Many of you are probably unaware of the number of different types of bridges. Just the basic list includes beam bridges, cantilever, arch, truss, suspension, cable-stayed, movable, floating and combination bridges. Within these basic categories there are a number of subcategories as well. Movable bridges, for instance, include the bascule (drawbridge), vertical lift and the swing bridge to name just the most common.
The basic characteristic of movable bridges is that they can be moved to allow other traffic to pass. The drawbridges of medieval times were the original movable bridges. These were raised by ropes and pulleys. In the 16th century, Leonardo da Vinci designed a movable bridge for military purposes. The most famous bascule bridge in the word is probably Tower Bridge across the Thames in London (below). Tower Bridge was originally hydraulically operated using steam power from coal-burning boilers. Today, the hydraulic mechanism is driven by oil, and it is powered by electricity.