ADVANTAGES OF LIFT-BRIDGES.

The advantages of lift-bridges in comparison with rotating drawbridges are as follows:

1st.   A lift-bridge gives one wide channel for vessels instead of the two narrow ones afforded by a centre-pivoted swingbridge.

2d.   There are no land damages in the case of a lift-bridge, as the whole structure is confined to the width of the street. These land damages in the case of some swing-bridges amount to a large percentage of the total cost of structure.

3d.   Vessels can lie at the docks close to a lift-bridge, which they cannot do in the case of a swing-bridge; consequently with the former the dock-front can be made available for a much greater length between streets than it can with the latter.

4th.   The time of operation for a lift-bridge is about 30% less than that for a corresponding swing-bridge.

The advantages of a lift-bridge in comparison with a bascule or a jack-knife draw, both of these being supposed to be without a centre pier, are as follows:

1st.   The lift-bridge can be made of any desired span, while in tile case of the others the span is necessarily quite limited in length.

2d.   A lift-bridge can be paved, while the others cannot.

3d.   The lift-bridge is very much more rigid than any structure composed of two or more partially or wholly independent parts, a feature characteristic of the jack-knife bridge or the bascule without a centre pier.

4th.   In a lift-bridge the operating machinery is much more simple; and, in case that it should ever get out of order, the span can be raised or lowered either by unbalancing, or by simple hand mechanism, or by both combined.

If the author were to design another lift-bridge similar to the Halsted Street structure, and if he were given carte blanche in the designing, he would make the following improvements:

1.   Curve the rear columns and arch the overhead girders at tops of

towers, so as to improve the general appearance.

2.   Operate by electricity instead of by steam.

3. Place the machinery-house in one of the towers and dispense

with  the  operating-house  on  the  span,  letting the