13. Siege Machines.

A look at siege machines that convert energy into mechanical force to go over, under, or through fortified or fixed defenses too strong for conventional force. These engines range from man's first long-range missile weapon, the slingshot, to the laser cannons and satellite-destroying robots of the 21st century. All of these machines are designed to breach barriers–castle walls, entrenched troops, even outer space. When the going gets tough, the tough get siege machines.