RAILWAY CONSTRUCTION. 87 



adjoining span are made the same, and the girders for the two 

 spans are constructed in one continuous length. By means of 

 gearing attached to the fixed portion of the work, the continuous 

 length of girder, with its roadway, is first slightly raised or 

 lowered, and then drawn back on rollers sufficiently far to leave 

 the opening span quite clear for the passage of vessels. A 

 reverse movement of the gearing causes the movable girders 

 and roadway to travel back and return to their original position 

 ready for the train traffic. 



Opening bridges are sometimes constructed on this system 

 in cases where the level of the rails is only a few feet above the 

 level of the water, and where there is only one water opening, 

 and that not more than 20 to 30 feet wide. In such bridges 

 the movable portion is rolled back along iron rails, or plates 

 secured to masonry walls, or strong pile-work. This class of 

 bridge is cumbersome, slow to move, and is now but very rarely 

 adopted. 



Fig. 82 shows a type of simple lift bridge, of which there are 

 but few examples remaining. In this particular bridge the 

 girders and roadway form a solid framework, which rests on 

 the abutments during the passage of the trains. Strong chains, 

 secured to the corners of the framework, pass over large sheaves 

 on the top of the iron standards, and then round drums placed 

 below the level of the rails, and terminate by attachment to 

 heavy counter-weights suspended in iron cylinders. The counter- 

 weights are adjusted to approximately balance the bridge, so 

 that a moderate power applied to the wheel-gearing on the 

 drums is sufficient to raise the roadway to the required height. 

 This class of opening bridge is only suitable for the passage of 

 barges and small craft without masts; and it requires the re- 

 adjustment of the counter-weights when the roadway varies 

 in weight, in consequence of rain or repairs. 



Figs. 83, 84, and 85 are sketches of small swing-bridges 

 constructed for narrow waterways. Although differing in 

 appearance, they are all practically on the same principle, 

 with centre pin and roller path, and are similar in general 

 arrangement to the large-size-opening swing-bridges shown in 

 Figs. 78 and 79. 



The swing-bridge arrangement is so simple in construction, 

 convenient for inspection, and easy to maintain, that where 

 possible it is now generally adopted in preference to any other 



