sage of two vessels would not be inferior; but for small openings, of course, this percentage of increase would have to be much greater.

Second. The swing, on account of either its pivot pier or its draw protection, offers much more obstruction to the flow of water than does either of the other types.

Third. The cost of maintenance is more in a swing span than in either of the others on account of the upkeep and periodical replacement of a costly and perishable drawn protection.

Fourth. The swing, of necessity, occupies space outside of that required for the accommodation of land traffic, while the other types do not.

Fifth. The least practicable time of operation is usually twice or thrice as great for a swing as for a corresponding single-leaf bascule or vertical lift.

Sixth. Either the vertical lift or the single-leaf bascule affords better automatic adjustment of the railroad tracks thereon than does the swing span.

Seventh. In the case of future enlargement of bridge to accommodate an increase of traffic, the swing has to be torn down and rebuilt, but a vertical lift or a bascule can simply be duplicated alongside.

Eighth. The danger of the span's being struck, when in motion, by passing vessels is much greater in the case of a swing than in that of either of the other types.

Ninth. The wider the roadway of a swing the more obstructive does it become to navigation, whilst the widening of either a vertical lift or a bascule does no harm thereto whatsoever.

Tenth. In passing vessels with low masts, a swing has to open just as fully as for a high-masted craft, which is not the case with a vertical lift or a bascule.

Eleventh. In sand-bearing streams the protection-works for the moving span of a swing bridge cause a deposit of sediment, and thus often tend, to obstruct navigation.

Twelfth. In the case of a shifting channel, the two openings in a swing may score an advantage for that type over the other types, in that vessels might be able to pass through one opening after the other has been silted up; but under such conditions the silting is more than likely to block both openings. Moreover, for such conditions the vertical lift is far superior to the other two types, in that the design of the structure can be made so as to raise at any time any one of several similar spans, simply by shifting thereto the towers, the machinery, and the house or houses.

Vertical Lift versus Bascule

Comparing the vertical lift with the bascule, the former has several advantages, amongst which may be mentioned the following:

First. The floor is always horizontal, permitting the employment of