Railway bridges of this pattern are now very rarely adopted. They have the great drawback that when raised to the vertical position, a very large area is presented to the action of the wind, and this defect might lead to very serious consequences in the case of a bridge situated in an exposed locality. An open-work floor diminishes the wind area, but a very large surface must necessarily remain. [Illustration: Fig. 80, 81, 82, 83, 84, 85] Fig. 81 illustrates what is known as a _traversing bridge_. In this case the width of the opening passage-way and the adjoining span are 87 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. [Illustration: Fig. 86, 87, 88]