 lbs. per square inch and an ultimate strength of 115,000 lbs. per square inch.

Fifth. Heat-treated, alloy-steel eye-bar-cables having an elastic limit of at least 100,000 lbs. per square inch and an ultimate strength of 150,000 lbs. per square inch.

The specifications used for the designing, as far as they would apply, were those given in Chapter LXXVIII of "Bridge Engineering," and the