July 4, 1902.] 



SCIENCE. 



21 



The Pennsylvania Railroad bi^ilt a stone 

 bridge, consisting of 48 segmental arches 

 of 70 feet span, at the crossing of the Sus- 

 quehanna River at Rockville, Pa. It is 

 52 feet wide, accommodates four tracks 

 and coat a million dollars. This bridge has 

 not only the advantage of almost entirely 

 eliminating the cost of maintenance, but 

 it also has sufficient mass to withstand the 

 floods which occasionally wreck the other 

 bridges on that river. This year the same 

 railroad is building a similar bridge over 

 the Raritan River at New Brunswick, N. J. 



Of movable bridges the largest swing 

 span existing was erected in 1893 at Oma- 

 ha over the Missouri River. Two years 

 later a four-track railroad swing bridge 

 was built by the New York Central Rail- 

 road over the Harlem River in New York 

 city, which is only 389 feet long between 

 centers of end pins, but which weighs about 

 2,500 tons and is accordingl3r the heaviest 

 drawbridge of any class in the world. 



During the past decade a remarkable 

 development was made in drawbridge con- 

 struction by the modification and improve- 

 ment of some of the older types of lift 

 bridges and the design of several new 

 types. At Soiith Halstead Street a direct- 

 lift bridge was built in 1893 over the Chi- 

 cago River, in which a simple span 130 

 feet long and 50 feet wide is lifted verti- 

 cally 1424 feet by means of cables to which 

 counterweights are attached. Formerly, 

 only very small bridges of this kind were 

 used, as those, for instance, over the Erie 

 Canal. 



In 1895 a rolling lift bridge over the 

 Chicago River was completed. In this new 

 design as each leaf of the bridge rotates 

 to a vertical position it rolls backward at 

 one end. "When closed the two leaves are 

 locked at the center, but they are supported 

 as cantilevers. This form has been found 

 to have so many advantages for the cross- 

 ings of relatively narrow streams, where 



an unobstructed water way is required and 

 the adjacent shores are needed for dock 

 room, that a score of important structures 

 of this class have been built in different 

 cities. The largest span that has been de- 

 signed is 275 feet between centers of sup- 

 ports, while the widest one is to accom- 

 modate eight railroad tracks crossing the 

 Chicago Main Drainage Canal. 



About the same time and under similar 

 conditions another type of bascule bridge 

 was built at Sixteenth Street, Milwaukee, 

 in which, as each leaf moves toward the 

 shore, one end rises and the other falls, so 

 that its center of gravity moves horizon- 

 tally, thus requiring a very small expen- 

 diture of power to operate the bridge. 



Several improved forms of hinged-lift 

 bridges have also been designed and built 

 in Chicago and elsewhere. In a small 

 bridge erected in 1896 on the Erie Rail- 

 road in the Hackensack meadows there is 

 only a single leaf hinged at one end and 

 lifted by a cable attached to the other end. 

 The counter weight rolls on a curved track 

 so designed as to make the counter balance 

 equally effective in all stages of opening 

 and closing the bridge. 



A novel bridge is now being built over 

 the ship canal at Duluth which is different 

 from any other type in this country. The 

 general scheme is similar to that of a de- 

 sign made by a French engineer who built 

 three of the structures in different coun- 

 tries. It consists of a simple truss bridge 

 393 feet 9 inches long, supported on tow- 

 ers at a clear height of 135 feet above high 

 water. Instead of supporting the usual 

 floor of a highway bridge it supports the 

 track of a suspended car which is properly 

 stiffened against wind pressure and lateral 

 vibration, the floor of the car being on a 

 level with the docks. This ferry is oper- 

 ated by electricity. The loaded car, its 

 hangers, trucks and machinery weigh 120 

 tons. In the French design a suspension 



