Fifth. The tops of the anchor-piers should be made absolutely water-tight without interfering with the longitudinal expansion of the anchor-arm, so as to prevent rusting of the interior metal-work.

Sixth. The net weight of masonry in any anchor-pier, after deducting the greatest buoyant effort of the displaced water, should be twice as great as the maximum uplift on the said anchor-pier, when the effect of impact is duly included.

A few observations concerning some of the largest cantilever bridges yet built may be of service to the reader:

The largest structure of this type in the world is the bridge at Queensferry over the Firth of Forth, the main portion of which consists of two spans of 1710 ft. each, with central spans of 350 ft. each, and two anchor-arms of 680 ft. each. The length of the tower-span over the centre pier is 260 ft., and that of each of the two other tower-spans is 145 ft., making the total length of the main structure 5410 ft. The design for this bridge and a complete history of its construction are given in a special work published by Engineering (London).

The exceptions which the author would take to this design are as follows:

First. The suspended spans are just about one half as long as they ought to be for both appearance and economy.

Second. The structure should have been made pin-connected for both ease of erection and certainty of stress distribution.

Third. A single system of cancellation for the webs of the girders would have been more scientific than the double system adopted, and would not have been any more expensive.

Fourth. The structure as a whole, from the point of view of American engineers, was unnecessarily expensive.

On the other hand, though, the labor involved in both the designing and building of this bridge was immense; and the successful completion of the structure is a great credit to all concerned in its designing and construction.

The cantilever bridge having the next longest span is the Lansdowne Bridge over the Indus River at Sukkur, India.