We have all read of the castles in olden days into which the owner could retire and raise a drawbridge across a ditch, thus putting a barrier in the way of his enemies.

That old style drawbridge, with, of course, many improvements, has been adopted in these modern times to use in permitting navigable rivers and channels to be crossed by railroads and other kinds of transportation, without preventing the passage of vessels up and down the rivers.

Modern drawbridges across rivers, canals, the entrances of docks, etc., are generally made to open vertically, and the movable portion is called a bascule, balance or lifting bridge; a turning, swivel or swing bridge; or a rolling bridge, in accordance with the mode in which it is made to open.

Swing bridges are usually divided into two parts meeting in the middle, and each moved on pivots on the opposite sides of the channel, or they may move as a whole on a pivot in the middle of the channel.

Rolling bridges are suspended from a structure high above the water, and are propelled backwards and forwards by means of rollers.

Bascule Bridge Open*.

Bascule Bridge Closed*.

The advantages of this type of bridge are that the entire width of the channel is available for navigation, and the draw may be opened and closed more readily than the swing type. * Courtesy of The Strauss Bascule Bridge Co.