Bascule spans may be divided into two general classes—single-leaf and double-leaf. The former type is superior to the latter in rigidity but inferior in appearance, because of lack of symmetry. In the opinion of most railway engineers, on account of the difficulty in properly connecting the outer ends of the two leaves, the double-leaf bascule ought not to be employed for steam-railway bridges, for the reason that the lack of rigidity and the great deflection involved are not compatible with truly-first-class construction.

There is an economic question in connection with bascules that is very difficult to solve, viz., what is the distance between centers of bearings at which it will save in first cost to change from a single-leaf to a double-leaf structure? In the case of a bridge in which the counterweights, the machinery, and their supporting metal are below the deck, the economic limit for the single-leaf span will almost always be less than it is when those parts of the structure are above; and, in the former, the closer the deck is to high-water level, the shorter will be this limiting economic distance. The reason for this is that, with a single-leaf structure and a small vertical distance between grade and high water, unless the moving span be short, either the counterweight will be excessively heavy, or else a pit will have to be provided to receive the tail end. The adoption of either of these expedients causes the cost of structure to rise rapidly.

One of the reasons why the cost of a two-leaf bascule tends to exceed that of the corresponding one-leaf structure is that in the former there must be a holding-down reaction at each end; and because that reaction involves the use of considerable extra metal in the flanking spans or over the piers-much of it being high priced. Since this anchorage is required for live load only, it follows that the condition of small live load and large dead load favors the double-leaf bascule, whereas that of large live load and small dead load favors the single-leaf type.

With counterweights, towers, and machinery above the deck, the clear opening for equal cost of one-leaf and two-leaf spans is probably so great as to exceed the length above which it becomes economic to pass from bascule to vertical lift. While the author has made no special figures to establish beyond all doubt the correctness of this statement, his experience with bascule designing warrants him in drawing the conclusion. He is of the opinion that, for the overhead-counterweight type, the length for equal cost lies between one hundred and fifty and two hundred feet; and, for such a span-length, the vertical lift, for the sake of economy, if for no other reason, should supplant the bascule.

It is also the author's opinion, based on practical experience rather than upon extensive special economic computations, that the double-leaf type of bascule  is  necessitated  only  by  reason  of  aesthetics  or because of a