In dealing with the economics of movable spans it will suffice to consider only those types thereof which have survived the test of time, relegating the others to oblivion. The description and history of all such types, good, bad, and indifferent, will be found in Chapters XXVIII to XXXI, inclusive, of "Bridge Engineering." As stated there, the surviving types are the swing, the bascule, and the vertical lift; and the first mentioned, as will be shown further on in this chapter, has no longer any real raison d'être. The choice today, consequently, is between the bascule and the vertical lift, with the preponderance of advantage and economy in most cases favoring the latter.

Before beginning a discussion of the comparative costs of the three surviving types, it will be well to consider thoroughly all their important advantages and disadvantages, excepting only those that relate to first cost of construction, plus capitalized cost of maintenance and repairs.

Swing-Span versus Either Bascule or Vertical Lift

First. The swing provides two openings, while either the bascule or the vertical lift affords only one. This is claimed by the advocates of the swing as an advantage; but it is not often such, because very seldom is there a location at which there exists a possibility of the water traffic being so great as to necessitate the simultaneous passage of vessels in opposite directions, or such a large amount thereof in one direction as to call for two openings. Probably not one location in a hundred would have so many craft passing that two openings would be utilized at the same time, excepting semi-occasionally. But if such were the case, the single opening could be enlarged so as safely to permit two vessels to pass at the crossing. The question would then arise as to how greatly the single opening should be increased in order to afford equal facility for passing, as compared with a structure having two openings. In the author's opinion, if the single opening in ordinary cases were made twenty-five per cent wider than either opening of the swing, the facility thus provided for the simultaneous pas-

* This chapter was presented as a memoir to the American Railway Engineering Association in Dec., 1920, and is now due to appear in its "Proceedings." It will be submitted to the leading bridge engineers of this country (both in and outside of the Association) for a thorough discussion.

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