In making the Detroit-Windsor Bridge study, a practical proof was given of the usefulness of the paper. No copy of "Bridge Engineering" was available for making an estimate of the cost of the suspension bridge and its approaches, but a copy of the paper was at hand; and, as a rough estimate was required immediately, the following procedure was adopted, it being recognized at the outset that all the assumptions made therein were upon the side of safety, and that, consequently, the resulting figures of cost would be somewhat too great:

Referring to Fig. 13j, the curve for costs of suspension bridges was extended on an enlarged cross-section sheet to a span of 1,700 feet, at which length the spans on Fig. 13c begin. The cost thus found was mul- tiplied by the ratio of the total combined clear widths of roadway and sidewalks for the two structures considered, and the product was multiplied by the average of the ratios of the unit costs of all substructure and superstructure materials in place for present conditions and the conditions assumed in the paper. Then, referring to Fig. 13c, it was noted that the cost of a 2,500 foot-span suspension-bridge and its approaches is almost exactly double that for a similar 1,700-foot span with its approaches; hence the cost just found was doubled, and to the result were added the cost of the entire flooring from entrance to exit of structure, an allowance for the greater length of the approaches involved, and the approximate cost of either elevators or an escalator and a stairway at the Detroit approach.

Later, a more exact estimate of cost was made from the various data in "Bridge Engineering," the result being some 5 per cent less than that of the first approximation: This more-exact estimate was computed in a single working day. Without the aid of the book mentioned, it would probably have required as many weeks of figuring as it actually took hours thereof, in order to obtain results of equal accuracy.

In the Appendix to the original paper there are given five pages of estimates of cost, covering fourteen structures out of the twenty-five that were computed. It has not been deemed worth while to reproduce them in this treatise; for probably they would not be of much interest to any reader. If, though, anyone desires to see them, he can do so by consulting the Transactions of the Western Society of Engineers.