railway bridges, excepting those of exceedingly long span-far longer than any that have been constructed-and the slow development of the American highway system have combined to keep the type in the background; but the time is approaching when it will be necessary to carry our rapidly developing network of highways across some of the largest of our rivers, and then the suspension bridge may have an opportunity to come into its own. It must not be forgotten, however, that the conditions warranting the building of a suspension bridge are not likely to be often encountered; because it is nearly always the fiat of the War Department which necessitates a span-length so great as to render economical the building of a structure of that type.

By means of a long and elaborate economic investigation made in 1918 and published in 1919 by the Western Society of Engineers under the caption "Comparative Economics of Cantilever and Suspension Bridges," the author has shown the span-lengths of equal cost for these two classes of structure. The said lengths usually vary from 1,000 feet for highway bridges to about 2,600 feet for steam-railway bridges, the lengths for combined steam-railway and highway structures being intermediate and directly interpolated in accordance with the division of total live load, including impact allowances, between railways and highways. Later it was found that the irregularity of the abnormal unit prices of materials at present governing has lengthened the spans of equal cost some two hundred feet for highway bridges and sixty feet for steam-railway bridges; but a return to normal market conditions will assuredly bring them back to about the limits first found.

Had the original investigation been based upon the assumption of anchored ends instead of free ends for the stiffening trusses, the span-lengths for equal cost would have been found about one hundred feet shorter, or 900 ft. for highway bridges and 2,500 ft. for railway bridges at ante-bellum unit prices, and 1,100 ft. for highway bridges and 2,560 ft. for railway bridges at the unit prices prevailing early in 1920.

As that investigation proved that the suspension bridge is less economical for steam-railway traffic than the cantilever structure up to the extreme practicable main-span-length of the latter, and as it is well known that the cantilever is always the superior of the two in respect to the important matter of rigidity, the present investigation has been limited to the consideration of highway bridges carrying also electric railway trains, such as those that cross the East River in New York City.

The comparative advantages and disadvantages of the two types of cables are as follows:

First. The wires for the cables have higher elastic limit and ultimate strength than can probably ever be developed in eye-bars.

Second. The percentage of weight of details is far lower for wire cables than for eye-bar cables.

Third. The cost of erection is inevitably less for wire cables than