Third. When, however, the problem is to determine the economic length of anchor arm for a fixed distance between main piers, the result will be quite different; because, within reasonable limits, the shorter the anchor arm the smaller will be its total weight of metal, and because trestle approach is much less expensive than anchor arm. It would not, for evident reasons, be advisable to make the length of anchor arm less than twenty per cent of that of the main opening, or say fifteen per cent of the total distance between centers of anchorages. With this length there would probably be no reversion of stress in the chords of the anchor arm, even when impact is considered. Generally, though, the appearance of the structure would be improved by using longer anchor arms than the inferior limit just suggested.

Fourth. In respect to the economic length of anchor-span in a succession of cantilever spans, it may be stated that, within reasonable limits, the shorter such anchor-spans are the greater will be the economy involved; but, generally, navigation interests will prevent their being built as short as might be desired. If permissible, they may be made so short that, as in the case of anchor arms, eye-bars may be used for the top chords, thus effecting a decided economy of metal, although shortening the anchor-span increases proportionately the stresses on the web members and the weights thereof.

In regard to truss-depths for cantilever bridges, the author's practice is to make that for the suspended span, when the chords are parallel, from one-fifth of its length for short spans to one-seventh of its length for very long ones, interpolating between these limits for intermediate lengths. If one of the chords be polygonal, a greater proportionate truss depth at mid-span and a smaller one at the ends would logically be employed. The height of the vertical posts over the main piers can be made about fifteen (15) per cent of the length of the main opening, or not to exceed three and a half (3.5) times the perpendicular distance between central planes of trusses over the main piers. In the new design for the Quebec bridge these posts were made 310 feet high for the sake of appearance, although the economic length was found to be only 290 feet. These figures correspond to percentages of main openings of about seventeen (17) and sixteen (16) respectively.

There are certain legitimate economies that may be employed in the designing of cantilever bridges, among which may be mentioned the following:

A. The wind pressure assumed in computing the erection stresses may be taken lower than that given in the specifications for the finished structure, provided that the full wind pressure would not overstress any of the metal seriously or involve any risk of disaster during erection. A stress of three-quarters of the elastic limit of the metal applied a few times during erection would do no harm; and the chance of there being in such [a] limited time any wind pressure at all approaching in magnitude that