design is illustrated in Engineering News of December 14, 1893.

Lift-bridges on a small scale have been used for many years for crossings of canals, lifting only high enough to let the canal-boats pass beneath. They have proved to be quite satisfactory and fairly economical in both first cost and operation, the method of the latter being usually man-power.

No large structure of this type was ever built until 1893, when the author designed for the city of Chicago the South Halsted Street Lift-bridge. This structure has been described in the principal engineering papers of America and Europe, and the author's description, written for the American Society of Civil Engineers, may be found in the Transactions of that Society for January 1895, from which the following description is taken:

The bridge consists of a single, Pratt truss, through-span of 130 ft., in seven equal panels, and having a truss depth of 23 ft. between centres of chord pins, so supported and constructed as to permit of being lifted vertically to a height of 155 ft. clear above mean low water. At its lowest position the clearance is about 15 ft., which is sufficient for the passage of tugs when their smokestacks are lowered. The span differs from ordinary bridges only in having provisions for attaching the sustaining and hoisting cables, guide-rollers, etc., and in the inclination of the end posts, which are battered slightly, so as to bring their upper ends at the proper distance from the tower columns, and their lower ends in the required position on the piers.

At each side of the river is a strong, thoroughly braced, steel tower, about 217 ft. high from the water to the top of the housing, exclusive of the flag-poles, carrying at its top four built-up steel and cast-iron sheaves, 12 ft. in diameter, which turn on 12-in. axles. Over these sheaves pass the 1 1/2-in. steel-wire ropes (32 in all) which sustain the span. These ropes are double, i.e., two of them are brought together where the span is suspended, and the ends are fastened by clamps, while, where they attach to the counterweights, they form a loop, which passes around a 15-in. wheel or pulley that acts as an equalizer in case the two adjacent ropes tend to stretch unequally.

The counterweights, which are intended to just balance the weight of the span, consist of a number of horizontal cast-iron blocks  about 10 X 12 in.  in  section, and 8 ft. 7 in.  long,